Improve const mismatch FulfillmentError

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1588,9 +1588,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             Mismatch::Variable(infer::ExpectedFound { expected, found }),
                         )
                     }
+                    ValuePairs::Terms(infer::ExpectedFound {
+                        expected: ty::Term::Const(_),
+                        found: ty::Term::Const(_),
+                    }) => (false, Mismatch::Fixed("constant")),
                     ValuePairs::TraitRefs(_) | ValuePairs::PolyTraitRefs(_) => {
                         (false, Mismatch::Fixed("trait"))
                     }
+                    ValuePairs::Regions(_) => (false, Mismatch::Fixed("lifetime")),
                     _ => (false, Mismatch::Fixed("type")),
                 };
                 let vals = match self.values_str(values) {

compiler/rustc_middle/src/traits/mod.rs

@@ -469,6 +469,13 @@ impl<'tcx> ObligationCauseCode<'tcx> {
             _ => None,
         }
     }
+
+    pub fn peel_match_impls(&self) -> &Self {
+        match self {
+            MatchImpl(cause, _) => cause.code(),
+            _ => self,
+        }
+    }
 }
 
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1506,13 +1506,28 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 .emit();
             }
             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
-                self.report_mismatched_consts(
+                let mut diag = self.report_mismatched_consts(
                     &error.obligation.cause,
                     expected_found.expected,
                     expected_found.found,
                     err.clone(),
-                )
-                .emit();
+                );
+                let code = error.obligation.cause.code().peel_derives().peel_match_impls();
+                if let ObligationCauseCode::BindingObligation(..)
+                | ObligationCauseCode::ItemObligation(..)
+                | ObligationCauseCode::ExprBindingObligation(..)
+                | ObligationCauseCode::ExprItemObligation(..) = code
+                {
+                    self.note_obligation_cause_code(
+                        &mut diag,
+                        &error.obligation.predicate,
+                        error.obligation.param_env,
+                        code,
+                        &mut vec![],
+                        &mut Default::default(),
+                    );
+                }
+                diag.emit();
             }
         }
     }

src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     Foo::<10, 12>
    |     ^^^^^^^^^^^^^ expected `11`, found `12`
    |
-   = note: expected type `11`
-              found type `12`
+   = note: expected constant `11`
+              found constant `12`
 
 error: aborting due to previous error
 

src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     Foo::<N, { N + 2 }>
    |     ^^^^^^^^^^^^^^^^^^^ expected `{ N + 1 }`, found `{ N + 2 }`
    |
-   = note: expected type `{ N + 1 }`
-              found type `{ N + 2 }`
+   = note: expected constant `{ N + 1 }`
+              found constant `{ N + 2 }`
 
 error: aborting due to previous error
 

src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr

@@ -22,8 +22,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as u128 }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as u128 }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:20:19
@@ -49,26 +54,41 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as _ }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as _ }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:23:5
    |
 LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12`, found `13`
    |
-   = note: expected type `12`
-              found type `13`
+   = note: expected constant `12`
+              found constant `13`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:25:5
    |
 LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13`, found `14`
    |
-   = note: expected type `13`
-              found type `14`
+   = note: expected constant `13`
+              found constant `14`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:35:19
@@ -94,8 +114,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as u128 }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as u128 }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:38:19
@@ -121,26 +146,41 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as _ }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as _ }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:41:5
    |
 LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12`, found `13`
    |
-   = note: expected type `12`
-              found type `13`
+   = note: expected constant `12`
+              found constant `13`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:43:5
    |
 LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13`, found `14`
    |
-   = note: expected type `13`
-              found type `14`
+   = note: expected constant `13`
+              found constant `14`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: aborting due to 12 previous errors
 

src/test/ui/const-generics/generic_const_exprs/different-fn.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     [0; size_of::<Foo<T>>()]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected `size_of::<T>()`, found `size_of::<Foo<T>>()`
    |
-   = note: expected type `size_of::<T>()`
-              found type `size_of::<Foo<T>>()`
+   = note: expected constant `size_of::<T>()`
+              found constant `size_of::<Foo<T>>()`
 
 error: unconstrained generic constant
   --> $DIR/different-fn.rs:10:9

src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |         ArrayHolder([0; Self::SIZE])
    |                     ^^^^^^^^^^^^^^^ expected `X`, found `Self::SIZE`
    |
-   = note: expected type `X`
-              found type `Self::SIZE`
+   = note: expected constant `X`
+              found constant `Self::SIZE`
 
 error: unconstrained generic constant
   --> $DIR/issue-62504.rs:18:25

src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr

@@ -4,17 +4,31 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |            ^^^^^^^^^^^^^^^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-72819-generic-in-const-eval.rs:20:32
    |
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |                                ^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr

@@ -4,8 +4,8 @@ error[E0308]: method not compatible with trait
 LL |     fn size(&self) -> [usize; DIM] {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Self::DIM`, found `DIM`
    |
-   = note: expected type `Self::DIM`
-              found type `DIM`
+   = note: expected constant `Self::DIM`
+              found constant `DIM`
 
 error: unconstrained generic constant
   --> $DIR/issue-83765.rs:32:24
@@ -26,8 +26,8 @@ error[E0308]: mismatched types
 LL |         self.reference.size()
    |         ^^^^^^^^^^^^^^^^^^^^^ expected `DIM`, found `Self::DIM`
    |
-   = note: expected type `DIM`
-              found type `Self::DIM`
+   = note: expected constant `DIM`
+              found constant `Self::DIM`
 
 error: aborting due to 3 previous errors
 

src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr

@@ -54,8 +54,8 @@ error[E0308]: mismatched types
 LL |     writes_to_specific_path(&cap);
    |     ^^^^^^^^^^^^^^^^^^^^^^^ expected `true`, found `{ contains::<T, U>() }`
    |
-   = note: expected type `true`
-              found type `{ contains::<T, U>() }`
+   = note: expected constant `true`
+              found constant `{ contains::<T, U>() }`
 
 error: aborting due to 3 previous errors
 

src/test/ui/const-generics/generic_const_exprs/obligation-cause.rs

@@ -0,0 +1,24 @@
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+trait True {}
+
+struct Is<const V: bool>;
+
+impl True for Is<true> {}
+
+fn g<T>()
+//~^ NOTE required by a bound in this
+where
+    Is<{ std::mem::size_of::<T>() == 0 }>: True,
+    //~^ NOTE required by a bound in `g`
+    //~| NOTE required by this bound in `g`
+{
+}
+
+fn main() {
+    g::<usize>();
+    //~^ ERROR mismatched types
+    //~| NOTE expected `false`, found `true`
+    //~| NOTE expected constant `false`
+}

src/test/ui/const-generics/generic_const_exprs/obligation-cause.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/obligation-cause.rs:20:5
+   |
+LL |     g::<usize>();
+   |     ^^^^^^^^^^ expected `false`, found `true`
+   |
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `g`
+  --> $DIR/obligation-cause.rs:13:44
+   |
+LL | fn g<T>()
+   |    - required by a bound in this
+...
+LL |     Is<{ std::mem::size_of::<T>() == 0 }>: True,
+   |                                            ^^^^ required by this bound in `g`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/const-generics/issues/issue-73260.stderr

@@ -4,17 +4,33 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |            ^^^^^^^^^^^^^^^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-73260.rs:16:32
    |
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |                                ^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors