Rollup merge of #100888 - spastorino:coherence-negative-impls-implied-bounds, r=lcnr

Coherence negative impls implied bounds

Fixes #93875

This PR is rebased on top of #100789 and it would need to include that one which is already r+ed.

r? ``@nikomatsakis``

cc ``@lcnr`` (which I've talked about https://github.com/rust-lang/rust/commit/3222f420d9d2312efe0735eb48160c7b070adc54, I guess after you finish your reordering of modules and work with OutlivesEnvironmentEnv this commit can just be reverted).

4 files changed, 146 insertions, 11 deletions

diff --git a/compiler/rustc_trait_selection/src/lib.rs b/compiler/rustc_trait_selection/src/lib.rs
index 9c252fcfe1c..ce48d4c99e9 100644
--- a/compiler/rustc_trait_selection/src/lib.rs
+++ b/compiler/rustc_trait_selection/src/lib.rs
@@ -20,6 +20,7 @@
 #![feature(let_else)]
 #![feature(if_let_guard)]
 #![feature(never_type)]
+#![feature(type_alias_impl_trait)]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 3bc08fba91a..292787d4dbb 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -6,16 +6,18 @@
 
 use crate::infer::outlives::env::OutlivesEnvironment;
 use crate::infer::{CombinedSnapshot, InferOk};
+use crate::traits::outlives_bounds::InferCtxtExt as _;
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::util::impl_subject_and_oblig;
 use crate::traits::SkipLeakCheck;
 use crate::traits::{
-    self, Normalized, Obligation, ObligationCause, PredicateObligation, PredicateObligations,
-    SelectionContext,
+    self, Normalized, Obligation, ObligationCause, ObligationCtxt, PredicateObligation,
+    PredicateObligations, SelectionContext,
 };
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::Diagnostic;
-use rustc_hir::def_id::{DefId, LOCAL_CRATE};
+use rustc_hir::def_id::{DefId, CRATE_DEF_ID, LOCAL_CRATE};
+use rustc_hir::CRATE_HIR_ID;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::util;
 use rustc_middle::traits::specialization_graph::OverlapMode;
@@ -322,7 +324,7 @@ fn negative_impl<'cx, 'tcx>(
         let (subject2, obligations) =
             impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
 
-        !equate(&infcx, impl_env, subject1, subject2, obligations)
+        !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
     })
 }
 
@@ -332,6 +334,7 @@ fn equate<'cx, 'tcx>(
     subject1: ImplSubject<'tcx>,
     subject2: ImplSubject<'tcx>,
     obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
+    body_def_id: DefId,
 ) -> bool {
     // do the impls unify? If not, not disjoint.
     let Ok(InferOk { obligations: more_obligations, .. }) =
@@ -342,8 +345,10 @@ fn equate<'cx, 'tcx>(
     };
 
     let selcx = &mut SelectionContext::new(&infcx);
-    let opt_failing_obligation =
-        obligations.into_iter().chain(more_obligations).find(|o| negative_impl_exists(selcx, o));
+    let opt_failing_obligation = obligations
+        .into_iter()
+        .chain(more_obligations)
+        .find(|o| negative_impl_exists(selcx, o, body_def_id));
 
     if let Some(failing_obligation) = opt_failing_obligation {
         debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
@@ -358,14 +363,15 @@ fn equate<'cx, 'tcx>(
 fn negative_impl_exists<'cx, 'tcx>(
     selcx: &SelectionContext<'cx, 'tcx>,
     o: &PredicateObligation<'tcx>,
+    body_def_id: DefId,
 ) -> bool {
-    if resolve_negative_obligation(selcx.infcx().fork(), o) {
+    if resolve_negative_obligation(selcx.infcx().fork(), o, body_def_id) {
         return true;
     }
 
     // Try to prove a negative obligation exists for super predicates
     for o in util::elaborate_predicates(selcx.tcx(), iter::once(o.predicate)) {
-        if resolve_negative_obligation(selcx.infcx().fork(), &o) {
+        if resolve_negative_obligation(selcx.infcx().fork(), &o, body_def_id) {
             return true;
         }
     }
@@ -377,6 +383,7 @@ fn negative_impl_exists<'cx, 'tcx>(
 fn resolve_negative_obligation<'cx, 'tcx>(
     infcx: InferCtxt<'cx, 'tcx>,
     o: &PredicateObligation<'tcx>,
+    body_def_id: DefId,
 ) -> bool {
     let tcx = infcx.tcx;
 
@@ -385,12 +392,24 @@ fn resolve_negative_obligation<'cx, 'tcx>(
     };
 
     let param_env = o.param_env;
-    let errors = super::fully_solve_obligation(&infcx, o);
-    if !errors.is_empty() {
+    if !super::fully_solve_obligation(&infcx, o).is_empty() {
         return false;
     }
 
-    let outlives_env = OutlivesEnvironment::new(param_env);
+    let (body_id, body_def_id) = if let Some(body_def_id) = body_def_id.as_local() {
+        (tcx.hir().local_def_id_to_hir_id(body_def_id), body_def_id)
+    } else {
+        (CRATE_HIR_ID, CRATE_DEF_ID)
+    };
+
+    let ocx = ObligationCtxt::new(&infcx);
+    let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
+    let outlives_env = OutlivesEnvironment::with_bounds(
+        param_env,
+        Some(&infcx),
+        infcx.implied_bounds_tys(param_env, body_id, wf_tys),
+    );
+
     infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
 
     infcx.resolve_regions(&outlives_env).is_empty()
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index d922f893321..904c81f9215 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -13,6 +13,7 @@ mod fulfill;
 pub mod misc;
 mod object_safety;
 mod on_unimplemented;
+pub mod outlives_bounds;
 mod project;
 pub mod query;
 pub(crate) mod relationships;
diff --git a/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
new file mode 100644
index 00000000000..a4b54018228
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
@@ -0,0 +1,114 @@
+use crate::infer::InferCtxt;
+use crate::traits::query::type_op::{self, TypeOp, TypeOpOutput};
+use crate::traits::query::NoSolution;
+use crate::traits::{ObligationCause, TraitEngine, TraitEngineExt};
+use rustc_data_structures::fx::FxHashSet;
+use rustc_hir as hir;
+use rustc_hir::HirId;
+use rustc_middle::ty::{self, ParamEnv, Ty};
+
+pub use rustc_middle::traits::query::OutlivesBound;
+
+type Bounds<'a, 'tcx: 'a> = impl Iterator<Item = OutlivesBound<'tcx>> + 'a;
+pub trait InferCtxtExt<'a, 'tcx> {
+    fn implied_outlives_bounds(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        body_id: hir::HirId,
+        ty: Ty<'tcx>,
+    ) -> Vec<OutlivesBound<'tcx>>;
+
+    fn implied_bounds_tys(
+        &'a self,
+        param_env: ty::ParamEnv<'tcx>,
+        body_id: hir::HirId,
+        tys: FxHashSet<Ty<'tcx>>,
+    ) -> Bounds<'a, 'tcx>;
+}
+
+impl<'a, 'cx, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'cx, 'tcx> {
+    /// Implied bounds are region relationships that we deduce
+    /// automatically. The idea is that (e.g.) a caller must check that a
+    /// function's argument types are well-formed immediately before
+    /// calling that fn, and hence the *callee* can assume that its
+    /// argument types are well-formed. This may imply certain relationships
+    /// between generic parameters. For example:
+    /// ```
+    /// fn foo<'a,T>(x: &'a T) {}
+    /// ```
+    /// can only be called with a `'a` and `T` such that `&'a T` is WF.
+    /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
+    ///
+    /// # Parameters
+    ///
+    /// - `param_env`, the where-clauses in scope
+    /// - `body_id`, the body-id to use when normalizing assoc types.
+    ///   Note that this may cause outlives obligations to be injected
+    ///   into the inference context with this body-id.
+    /// - `ty`, the type that we are supposed to assume is WF.
+    #[instrument(level = "debug", skip(self, param_env, body_id))]
+    fn implied_outlives_bounds(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        body_id: hir::HirId,
+        ty: Ty<'tcx>,
+    ) -> Vec<OutlivesBound<'tcx>> {
+        let span = self.tcx.hir().span(body_id);
+        let result = param_env
+            .and(type_op::implied_outlives_bounds::ImpliedOutlivesBounds { ty })
+            .fully_perform(self);
+        let result = match result {
+            Ok(r) => r,
+            Err(NoSolution) => {
+                self.tcx.sess.delay_span_bug(
+                    span,
+                    "implied_outlives_bounds failed to solve all obligations",
+                );
+                return vec![];
+            }
+        };
+
+        let TypeOpOutput { output, constraints, .. } = result;
+
+        if let Some(constraints) = constraints {
+            // Instantiation may have produced new inference variables and constraints on those
+            // variables. Process these constraints.
+            let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(self.tcx);
+            let cause = ObligationCause::misc(span, body_id);
+            for &constraint in &constraints.outlives {
+                let obligation = self.query_outlives_constraint_to_obligation(
+                    constraint,
+                    cause.clone(),
+                    param_env,
+                );
+                fulfill_cx.register_predicate_obligation(self, obligation);
+            }
+            if !constraints.member_constraints.is_empty() {
+                span_bug!(span, "{:#?}", constraints.member_constraints);
+            }
+            let errors = fulfill_cx.select_all_or_error(self);
+            if !errors.is_empty() {
+                self.tcx.sess.delay_span_bug(
+                    span,
+                    "implied_outlives_bounds failed to solve obligations from instantiation",
+                );
+            }
+        };
+
+        output
+    }
+
+    fn implied_bounds_tys(
+        &'a self,
+        param_env: ParamEnv<'tcx>,
+        body_id: HirId,
+        tys: FxHashSet<Ty<'tcx>>,
+    ) -> Bounds<'a, 'tcx> {
+        tys.into_iter()
+            .map(move |ty| {
+                let ty = self.resolve_vars_if_possible(ty);
+                self.implied_outlives_bounds(param_env, body_id, ty)
+            })
+            .flatten()
+    }
+}