Auto merge of #112351 - BoxyUwU:proof_tree_generation, r=lcnr

new solver proof tree generation

Adds a new `-Z` flag `-Zdump-solver-proof-tree` which causes us to generate proof trees for each call to `infcx.evaluate_root_goal`. Currently these just get `debug!`'d out rather than put in a file.

Callers of `infcx.evaluate_root_goal` get the proof tree returned to them and can force one to be generated by passing in `GenerateProofTree::Yes`. Currently we do not disable the global cache or do anything about the provisional cache when generating proof trees is enabled, this is optimal for debugging but not so much for diagnostics.

Personally I think the exact formatting of proof trees here is not ideal but it's easy to change after this PR lands and would like to avoid bikeshedding the exact way we output this before we even have any of the logic merged to get all the information out of the solver.

This PR is best reviewed with whitespace changes hidden, there is also probably no point going commit by commit

r? `@lcnr`

Below is the proof tree output for the `AliasRelate(TAIT, sub, Alias)` case in rust-lang/trait-system-refactor-initiative#25 from the following command:
``RUSTC_LOG="rustc_trait_selection::solve::inspect::dump=debug" RUSTFLAGS="-Ztrait-solver=next -Zverbose -Zdump-solver-proof-tree" cargo +stage1 check``

<details><summary><strong>Proof tree</strong></summary>
<p>

```

DEBUG rustc_trait_selection::solve::inspect::dump tree=GOAL: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), [])), Subtype, Term::Ty(<?8t as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), [])), Subtype, Term::Ty(<^1_0 as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
    INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), [])), Subtype, Term::Ty(<?0t as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }
        CANDIDATE normalizes-to: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
            TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
            REVISION 0
                GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                    INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(<?0t as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }
                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                        REVISION 0
                            GOAL: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<^1_0 as std::iter::IntoIterator>::IntoIter as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                    NORMALIZING SELF TY FOR ASSEMBLY:
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                                    REVISION 0
                                            RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                    CANDIDATE constituent tys: Err(NoSolution)
                                    CANDIDATE assumption: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                        REVISION 0
                            RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                            GOAL: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::iter::Iterator>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<^1_0 as std::iter::IntoIterator>::IntoIter as std::iter::Iterator>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::iter::Iterator>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                    NORMALIZING SELF TY FOR ASSEMBLY:
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                    CANDIDATE assumption: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                        REVISION 0
                            RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<?0t as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<^1_0 as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<?0t as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                    REVISION 0
                                        NORMALIZES-TO HACK GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<?0t as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<^1_0 as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                        REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, <^0 as std::iter::IntoIterator>::Item] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                            INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<?0t as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                                NORMALIZING SELF TY FOR ASSEMBLY:
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                    REVISION 0
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?2t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                    REVISION 0
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?2t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                CANDIDATE assumption: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, <^0 as std::iter::IntoIterator>::Item] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                                    REVISION 0
                                        RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, <^0 as std::iter::IntoIterator>::Item] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        GOAL: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), [])), Equate, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), [])), Equate, Term::Ty(<^1_0 as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                        REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                            INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), [])), Equate, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                                CANDIDATE normalizes-to: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                    REVISION 0
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                            INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                                                TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                                REVISION 0
                                                                    GOAL: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::Item as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                                    CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<^1_0 as std::iter::IntoIterator>::Item as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                                    REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                                        INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::Item as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                                                                            NORMALIZING SELF TY FOR ASSEMBLY:
                                                                                TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                                                REVISION 0
                                                                                    GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                                                    CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                                                    REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                                                        INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                                                                                            TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                                                                            REVISION 0
                                                                                    RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                                                TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                                                REVISION 0
                                                                                    GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                                                    CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                                                    GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                                            CANDIDATE constituent tys: Err(NoSolution)
                                                                            CANDIDATE assumption: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                                                TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                                                                REVISION 0
                                                                    RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                                    GOAL: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::Item as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                                    CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<^1_0 as std::iter::IntoIterator>::Item as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                                    GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                        RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    REVISION 1
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                CANDIDATE normalizes-to: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                    REVISION 0
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                            INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                                                TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                                REVISION 0
                                                                    NORMALIZES-TO HACK GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                                    CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                                    REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                                        INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }
                                                                            TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                                                            REVISION 0
                                                                    RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                        RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                CANDIDATE substs relate: Err(NoSolution)
                                                CANDIDATE bidir normalizes-to: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                    REVISION 0
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                            INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                                                                TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                                REVISION 0
                                                                    NORMALIZES-TO HACK GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                                    CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                                    GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                        RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    REVISION 1
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7203 ~ core[b962]::iter::traits::collect::IntoIterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                    REVISION 1
                                        NORMALIZES-TO HACK GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<?0t as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(?2t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<^1_0 as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                        REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, <^0 as std::iter::IntoIterator>::Item] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                            INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<?0t as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                                                NORMALIZING SELF TY FOR ASSEMBLY:
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                    REVISION 0
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?2t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                            INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                                                                TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                                                REVISION 0
                                                        RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                                    REVISION 0
                                                        GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?2t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                CANDIDATE assumption: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, <^0 as std::iter::IntoIterator>::Item] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                                    TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                                    REVISION 0
                                        RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, <^0 as std::iter::IntoIterator>::Item] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        GOAL: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), [])), Equate, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), [])), Equate, Term::Ty(<^1_0 as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        GOAL: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), [])), Equate, Term::Ty(<?0t as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                        CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(AliasRelate(Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), [])), Equate, Term::Ty(<^1_0 as std::iter::IntoIterator>::Item)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                        GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                            RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                        REVISION 1
                            GOAL: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<^1_0 as std::iter::IntoIterator>::IntoIter as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::marker::Sized>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                                    NORMALIZING SELF TY FOR ASSEMBLY:
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                    CANDIDATE constituent tys: Err(NoSolution)
                                    CANDIDATE assumption: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                        REVISION 0
                            RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                            GOAL: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::iter::Iterator>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<^1_0 as std::iter::IntoIterator>::IntoIter as std::iter::Iterator>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(TraitPredicate(<<?0t as std::iter::IntoIterator>::IntoIter as std::iter::Iterator>, polarity:Positive), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                                    NORMALIZING SELF TY FOR ASSEMBLY:
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                                        REVISION 0
                                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                    CANDIDATE assumption: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                                        TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                                        REVISION 0
                            RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                            GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<?0t as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [<^1_0 as std::iter::IntoIterator>::IntoIter], def_id: DefId(2:7319 ~ core[b962]::iter::traits::iterator::Iterator::Item) }, Term::Ty(Opaque(DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
            REVISION 1
                GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
        CANDIDATE normalizes-to: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
            TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
            REVISION 0
                GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                    INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }
                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                        REVISION 0
                            NORMALIZES-TO HACK GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
        CANDIDATE substs relate: Err(NoSolution)
        CANDIDATE bidir normalizes-to: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
            TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
            REVISION 0
                GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?2t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                REVISION 0: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                    INSTANTIATED: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <?0t as std::iter::IntoIterator>::Item)] }, PrivateZst)) }
                        TRY_EVALUATE_ADDED_GOALS: Ok(Maybe(Ambiguity))
                        REVISION 0
                            NORMALIZES-TO HACK GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                            CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(^1_1)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] }
                            GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0, ^1] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }, CanonicalVarInfo { kind: Ty(General(U0)) }] })
                RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
            REVISION 1
                GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(?1t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [], def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}) }, Term::Ty(<^1_0 as std::iter::IntoIterator>::IntoIter)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
                GOAL: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [?0t], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(?2t)), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }
                CANONICALIZED: Canonical { value: QueryInput { goal: Goal { predicate: Binder(ProjectionPredicate(AliasTy { substs: [^1_0], def_id: DefId(2:7204 ~ core[b962]::iter::traits::collect::IntoIterator::IntoIter) }, Term::Ty(Opaque(DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), []))), []), param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst } }, anchor: Bind(DefId(0:6 ~ playground_proof_tree[b494]::a)), predefined_opaques_in_body: PredefinedOpaques(Interned(PredefinedOpaquesData { opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] }
                GLOBAL CACHE HIT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })
RESULT: Ok(Canonical { value: Response { certainty: Maybe(Ambiguity), var_values: CanonicalVarValues { var_values: [^0] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [(OpaqueTypeKey { def_id: DefId(0:8 ~ playground_proof_tree[b494]::Tait::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::IntoIter), (OpaqueTypeKey { def_id: DefId(0:9 ~ playground_proof_tree[b494]::Tait::{opaque#0}::{opaque#0}), substs: [] }, <^0 as std::iter::IntoIterator>::Item)] }, PrivateZst)) }, max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }] })

```

</p>
</details>

1 files changed, 202 insertions, 174 deletions

diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index 279fc1229d4..d9f24455a81 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -6,6 +6,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::{LangItem, Movability};
 use rustc_infer::traits::query::NoSolution;
 use rustc_infer::traits::util::supertraits;
+use rustc_middle::traits::solve::inspect::CandidateKind;
 use rustc_middle::traits::solve::{CanonicalResponse, Certainty, Goal, QueryResult};
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams, TreatProjections};
 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
@@ -61,21 +62,24 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             },
         };
 
-        ecx.probe(|ecx| {
-            let impl_substs = ecx.fresh_substs_for_item(impl_def_id);
-            let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
-
-            ecx.eq(goal.param_env, goal.predicate.trait_ref, impl_trait_ref)?;
-            let where_clause_bounds = tcx
-                .predicates_of(impl_def_id)
-                .instantiate(tcx, impl_substs)
-                .predicates
-                .into_iter()
-                .map(|pred| goal.with(tcx, pred));
-            ecx.add_goals(where_clause_bounds);
-
-            ecx.evaluate_added_goals_and_make_canonical_response(maximal_certainty)
-        })
+        ecx.probe(
+            |ecx| {
+                let impl_substs = ecx.fresh_substs_for_item(impl_def_id);
+                let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
+
+                ecx.eq(goal.param_env, goal.predicate.trait_ref, impl_trait_ref)?;
+                let where_clause_bounds = tcx
+                    .predicates_of(impl_def_id)
+                    .instantiate(tcx, impl_substs)
+                    .predicates
+                    .into_iter()
+                    .map(|pred| goal.with(tcx, pred));
+                ecx.add_goals(where_clause_bounds);
+
+                ecx.evaluate_added_goals_and_make_canonical_response(maximal_certainty)
+            },
+            |r| CandidateKind::Candidate { name: "impl".into(), result: *r },
+        )
     }
 
     fn probe_and_match_goal_against_assumption(
@@ -84,21 +88,26 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         assumption: ty::Binder<'tcx, ty::Clause<'tcx>>,
         then: impl FnOnce(&mut EvalCtxt<'_, 'tcx>) -> QueryResult<'tcx>,
     ) -> QueryResult<'tcx> {
-        if let Some(trait_clause) = assumption.as_trait_clause()
-            && trait_clause.def_id() == goal.predicate.def_id()
-            && trait_clause.polarity() == goal.predicate.polarity
-        {
-            // FIXME: Constness
-            ecx.probe(|ecx| {
-                let assumption_trait_pred =
-                    ecx.instantiate_binder_with_infer(trait_clause);
-                ecx.eq(
-                    goal.param_env,
-                    goal.predicate.trait_ref,
-                    assumption_trait_pred.trait_ref,
-                )?;
-                then(ecx)
-            })
+        if let Some(trait_clause) = assumption.as_trait_clause() {
+            if trait_clause.def_id() == goal.predicate.def_id()
+                && trait_clause.polarity() == goal.predicate.polarity
+            {
+                // FIXME: Constness
+                ecx.probe(
+                    |ecx| {
+                        let assumption_trait_pred = ecx.instantiate_binder_with_infer(trait_clause);
+                        ecx.eq(
+                            goal.param_env,
+                            goal.predicate.trait_ref,
+                            assumption_trait_pred.trait_ref,
+                        )?;
+                        then(ecx)
+                    },
+                    |r| CandidateKind::Candidate { name: "assumption".into(), result: *r },
+                )
+            } else {
+                Err(NoSolution)
+            }
         } else {
             Err(NoSolution)
         }
@@ -132,13 +141,16 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
 
         let tcx = ecx.tcx();
 
-        ecx.probe(|ecx| {
-            let nested_obligations = tcx
-                .predicates_of(goal.predicate.def_id())
-                .instantiate(tcx, goal.predicate.trait_ref.substs);
-            ecx.add_goals(nested_obligations.predicates.into_iter().map(|p| goal.with(tcx, p)));
-            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-        })
+        ecx.probe(
+            |ecx| {
+                let nested_obligations = tcx
+                    .predicates_of(goal.predicate.def_id())
+                    .instantiate(tcx, goal.predicate.trait_ref.substs);
+                ecx.add_goals(nested_obligations.predicates.into_iter().map(|p| goal.with(tcx, p)));
+                ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            },
+            |r| CandidateKind::Candidate { name: "trait alias".into(), result: *r },
+        )
     }
 
     fn consider_builtin_sized_candidate(
@@ -344,109 +356,116 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         if b_ty.is_ty_var() {
             return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS);
         }
-        ecx.probe(|ecx| {
-            match (a_ty.kind(), b_ty.kind()) {
-                // Trait upcasting, or `dyn Trait + Auto + 'a` -> `dyn Trait + 'b`
-                (&ty::Dynamic(_, _, ty::Dyn), &ty::Dynamic(_, _, ty::Dyn)) => {
-                    // Dyn upcasting is handled separately, since due to upcasting,
-                    // when there are two supertraits that differ by substs, we
-                    // may return more than one query response.
-                    Err(NoSolution)
-                }
-                // `T` -> `dyn Trait` unsizing
-                (_, &ty::Dynamic(data, region, ty::Dyn)) => {
-                    // Can only unsize to an object-safe type
-                    if data
-                        .principal_def_id()
-                        .is_some_and(|def_id| !tcx.check_is_object_safe(def_id))
-                    {
-                        return Err(NoSolution);
+        ecx.probe(
+            |ecx| {
+                match (a_ty.kind(), b_ty.kind()) {
+                    // Trait upcasting, or `dyn Trait + Auto + 'a` -> `dyn Trait + 'b`
+                    (&ty::Dynamic(_, _, ty::Dyn), &ty::Dynamic(_, _, ty::Dyn)) => {
+                        // Dyn upcasting is handled separately, since due to upcasting,
+                        // when there are two supertraits that differ by substs, we
+                        // may return more than one query response.
+                        Err(NoSolution)
                     }
-
-                    let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
+                    // `T` -> `dyn Trait` unsizing
+                    (_, &ty::Dynamic(data, region, ty::Dyn)) => {
+                        // Can only unsize to an object-safe type
+                        if data
+                            .principal_def_id()
+                            .is_some_and(|def_id| !tcx.check_is_object_safe(def_id))
+                        {
+                            return Err(NoSolution);
+                        }
+
+                        let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
                         return Err(NoSolution);
                     };
-                    // Check that the type implements all of the predicates of the def-id.
-                    // (i.e. the principal, all of the associated types match, and any auto traits)
-                    ecx.add_goals(
-                        data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))),
-                    );
-                    // The type must be Sized to be unsized.
-                    ecx.add_goal(goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])));
-                    // The type must outlive the lifetime of the `dyn` we're unsizing into.
-                    ecx.add_goal(
-                        goal.with(tcx, ty::Binder::dummy(ty::OutlivesPredicate(a_ty, region))),
-                    );
-                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-                }
-                // `[T; n]` -> `[T]` unsizing
-                (&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => {
-                    // We just require that the element type stays the same
-                    ecx.eq(goal.param_env, a_elem_ty, b_elem_ty)?;
-                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-                }
-                // Struct unsizing `Struct<T>` -> `Struct<U>` where `T: Unsize<U>`
-                (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs))
-                    if a_def.is_struct() && a_def.did() == b_def.did() =>
-                {
-                    let unsizing_params = tcx.unsizing_params_for_adt(a_def.did());
-                    // We must be unsizing some type parameters. This also implies
-                    // that the struct has a tail field.
-                    if unsizing_params.is_empty() {
-                        return Err(NoSolution);
+                        // Check that the type implements all of the predicates of the def-id.
+                        // (i.e. the principal, all of the associated types match, and any auto traits)
+                        ecx.add_goals(
+                            data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))),
+                        );
+                        // The type must be Sized to be unsized.
+                        ecx.add_goal(goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])));
+                        // The type must outlive the lifetime of the `dyn` we're unsizing into.
+                        ecx.add_goal(
+                            goal.with(tcx, ty::Binder::dummy(ty::OutlivesPredicate(a_ty, region))),
+                        );
+                        ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
                     }
-
-                    let tail_field = a_def
-                        .non_enum_variant()
-                        .fields
-                        .raw
-                        .last()
-                        .expect("expected unsized ADT to have a tail field");
-                    let tail_field_ty = tcx.type_of(tail_field.did);
-
-                    let a_tail_ty = tail_field_ty.subst(tcx, a_substs);
-                    let b_tail_ty = tail_field_ty.subst(tcx, b_substs);
-
-                    // Substitute just the unsizing params from B into A. The type after
-                    // this substitution must be equal to B. This is so we don't unsize
-                    // unrelated type parameters.
-                    let new_a_substs =
-                        tcx.mk_substs_from_iter(a_substs.iter().enumerate().map(|(i, a)| {
-                            if unsizing_params.contains(i as u32) { b_substs[i] } else { a }
-                        }));
-                    let unsized_a_ty = tcx.mk_adt(a_def, new_a_substs);
-
-                    // Finally, we require that `TailA: Unsize<TailB>` for the tail field
-                    // types.
-                    ecx.eq(goal.param_env, unsized_a_ty, b_ty)?;
-                    ecx.add_goal(goal.with(
-                        tcx,
-                        ty::TraitRef::new(tcx, goal.predicate.def_id(), [a_tail_ty, b_tail_ty]),
-                    ));
-                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-                }
-                // Tuple unsizing `(.., T)` -> `(.., U)` where `T: Unsize<U>`
-                (&ty::Tuple(a_tys), &ty::Tuple(b_tys))
-                    if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
-                {
-                    let (a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
-                    let b_last_ty = b_tys.last().unwrap();
-
-                    // Substitute just the tail field of B., and require that they're equal.
-                    let unsized_a_ty =
-                        tcx.mk_tup_from_iter(a_rest_tys.iter().chain([b_last_ty]).copied());
-                    ecx.eq(goal.param_env, unsized_a_ty, b_ty)?;
-
-                    // Similar to ADTs, require that the rest of the fields are equal.
-                    ecx.add_goal(goal.with(
-                        tcx,
-                        ty::TraitRef::new(tcx, goal.predicate.def_id(), [*a_last_ty, *b_last_ty]),
-                    ));
-                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                    // `[T; n]` -> `[T]` unsizing
+                    (&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => {
+                        // We just require that the element type stays the same
+                        ecx.eq(goal.param_env, a_elem_ty, b_elem_ty)?;
+                        ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                    }
+                    // Struct unsizing `Struct<T>` -> `Struct<U>` where `T: Unsize<U>`
+                    (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs))
+                        if a_def.is_struct() && a_def.did() == b_def.did() =>
+                    {
+                        let unsizing_params = tcx.unsizing_params_for_adt(a_def.did());
+                        // We must be unsizing some type parameters. This also implies
+                        // that the struct has a tail field.
+                        if unsizing_params.is_empty() {
+                            return Err(NoSolution);
+                        }
+
+                        let tail_field = a_def
+                            .non_enum_variant()
+                            .fields
+                            .raw
+                            .last()
+                            .expect("expected unsized ADT to have a tail field");
+                        let tail_field_ty = tcx.type_of(tail_field.did);
+
+                        let a_tail_ty = tail_field_ty.subst(tcx, a_substs);
+                        let b_tail_ty = tail_field_ty.subst(tcx, b_substs);
+
+                        // Substitute just the unsizing params from B into A. The type after
+                        // this substitution must be equal to B. This is so we don't unsize
+                        // unrelated type parameters.
+                        let new_a_substs =
+                            tcx.mk_substs_from_iter(a_substs.iter().enumerate().map(|(i, a)| {
+                                if unsizing_params.contains(i as u32) { b_substs[i] } else { a }
+                            }));
+                        let unsized_a_ty = tcx.mk_adt(a_def, new_a_substs);
+
+                        // Finally, we require that `TailA: Unsize<TailB>` for the tail field
+                        // types.
+                        ecx.eq(goal.param_env, unsized_a_ty, b_ty)?;
+                        ecx.add_goal(goal.with(
+                            tcx,
+                            ty::TraitRef::new(tcx, goal.predicate.def_id(), [a_tail_ty, b_tail_ty]),
+                        ));
+                        ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                    }
+                    // Tuple unsizing `(.., T)` -> `(.., U)` where `T: Unsize<U>`
+                    (&ty::Tuple(a_tys), &ty::Tuple(b_tys))
+                        if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
+                    {
+                        let (a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
+                        let b_last_ty = b_tys.last().unwrap();
+
+                        // Substitute just the tail field of B., and require that they're equal.
+                        let unsized_a_ty =
+                            tcx.mk_tup_from_iter(a_rest_tys.iter().chain([b_last_ty]).copied());
+                        ecx.eq(goal.param_env, unsized_a_ty, b_ty)?;
+
+                        // Similar to ADTs, require that the rest of the fields are equal.
+                        ecx.add_goal(goal.with(
+                            tcx,
+                            ty::TraitRef::new(
+                                tcx,
+                                goal.predicate.def_id(),
+                                [*a_last_ty, *b_last_ty],
+                            ),
+                        ));
+                        ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                    }
+                    _ => Err(NoSolution),
                 }
-                _ => Err(NoSolution),
-            }
-        })
+            },
+            |r| CandidateKind::Candidate { name: "builtin unsize".into(), result: *r },
+        )
     }
 
     fn consider_builtin_dyn_upcast_candidates(
@@ -476,32 +495,38 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         }
 
         let mut unsize_dyn_to_principal = |principal: Option<ty::PolyExistentialTraitRef<'tcx>>| {
-            ecx.probe(|ecx| -> Result<_, NoSolution> {
-                // Require that all of the trait predicates from A match B, except for
-                // the auto traits. We do this by constructing a new A type with B's
-                // auto traits, and equating these types.
-                let new_a_data = principal
-                    .into_iter()
-                    .map(|trait_ref| trait_ref.map_bound(ty::ExistentialPredicate::Trait))
-                    .chain(a_data.iter().filter(|a| {
-                        matches!(a.skip_binder(), ty::ExistentialPredicate::Projection(_))
-                    }))
-                    .chain(
-                        b_data
-                            .auto_traits()
-                            .map(ty::ExistentialPredicate::AutoTrait)
-                            .map(ty::Binder::dummy),
+            ecx.probe(
+                |ecx| -> Result<_, NoSolution> {
+                    // Require that all of the trait predicates from A match B, except for
+                    // the auto traits. We do this by constructing a new A type with B's
+                    // auto traits, and equating these types.
+                    let new_a_data = principal
+                        .into_iter()
+                        .map(|trait_ref| trait_ref.map_bound(ty::ExistentialPredicate::Trait))
+                        .chain(a_data.iter().filter(|a| {
+                            matches!(a.skip_binder(), ty::ExistentialPredicate::Projection(_))
+                        }))
+                        .chain(
+                            b_data
+                                .auto_traits()
+                                .map(ty::ExistentialPredicate::AutoTrait)
+                                .map(ty::Binder::dummy),
+                        );
+                    let new_a_data = tcx.mk_poly_existential_predicates_from_iter(new_a_data);
+                    let new_a_ty = tcx.mk_dynamic(new_a_data, b_region, ty::Dyn);
+
+                    // We also require that A's lifetime outlives B's lifetime.
+                    ecx.eq(goal.param_env, new_a_ty, b_ty)?;
+                    ecx.add_goal(
+                        goal.with(
+                            tcx,
+                            ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region)),
+                        ),
                     );
-                let new_a_data = tcx.mk_poly_existential_predicates_from_iter(new_a_data);
-                let new_a_ty = tcx.mk_dynamic(new_a_data, b_region, ty::Dyn);
-
-                // We also require that A's lifetime outlives B's lifetime.
-                ecx.eq(goal.param_env, new_a_ty, b_ty)?;
-                ecx.add_goal(
-                    goal.with(tcx, ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region))),
-                );
-                ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-            })
+                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                },
+                |r| CandidateKind::Candidate { name: "upcast dyn to principle".into(), result: *r },
+            )
         };
 
         let mut responses = vec![];
@@ -698,20 +723,23 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         goal: Goal<'tcx, TraitPredicate<'tcx>>,
         constituent_tys: impl Fn(&EvalCtxt<'_, 'tcx>, Ty<'tcx>) -> Result<Vec<Ty<'tcx>>, NoSolution>,
     ) -> QueryResult<'tcx> {
-        self.probe(|ecx| {
-            ecx.add_goals(
-                constituent_tys(ecx, goal.predicate.self_ty())?
-                    .into_iter()
-                    .map(|ty| {
-                        goal.with(
-                            ecx.tcx(),
-                            ty::Binder::dummy(goal.predicate.with_self_ty(ecx.tcx(), ty)),
-                        )
-                    })
-                    .collect::<Vec<_>>(),
-            );
-            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-        })
+        self.probe(
+            |ecx| {
+                ecx.add_goals(
+                    constituent_tys(ecx, goal.predicate.self_ty())?
+                        .into_iter()
+                        .map(|ty| {
+                            goal.with(
+                                ecx.tcx(),
+                                ty::Binder::dummy(goal.predicate.with_self_ty(ecx.tcx(), ty)),
+                            )
+                        })
+                        .collect::<Vec<_>>(),
+                );
+                ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            },
+            |r| CandidateKind::Candidate { name: "constituent tys".into(), result: *r },
+        )
     }
 
     #[instrument(level = "debug", skip(self))]