Delegation refactoring: add builders for generics inheritance

3 files changed, 214 insertions, 110 deletions

diff --git a/compiler/rustc_hir_analysis/src/delegation.rs b/compiler/rustc_hir_analysis/src/delegation.rs
index 20aaa43219f..d7a0698b6b3 100644
--- a/compiler/rustc_hir_analysis/src/delegation.rs
+++ b/compiler/rustc_hir_analysis/src/delegation.rs
@@ -76,41 +76,153 @@ fn fn_kind<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> FnKind {
     }
 }
 
+struct GenericsBuilder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    sig_id: DefId,
+    parent: Option<DefId>,
+}
+
+impl<'tcx> GenericsBuilder<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, sig_id: DefId) -> GenericsBuilder<'tcx> {
+        GenericsBuilder { tcx, sig_id, parent: None }
+    }
+
+    fn build(self) -> ty::Generics {
+        let mut own_params = vec![];
+
+        let sig_generics = self.tcx.generics_of(self.sig_id);
+        if let Some(parent_def_id) = sig_generics.parent {
+            let sig_parent_generics = self.tcx.generics_of(parent_def_id);
+            own_params.append(&mut sig_parent_generics.own_params.clone());
+        }
+        own_params.append(&mut sig_generics.own_params.clone());
+
+        // Lifetime parameters must be declared before type and const parameters.
+        // Therefore, When delegating from a free function to a associated function,
+        // generic parameters need to be reordered:
+        //
+        // trait Trait<'a, A> {
+        //     fn foo<'b, B>(...) {...}
+        // }
+        //
+        // reuse Trait::foo;
+        // desugaring:
+        // fn foo<'a, 'b, This: Trait<'a, A>, A, B>(...) {
+        //     Trait::foo(...)
+        // }
+        own_params.sort_by_key(|key| key.kind.is_ty_or_const());
+
+        let param_def_id_to_index =
+            own_params.iter().map(|param| (param.def_id, param.index)).collect();
+
+        for (idx, param) in own_params.iter_mut().enumerate() {
+            param.index = idx as u32;
+            // FIXME(fn_delegation): Default parameters are not inherited, because they are
+            // not permitted in functions. Therefore, there are 2 options here:
+            //
+            // - We can create non-default generic parameters.
+            // - We can substitute default parameters into the signature.
+            //
+            // At the moment, first option has been selected as the most general.
+            if let ty::GenericParamDefKind::Type { has_default, .. }
+            | ty::GenericParamDefKind::Const { has_default, .. } = &mut param.kind
+            {
+                *has_default = false;
+            }
+        }
+
+        ty::Generics {
+            parent: self.parent,
+            parent_count: 0,
+            own_params,
+            param_def_id_to_index,
+            has_self: false,
+            has_late_bound_regions: sig_generics.has_late_bound_regions,
+            host_effect_index: sig_generics.host_effect_index,
+        }
+    }
+}
+
+struct PredicatesBuilder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    args: ty::GenericArgsRef<'tcx>,
+    parent: Option<DefId>,
+    sig_id: DefId,
+}
+
+impl<'tcx> PredicatesBuilder<'tcx> {
+    fn new(
+        tcx: TyCtxt<'tcx>,
+        args: ty::GenericArgsRef<'tcx>,
+        sig_id: DefId,
+    ) -> PredicatesBuilder<'tcx> {
+        PredicatesBuilder { tcx, args, parent: None, sig_id }
+    }
+
+    fn build(self) -> ty::GenericPredicates<'tcx> {
+        let mut preds = vec![];
+
+        let sig_predicates = self.tcx.predicates_of(self.sig_id);
+        if let Some(parent) = sig_predicates.parent {
+            let sig_parent_preds = self.tcx.predicates_of(parent);
+            preds.extend(sig_parent_preds.instantiate_own(self.tcx, self.args));
+        }
+        preds.extend(sig_predicates.instantiate_own(self.tcx, self.args));
+
+        ty::GenericPredicates {
+            parent: self.parent,
+            predicates: self.tcx.arena.alloc_from_iter(preds),
+            // FIXME(fn_delegation): Support effects.
+            effects_min_tys: ty::List::empty(),
+        }
+    }
+}
+
+struct GenericArgsBuilder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    remap_table: RemapTable,
+    sig_id: DefId,
+    def_id: LocalDefId,
+}
+
+impl<'tcx> GenericArgsBuilder<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, sig_id: DefId, def_id: LocalDefId) -> GenericArgsBuilder<'tcx> {
+        GenericArgsBuilder { tcx, remap_table: FxHashMap::default(), sig_id, def_id }
+    }
+
+    fn build_from_args(mut self, args: ty::GenericArgsRef<'tcx>) -> ty::GenericArgsRef<'tcx> {
+        let caller_generics = self.tcx.generics_of(self.def_id);
+        let callee_generics = self.tcx.generics_of(self.sig_id);
+
+        for caller_param in &caller_generics.own_params {
+            let callee_index =
+                callee_generics.param_def_id_to_index(self.tcx, caller_param.def_id).unwrap();
+            self.remap_table.insert(callee_index, caller_param.index);
+        }
+
+        let mut folder = ParamIndexRemapper { tcx: self.tcx, remap_table: self.remap_table };
+        args.fold_with(&mut folder)
+    }
+}
+
 fn create_generic_args<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
     sig_id: DefId,
 ) -> ty::GenericArgsRef<'tcx> {
-    let caller_generics = tcx.generics_of(def_id);
-    let callee_generics = tcx.generics_of(sig_id);
+    let builder = GenericArgsBuilder::new(tcx, sig_id, def_id);
 
     let caller_kind = fn_kind(tcx, def_id.into());
     let callee_kind = fn_kind(tcx, sig_id);
-    // FIXME(fn_delegation): Support generics on associated delegation items.
-    // Error will be reported in `check_constraints`.
+
     match (caller_kind, callee_kind) {
-        (FnKind::Free, _) => {
-            // Lifetime parameters must be declared before type and const parameters.
-            // Therefore, When delegating from a free function to a associated function,
-            // generic parameters need to be reordered:
-            //
-            // trait Trait<'a, A> {
-            //     fn foo<'b, B>(...) {...}
-            // }
-            //
-            // reuse Trait::foo;
-            // desugaring:
-            // fn foo<'a, 'b, This: Trait<'a, A>, A, B>(...) {
-            //     Trait::foo(...)
-            // }
-            let mut remap_table = RemapTable::default();
-            for caller_param in &caller_generics.own_params {
-                let callee_index =
-                    callee_generics.param_def_id_to_index(tcx, caller_param.def_id).unwrap();
-                remap_table.insert(callee_index, caller_param.index);
-            }
-            let mut folder = ParamIndexRemapper { tcx, remap_table };
-            ty::GenericArgs::identity_for_item(tcx, sig_id).fold_with(&mut folder)
+        (FnKind::Free, FnKind::Free)
+        | (FnKind::Free, FnKind::AssocTrait)
+        | (FnKind::AssocInherentImpl, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::AssocTrait) => {
+            let args = ty::GenericArgs::identity_for_item(tcx, sig_id);
+            builder.build_from_args(args)
         }
         // FIXME(fn_delegation): Only `Self` param supported here.
         (FnKind::AssocTraitImpl, FnKind::AssocTrait)
@@ -120,7 +232,11 @@ fn create_generic_args<'tcx>(
             let generic_self_ty = ty::GenericArg::from(self_ty);
             tcx.mk_args_from_iter(std::iter::once(generic_self_ty))
         }
-        _ => ty::GenericArgs::identity_for_item(tcx, sig_id),
+        // For trait impl's `sig_id` is always equal to the corresponding trait method.
+        (FnKind::AssocTraitImpl, _)
+        | (_, FnKind::AssocTraitImpl)
+        // Delegation to inherent methods is not yet supported.
+        | (_, FnKind::AssocInherentImpl) => unreachable!(),
     }
 }
 
@@ -129,47 +245,29 @@ pub(crate) fn inherit_generics_for_delegation_item<'tcx>(
     def_id: LocalDefId,
     sig_id: DefId,
 ) -> Option<ty::Generics> {
-    // FIXME(fn_delegation): Support generics on associated delegation items.
-    // Error will be reported in `check_constraints`.
-    if fn_kind(tcx, def_id.into()) != FnKind::Free {
-        return None;
-    }
+    let builder = GenericsBuilder::new(tcx, sig_id);
 
-    let mut own_params = vec![];
-
-    let callee_generics = tcx.generics_of(sig_id);
-    if let Some(parent_sig_id) = callee_generics.parent {
-        let parent_sig_generics = tcx.generics_of(parent_sig_id);
-        own_params.append(&mut parent_sig_generics.own_params.clone());
-    }
-    own_params.append(&mut callee_generics.own_params.clone());
+    let caller_kind = fn_kind(tcx, def_id.into());
+    let callee_kind = fn_kind(tcx, sig_id);
 
-    // Lifetimes go first.
-    own_params.sort_by_key(|key| key.kind.is_ty_or_const());
+    // FIXME(fn_delegation): Support generics on associated delegation items.
+    // Error will be reported in `check_constraints`.
+    match (caller_kind, callee_kind) {
+        (FnKind::Free, FnKind::Free)
+        | (FnKind::Free, FnKind::AssocTrait) => Some(builder.build()),
 
-    for (idx, param) in own_params.iter_mut().enumerate() {
-        param.index = idx as u32;
-        // Default parameters are not inherited: they are not allowed
-        // in fn's.
-        if let ty::GenericParamDefKind::Type { has_default, .. }
-        | ty::GenericParamDefKind::Const { has_default, .. } = &mut param.kind
-        {
-            *has_default = false;
-        }
+        (FnKind::AssocTraitImpl, FnKind::AssocTrait)
+        | (FnKind::AssocInherentImpl, FnKind::AssocTrait)
+        | (FnKind::AssocTrait, FnKind::AssocTrait)
+        | (FnKind::AssocInherentImpl, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::Free) => None,
+
+        // For trait impl's `sig_id` is always equal to the corresponding trait method.
+        (FnKind::AssocTraitImpl, _)
+        | (_, FnKind::AssocTraitImpl)
+        // Delegation to inherent methods is not yet supported.
+        | (_, FnKind::AssocInherentImpl) => unreachable!(),
     }
-
-    let param_def_id_to_index =
-        own_params.iter().map(|param| (param.def_id, param.index)).collect();
-
-    Some(ty::Generics {
-        parent: None,
-        parent_count: 0,
-        own_params,
-        param_def_id_to_index,
-        has_self: false,
-        has_late_bound_regions: callee_generics.has_late_bound_regions,
-        host_effect_index: callee_generics.host_effect_index,
-    })
 }
 
 pub(crate) fn inherit_predicates_for_delegation_item<'tcx>(
@@ -177,26 +275,32 @@ pub(crate) fn inherit_predicates_for_delegation_item<'tcx>(
     def_id: LocalDefId,
     sig_id: DefId,
 ) -> Option<ty::GenericPredicates<'tcx>> {
+    let args = create_generic_args(tcx, def_id, sig_id);
+    let builder = PredicatesBuilder::new(tcx, args, sig_id);
+
+    let caller_kind = fn_kind(tcx, def_id.into());
+    let callee_kind = fn_kind(tcx, sig_id);
+
     // FIXME(fn_delegation): Support generics on associated delegation items.
     // Error will be reported in `check_constraints`.
-    if fn_kind(tcx, def_id.into()) != FnKind::Free {
-        return None;
-    }
-
-    let callee_predicates = tcx.predicates_of(sig_id);
-    let args = create_generic_args(tcx, def_id, sig_id);
+    match (caller_kind, callee_kind) {
+        (FnKind::Free, FnKind::Free)
+        | (FnKind::Free, FnKind::AssocTrait) => {
+            Some(builder.build())
+        }
 
-    let mut preds = vec![];
-    if let Some(parent_id) = callee_predicates.parent {
-        preds.extend(tcx.predicates_of(parent_id).instantiate_own(tcx, args));
+        (FnKind::AssocTraitImpl, FnKind::AssocTrait)
+        | (FnKind::AssocInherentImpl, FnKind::AssocTrait)
+        | (FnKind::AssocTrait, FnKind::AssocTrait)
+        | (FnKind::AssocInherentImpl, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::Free) => None,
+
+        // For trait impl's `sig_id` is always equal to the corresponding trait method.
+        (FnKind::AssocTraitImpl, _)
+        | (_, FnKind::AssocTraitImpl)
+        // Delegation to inherent methods is not yet supported.
+        | (_, FnKind::AssocInherentImpl) => unreachable!(),
     }
-    preds.extend(callee_predicates.instantiate_own(tcx, args));
-
-    Some(ty::GenericPredicates {
-        parent: None,
-        predicates: tcx.arena.alloc_from_iter(preds),
-        effects_min_tys: ty::List::empty(),
-    })
 }
 
 fn check_constraints<'tcx>(
diff --git a/tests/ui/delegation/not-supported.rs b/tests/ui/delegation/unsupported.rs
index d5ac68ecf1b..d5ac68ecf1b 100644
--- a/tests/ui/delegation/not-supported.rs
+++ b/tests/ui/delegation/unsupported.rs
diff --git a/tests/ui/delegation/not-supported.stderr b/tests/ui/delegation/unsupported.stderr
index 14d6b374bd2..bb150254f48 100644
--- a/tests/ui/delegation/not-supported.stderr
+++ b/tests/ui/delegation/unsupported.stderr
@@ -4,7 +4,7 @@ error: using `#![feature(effects)]` without enabling next trait solver globally
    = help: use `-Znext-solver` to enable
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:18:29
+  --> $DIR/unsupported.rs:18:29
    |
 LL |         fn bar(&self, x: T) -> T { x }
    |         ------------------------ callee defined here
@@ -13,7 +13,7 @@ LL |         reuse GenericTrait::bar;
    |                             ^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:20:29
+  --> $DIR/unsupported.rs:20:29
    |
 LL |         fn bar1() {}
    |         --------- callee defined here
@@ -22,7 +22,7 @@ LL |         reuse GenericTrait::bar1;
    |                             ^^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:31:39
+  --> $DIR/unsupported.rs:31:39
    |
 LL |         fn bar(&self, x: T) -> T { x }
    |         ------------------------ callee defined here
@@ -31,7 +31,7 @@ LL |         reuse <F as GenericTrait<T>>::bar { &self.0 }
    |                                       ^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:33:34
+  --> $DIR/unsupported.rs:33:34
    |
 LL |         fn bar1() {}
    |         --------- callee defined here
@@ -40,7 +40,7 @@ LL |         reuse GenericTrait::<T>::bar1;
    |                                  ^^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:38:29
+  --> $DIR/unsupported.rs:38:29
    |
 LL |         fn bar(&self, x: T) -> T { x }
    |         ------------------------ callee defined here
@@ -49,7 +49,7 @@ LL |         reuse GenericTrait::bar { &F }
    |                             ^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:40:29
+  --> $DIR/unsupported.rs:40:29
    |
 LL |         fn bar1() {}
    |         --------- callee defined here
@@ -58,7 +58,7 @@ LL |         reuse GenericTrait::bar1;
    |                             ^^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:45:22
+  --> $DIR/unsupported.rs:45:22
    |
 LL |         fn foo(&self, x: i32) -> i32 { x }
    |         ---------------------------- callee defined here
@@ -67,7 +67,7 @@ LL |         reuse Trait::foo;
    |                      ^^^
 
 error[E0049]: method `foo2` has 0 type parameters but its trait declaration has 1 type parameter
-  --> $DIR/not-supported.rs:51:22
+  --> $DIR/unsupported.rs:51:22
    |
 LL |         fn foo2<T>(&self, x: T) -> T { x }
    |                 - expected 1 type parameter
@@ -76,7 +76,7 @@ LL |         reuse Trait::foo2 { &self.0 }
    |                      ^^^^ found 0 type parameters
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:54:29
+  --> $DIR/unsupported.rs:54:29
    |
 LL |         fn foo3<'a: 'a>(_: &'a u32) {}
    |         --------------------------- callee defined here
@@ -85,7 +85,7 @@ LL |         reuse <F as Trait>::foo3;
    |                             ^^^^
 
 error[E0195]: lifetime parameters or bounds on associated function `foo3` do not match the trait declaration
-  --> $DIR/not-supported.rs:54:29
+  --> $DIR/unsupported.rs:54:29
    |
 LL |         fn foo3<'a: 'a>(_: &'a u32) {}
    |                -------- lifetimes in impl do not match this associated function in trait
@@ -94,7 +94,7 @@ LL |         reuse <F as Trait>::foo3;
    |                             ^^^^ lifetimes do not match associated function in trait
 
 error: delegation to a function with effect parameter is not supported yet
-  --> $DIR/not-supported.rs:112:18
+  --> $DIR/unsupported.rs:112:18
    |
 LL |         fn foo();
    |         --------- callee defined here
@@ -103,7 +103,7 @@ LL |     reuse Trait::foo;
    |                  ^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:61:22
+  --> $DIR/unsupported.rs:61:22
    |
 LL |         fn foo(&self, x: i32) -> i32 { x }
    |         ---------------------------- callee defined here
@@ -112,7 +112,7 @@ LL |         reuse Trait::foo { &self.0 }
    |                      ^^^
 
 error: early bound generics are not supported for associated delegation items
-  --> $DIR/not-supported.rs:51:22
+  --> $DIR/unsupported.rs:51:22
    |
 LL |         fn foo2<T>(&self, x: T) -> T { x }
    |         ---------------------------- callee defined here
@@ -121,7 +121,7 @@ LL |         reuse Trait::foo2 { &self.0 }
    |                      ^^^^
 
 warning: this function depends on never type fallback being `()`
-  --> $DIR/not-supported.rs:79:9
+  --> $DIR/unsupported.rs:79:9
    |
 LL |         fn opaque_ret() -> impl Trait { unimplemented!() }
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -130,33 +130,33 @@ LL |         fn opaque_ret() -> impl Trait { unimplemented!() }
    = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
    = help: specify the types explicitly
 note: in edition 2024, the requirement `!: opaque::Trait` will fail
-  --> $DIR/not-supported.rs:79:28
+  --> $DIR/unsupported.rs:79:28
    |
 LL |         fn opaque_ret() -> impl Trait { unimplemented!() }
    |                            ^^^^^^^^^^
    = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:24>::{synthetic#0}`
-  --> $DIR/not-supported.rs:86:25
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/unsupported.rs:85:5: 85:24>::{synthetic#0}`
+  --> $DIR/unsupported.rs:86:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:86:25
+  --> $DIR/unsupported.rs:86:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:24>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:85:5: 85:24>` is well-formed
-  --> $DIR/not-supported.rs:85:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/unsupported.rs:85:5: 85:24>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/unsupported.rs:85:5: 85:24>` is well-formed
+  --> $DIR/unsupported.rs:85:5
    |
 LL |     impl ToReuse for u8 {
    |     ^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
 warning: this function depends on never type fallback being `()`
-  --> $DIR/not-supported.rs:73:9
+  --> $DIR/unsupported.rs:73:9
    |
 LL |         pub fn opaque_ret() -> impl Trait { unimplemented!() }
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -165,32 +165,32 @@ LL |         pub fn opaque_ret() -> impl Trait { unimplemented!() }
    = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
    = help: specify the types explicitly
 note: in edition 2024, the requirement `!: opaque::Trait` will fail
-  --> $DIR/not-supported.rs:73:32
+  --> $DIR/unsupported.rs:73:32
    |
 LL |         pub fn opaque_ret() -> impl Trait { unimplemented!() }
    |                                ^^^^^^^^^^
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:88:5: 88:25>::{synthetic#0}`
-  --> $DIR/not-supported.rs:89:24
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/unsupported.rs:88:5: 88:25>::{synthetic#0}`
+  --> $DIR/unsupported.rs:89:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:89:24
+  --> $DIR/unsupported.rs:89:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:88:5: 88:25>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:88:5: 88:25>` is well-formed
-  --> $DIR/not-supported.rs:88:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/unsupported.rs:88:5: 88:25>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/unsupported.rs:88:5: 88:25>` is well-formed
+  --> $DIR/unsupported.rs:88:5
    |
 LL |     impl ToReuse for u16 {
    |     ^^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
 error: recursive delegation is not supported yet
-  --> $DIR/not-supported.rs:102:22
+  --> $DIR/unsupported.rs:102:22
    |
 LL |         pub reuse to_reuse2::foo;
    |                              --- callee defined here