make compare_const_impl a query and use it in instance.rs

compiler/rustc_hir_analysis/src/check/check.rs

@@ -2,7 +2,7 @@ use crate::check::intrinsicck::InlineAsmCtxt;
 
 use super::coercion::CoerceMany;
 use super::compare_method::check_type_bounds;
-use super::compare_method::{compare_const_impl, compare_impl_method, compare_ty_impl};
+use super::compare_method::{compare_impl_method, compare_ty_impl};
 use super::*;
 use rustc_attr as attr;
 use rustc_errors::{Applicability, ErrorGuaranteed, MultiSpan};
@@ -1044,14 +1044,10 @@ fn check_impl_items_against_trait<'tcx>(
         let impl_item_full = tcx.hir().impl_item(impl_item.id);
         match impl_item_full.kind {
             hir::ImplItemKind::Const(..) => {
-                // Find associated const definition.
-                compare_const_impl(
-                    tcx,
-                    &ty_impl_item,
-                    impl_item.span,
-                    &ty_trait_item,
-                    impl_trait_ref,
-                );
+                let _ = tcx.compare_assoc_const_impl_item_with_trait_item((
+                    impl_item.id.def_id.def_id,
+                    ty_impl_item.trait_item_def_id.unwrap(),
+                ));
             }
             hir::ImplItemKind::Fn(..) => {
                 let opt_trait_span = tcx.hir().span_if_local(ty_trait_item.def_id);

compiler/rustc_hir_analysis/src/check/compare_method.rs

@@ -1,6 +1,6 @@
 use super::potentially_plural_count;
 use crate::errors::LifetimesOrBoundsMismatchOnTrait;
-use hir::def_id::DefId;
+use hir::def_id::{DefId, LocalDefId};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticId, ErrorGuaranteed};
 use rustc_hir as hir;
@@ -1300,17 +1300,20 @@ fn compare_generic_param_kinds<'tcx>(
     Ok(())
 }
 
-pub(crate) fn compare_const_impl<'tcx>(
+/// Use `tcx.compare_assoc_const_impl_item_with_trait_item` instead
+pub(crate) fn raw_compare_const_impl<'tcx>(
     tcx: TyCtxt<'tcx>,
-    impl_c: &ty::AssocItem,
-    impl_c_span: Span,
-    trait_c: &ty::AssocItem,
-    impl_trait_ref: ty::TraitRef<'tcx>,
-) {
+    (impl_const_item_def, trait_const_item_def): (LocalDefId, DefId),
+) -> Result<(), ErrorGuaranteed> {
+    let impl_const_item = tcx.associated_item(impl_const_item_def);
+    let trait_const_item = tcx.associated_item(trait_const_item_def);
+    let impl_trait_ref = tcx.impl_trait_ref(impl_const_item.container_id(tcx)).unwrap();
     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 
+    let impl_c_span = tcx.def_span(impl_const_item_def.to_def_id());
+
     tcx.infer_ctxt().enter(|infcx| {
-        let param_env = tcx.param_env(impl_c.def_id);
+        let param_env = tcx.param_env(impl_const_item_def.to_def_id());
         let ocx = ObligationCtxt::new(&infcx);
 
         // The below is for the most part highly similar to the procedure
@@ -1322,18 +1325,18 @@ pub(crate) fn compare_const_impl<'tcx>(
 
         // Create a parameter environment that represents the implementation's
         // method.
-        let impl_c_hir_id = tcx.hir().local_def_id_to_hir_id(impl_c.def_id.expect_local());
+        let impl_c_hir_id = tcx.hir().local_def_id_to_hir_id(impl_const_item_def);
 
         // Compute placeholder form of impl and trait const tys.
-        let impl_ty = tcx.type_of(impl_c.def_id);
-        let trait_ty = tcx.bound_type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
+        let impl_ty = tcx.type_of(impl_const_item_def.to_def_id());
+        let trait_ty = tcx.bound_type_of(trait_const_item_def).subst(tcx, trait_to_impl_substs);
         let mut cause = ObligationCause::new(
             impl_c_span,
             impl_c_hir_id,
             ObligationCauseCode::CompareImplItemObligation {
-                impl_item_def_id: impl_c.def_id.expect_local(),
-                trait_item_def_id: trait_c.def_id,
-                kind: impl_c.kind,
+                impl_item_def_id: impl_const_item_def,
+                trait_item_def_id: trait_const_item_def,
+                kind: impl_const_item.kind,
             },
         );
 
@@ -1358,24 +1361,24 @@ pub(crate) fn compare_const_impl<'tcx>(
             );
 
             // Locate the Span containing just the type of the offending impl
-            match tcx.hir().expect_impl_item(impl_c.def_id.expect_local()).kind {
+            match tcx.hir().expect_impl_item(impl_const_item_def).kind {
                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
-                _ => bug!("{:?} is not a impl const", impl_c),
+                _ => bug!("{:?} is not a impl const", impl_const_item),
             }
 
             let mut diag = struct_span_err!(
                 tcx.sess,
                 cause.span,
                 E0326,
                 "implemented const `{}` has an incompatible type for trait",
-                trait_c.name
+                trait_const_item.name
             );
 
-            let trait_c_span = trait_c.def_id.as_local().map(|trait_c_def_id| {
+            let trait_c_span = trait_const_item_def.as_local().map(|trait_c_def_id| {
                 // Add a label to the Span containing just the type of the const
                 match tcx.hir().expect_trait_item(trait_c_def_id).kind {
                     TraitItemKind::Const(ref ty, _) => ty.span,
-                    _ => bug!("{:?} is not a trait const", trait_c),
+                    _ => bug!("{:?} is not a trait const", trait_const_item),
                 }
             });
 
@@ -1391,23 +1394,22 @@ pub(crate) fn compare_const_impl<'tcx>(
                 false,
                 false,
             );
-            diag.emit();
-        }
+            return Err(diag.emit());
+        };
 
         // Check that all obligations are satisfied by the implementation's
         // version.
         let errors = ocx.select_all_or_error();
         if !errors.is_empty() {
-            infcx.report_fulfillment_errors(&errors, None, false);
-            return;
+            return Err(infcx.report_fulfillment_errors(&errors, None, false));
         }
 
+        // FIXME return `ErrorReported` if region obligations error?
         let outlives_environment = OutlivesEnvironment::new(param_env);
-        infcx.check_region_obligations_and_report_errors(
-            impl_c.def_id.expect_local(),
-            &outlives_environment,
-        );
-    });
+        infcx
+            .check_region_obligations_and_report_errors(impl_const_item_def, &outlives_environment);
+        Ok(())
+    })
 }
 
 pub(crate) fn compare_ty_impl<'tcx>(

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -251,6 +251,7 @@ pub fn provide(providers: &mut Providers) {
         check_mod_item_types,
         region_scope_tree,
         collect_trait_impl_trait_tys,
+        compare_assoc_const_impl_item_with_trait_item: compare_method::raw_compare_const_impl,
         ..*providers
     };
 }

compiler/rustc_middle/src/query/mod.rs

@@ -2100,4 +2100,10 @@ rustc_queries! {
     query permits_zero_init(key: TyAndLayout<'tcx>) -> bool {
         desc { "checking to see if {:?} permits being left zeroed", key.ty }
     }
+
+    query compare_assoc_const_impl_item_with_trait_item(
+        key: (LocalDefId, DefId)
+    ) -> Result<(), ErrorGuaranteed> {
+        desc { |tcx| "checking assoc const `{}` has the same type as trait item", tcx.def_path_str(key.0.to_def_id()) }
+    }
 }

compiler/rustc_ty_utils/src/instance.rs

@@ -182,40 +182,14 @@ fn resolve_associated_item<'tcx>(
             // a `trait` to an associated `const` definition in an `impl`, where
             // the definition in the `impl` has the wrong type (for which an
             // error has already been/will be emitted elsewhere).
-            //
-            // NB: this may be expensive, we try to skip it in all the cases where
-            // we know the error would've been caught (e.g. in an upstream crate).
-            //
-            // A better approach might be to just introduce a query (returning
-            // `Result<(), ErrorGuaranteed>`) for the check that `rustc_hir_analysis`
-            // performs (i.e. that the definition's type in the `impl` matches
-            // the declaration in the `trait`), so that we can cheaply check
-            // here if it failed, instead of approximating it.
             if leaf_def.item.kind == ty::AssocKind::Const
                 && trait_item_id != leaf_def.item.def_id
-                && leaf_def.item.def_id.is_local()
+                && let Some(leaf_def_item) = leaf_def.item.def_id.as_local()
             {
-                let normalized_type_of = |def_id, substs| {
-                    tcx.subst_and_normalize_erasing_regions(substs, param_env, tcx.type_of(def_id))
-                };
-
-                let original_ty = normalized_type_of(trait_item_id, rcvr_substs);
-                let resolved_ty = normalized_type_of(leaf_def.item.def_id, substs);
-
-                if original_ty != resolved_ty {
-                    let msg = format!(
-                        "Instance::resolve: inconsistent associated `const` type: \
-                         was `{}: {}` but resolved to `{}: {}`",
-                        tcx.def_path_str_with_substs(trait_item_id, rcvr_substs),
-                        original_ty,
-                        tcx.def_path_str_with_substs(leaf_def.item.def_id, substs),
-                        resolved_ty,
-                    );
-                    let span = tcx.def_span(leaf_def.item.def_id);
-                    let reported = tcx.sess.delay_span_bug(span, &msg);
-
-                    return Err(reported);
-                }
+                tcx.compare_assoc_const_impl_item_with_trait_item((
+                    leaf_def_item,
+                    trait_item_id,
+                ))?;
             }
 
             Some(ty::Instance::new(leaf_def.item.def_id, substs))

compiler/rustc_ty_utils/src/lib.rs

@@ -5,6 +5,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(let_chains)]
 #![feature(control_flow_enum)]
 #![feature(never_type)]
 #![feature(box_patterns)]

src/test/ui/associated-consts/associated-const-impl-wrong-lifetime.stderr

@@ -2,7 +2,7 @@ error[E0308]: const not compatible with trait
   --> $DIR/associated-const-impl-wrong-lifetime.rs:7:5
    |
 LL |     const NAME: &'a str = "unit";
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
+   |     ^^^^^^^^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected reference `&'static str`
               found reference `&'a str`

src/test/ui/associated-consts/mismatched_impl_ty_1.rs

@@ -0,0 +1,18 @@
+// run-pass
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+trait MyTrait {
+    type ArrayType;
+    const SIZE: usize;
+    const ARRAY: Self::ArrayType;
+}
+impl MyTrait for () {
+    type ArrayType = [u8; Self::SIZE];
+    const SIZE: usize = 4;
+    const ARRAY: [u8; Self::SIZE] = [1, 2, 3, 4];
+}
+
+fn main() {
+    let _ = <() as MyTrait>::ARRAY;
+}

src/test/ui/associated-consts/mismatched_impl_ty_2.rs

@@ -0,0 +1,11 @@
+// run-pass
+trait Trait {
+    const ASSOC: fn(&'static u32);
+}
+impl Trait for () {
+    const ASSOC: for<'a> fn(&'a u32) = |_| ();
+}
+
+fn main() {
+    let _ = <() as Trait>::ASSOC;
+}

src/test/ui/associated-consts/mismatched_impl_ty_3.rs

@@ -0,0 +1,11 @@
+// run-pass
+trait Trait {
+    const ASSOC: for<'a, 'b> fn(&'a u32, &'b u32);
+}
+impl Trait for () {
+    const ASSOC: for<'a> fn(&'a u32, &'a u32) = |_, _| ();
+}
+
+fn main() {
+    let _ = <() as Trait>::ASSOC;
+}

src/test/ui/nll/trait-associated-constant.stderr

@@ -2,7 +2,7 @@ error[E0308]: const not compatible with trait
   --> $DIR/trait-associated-constant.rs:21:5
    |
 LL |     const AC: Option<&'c str> = None;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected enum `Option<&'b str>`
               found enum `Option<&'c str>`