Point out the actual mismatch error

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

@@ -1,3 +1,5 @@
+// ignore-tidy-filelength :(
+
 mod ambiguity;
 pub mod on_unimplemented;
 pub mod suggestions;
@@ -1943,6 +1945,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         other: bool,
         param_env: ty::ParamEnv<'tcx>,
     ) -> bool {
+        // If we have a single implementation, try to unify it with the trait ref
+        // that failed. This should uncover a better hint for what *is* implemented.
         if let [single] = &impl_candidates {
             if self.probe(|_| {
                 let ocx = ObligationCtxt::new(self);
@@ -1972,7 +1976,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     std::iter::zip(obligation_trait_ref.args, impl_trait_ref.args)
                 {
                     if let Err(terr) =
-                        ocx.eq(&ObligationCause::dummy(), param_env, obligation_arg, impl_arg)
+                        ocx.eq(&ObligationCause::dummy(), param_env, impl_arg, obligation_arg)
                     {
                         terrs.push(terr);
                     }
@@ -2000,6 +2004,15 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     (cand.self_ty().to_string(), Style::Highlight),
                     ("`".to_string(), Style::NoStyle),
                 ]);
+
+                if let [TypeError::Sorts(exp_found)] = &terrs[..] {
+                    let exp_found = self.resolve_vars_if_possible(*exp_found);
+                    err.help(format!(
+                        "for that trait implementation, expected `{}`, found `{}`",
+                        exp_found.expected, exp_found.found
+                    ));
+                }
+
                 true
             }) {
                 return true;

tests/ui/generic-const-items/unsatisfied-bounds.stderr

@@ -17,6 +17,7 @@ LL |     let () = K::<()>;
    |                  ^^ the trait `From<()>` is not implemented for `Infallible`
    |
    = help: the trait `From<!>` is implemented for `Infallible`
+   = help: for that trait implementation, expected `!`, found `()`
 note: required by a bound in `K`
   --> $DIR/unsatisfied-bounds.rs:12:17
    |
@@ -48,6 +49,7 @@ LL |     let _ = <() as Trait<&'static str>>::B::<()>;
    |                                              ^^ the trait `From<()>` is not implemented for `Infallible`
    |
    = help: the trait `From<!>` is implemented for `Infallible`
+   = help: for that trait implementation, expected `!`, found `()`
 note: required by a bound in `Trait::B`
   --> $DIR/unsatisfied-bounds.rs:21:21
    |

tests/ui/impl-trait/issues/issue-62742.stderr

@@ -43,6 +43,7 @@ LL |     WrongImpl::<()>::foo(0i32);
    |     ^^^^^^^^^^^^^^^ the trait `Raw<()>` is not implemented for `RawImpl<()>`
    |
    = help: the trait `Raw<[()]>` is implemented for `RawImpl<()>`
+   = help: for that trait implementation, expected `[()]`, found `()`
 note: required by a bound in `SafeImpl`
   --> $DIR/issue-62742.rs:26:35
    |

tests/ui/indexing/index-help.stderr

@@ -6,6 +6,7 @@ LL |     x[0i32];
    |
    = help: the trait `SliceIndex<[{integer}]>` is not implemented for `i32`
    = help: the trait `SliceIndex<[{integer}]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `i32`
    = note: required for `Vec<{integer}>` to implement `Index<i32>`
 
 error: aborting due to previous error

tests/ui/indexing/indexing-requires-a-uint.stderr

@@ -6,6 +6,7 @@ LL |     [0][0u8];
    |
    = help: the trait `SliceIndex<[{integer}]>` is not implemented for `u8`
    = help: the trait `SliceIndex<[{integer}]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `u8`
    = note: required for `[{integer}]` to implement `Index<u8>`
 
 error[E0308]: mismatched types

tests/ui/integral-indexing.stderr

@@ -6,6 +6,7 @@ LL |     v[3u8];
    |
    = help: the trait `SliceIndex<[isize]>` is not implemented for `u8`
    = help: the trait `SliceIndex<[isize]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `u8`
    = note: required for `Vec<isize>` to implement `Index<u8>`
 
 error[E0277]: the type `[isize]` cannot be indexed by `i8`
@@ -16,6 +17,7 @@ LL |     v[3i8];
    |
    = help: the trait `SliceIndex<[isize]>` is not implemented for `i8`
    = help: the trait `SliceIndex<[isize]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `i8`
    = note: required for `Vec<isize>` to implement `Index<i8>`
 
 error[E0277]: the type `[isize]` cannot be indexed by `u32`
@@ -26,6 +28,7 @@ LL |     v[3u32];
    |
    = help: the trait `SliceIndex<[isize]>` is not implemented for `u32`
    = help: the trait `SliceIndex<[isize]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `u32`
    = note: required for `Vec<isize>` to implement `Index<u32>`
 
 error[E0277]: the type `[isize]` cannot be indexed by `i32`
@@ -36,6 +39,7 @@ LL |     v[3i32];
    |
    = help: the trait `SliceIndex<[isize]>` is not implemented for `i32`
    = help: the trait `SliceIndex<[isize]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `i32`
    = note: required for `Vec<isize>` to implement `Index<i32>`
 
 error[E0277]: the type `[u8]` cannot be indexed by `u8`
@@ -46,6 +50,7 @@ LL |     s.as_bytes()[3u8];
    |
    = help: the trait `SliceIndex<[u8]>` is not implemented for `u8`
    = help: the trait `SliceIndex<[u8]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `u8`
    = note: required for `[u8]` to implement `Index<u8>`
 
 error[E0277]: the type `[u8]` cannot be indexed by `i8`
@@ -56,6 +61,7 @@ LL |     s.as_bytes()[3i8];
    |
    = help: the trait `SliceIndex<[u8]>` is not implemented for `i8`
    = help: the trait `SliceIndex<[u8]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `i8`
    = note: required for `[u8]` to implement `Index<i8>`
 
 error[E0277]: the type `[u8]` cannot be indexed by `u32`
@@ -66,6 +72,7 @@ LL |     s.as_bytes()[3u32];
    |
    = help: the trait `SliceIndex<[u8]>` is not implemented for `u32`
    = help: the trait `SliceIndex<[u8]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `u32`
    = note: required for `[u8]` to implement `Index<u32>`
 
 error[E0277]: the type `[u8]` cannot be indexed by `i32`
@@ -76,6 +83,7 @@ LL |     s.as_bytes()[3i32];
    |
    = help: the trait `SliceIndex<[u8]>` is not implemented for `i32`
    = help: the trait `SliceIndex<[u8]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `i32`
    = note: required for `[u8]` to implement `Index<i32>`
 
 error: aborting due to 8 previous errors

tests/ui/issues/issue-34334.stderr

@@ -20,6 +20,7 @@ LL |     let sr2: Vec<(u32, _, _)> = sr.iter().map(|(faction, th_sender, th_rece
    |
    = help: the trait `FromIterator<()>` is not implemented for `Vec<(u32, _, _)>`
    = help: the trait `FromIterator<(u32, _, _)>` is implemented for `Vec<(u32, _, _)>`
+   = help: for that trait implementation, expected `(u32, _, _)`, found `()`
 note: the method call chain might not have had the expected associated types
   --> $DIR/issue-34334.rs:5:43
    |

tests/ui/issues/issue-45801.stderr

@@ -5,6 +5,7 @@ LL |     req.get_ref::<Params>();
    |         ^^^^^^^ the trait `Plugin<i32>` is not implemented for `Params`
    |
    = help: the trait `Plugin<Foo>` is implemented for `Params`
+   = help: for that trait implementation, expected `Foo`, found `i32`
 
 error: aborting due to previous error
 

tests/ui/issues/issue-66923-show-error-for-correct-call.stderr

@@ -6,6 +6,7 @@ LL |     let x2: Vec<f64> = x1.into_iter().collect();
    |
    = help: the trait `FromIterator<&f64>` is not implemented for `Vec<f64>`
    = help: the trait `FromIterator<f64>` is implemented for `Vec<f64>`
+   = help: for that trait implementation, expected `f64`, found `&f64`
 note: the method call chain might not have had the expected associated types
   --> $DIR/issue-66923-show-error-for-correct-call.rs:8:27
    |
@@ -26,6 +27,7 @@ LL |     let x3 = x1.into_iter().collect::<Vec<f64>>();
    |
    = help: the trait `FromIterator<&f64>` is not implemented for `Vec<f64>`
    = help: the trait `FromIterator<f64>` is implemented for `Vec<f64>`
+   = help: for that trait implementation, expected `f64`, found `&f64`
 note: the method call chain might not have had the expected associated types
   --> $DIR/issue-66923-show-error-for-correct-call.rs:12:17
    |

tests/ui/iterators/invalid-iterator-chain.stderr

@@ -6,6 +6,7 @@ LL |     i.collect()
    |
    = help: the trait `FromIterator<&X>` is not implemented for `Vec<X>`
    = help: the trait `FromIterator<X>` is implemented for `Vec<X>`
+   = help: for that trait implementation, expected `X`, found `&X`
 note: the method call chain might not have had the expected associated types
   --> $DIR/invalid-iterator-chain.rs:4:26
    |
@@ -160,6 +161,7 @@ LL |     let g: Vec<i32> = f.collect();
    |
    = help: the trait `FromIterator<()>` is not implemented for `Vec<i32>`
    = help: the trait `FromIterator<i32>` is implemented for `Vec<i32>`
+   = help: for that trait implementation, expected `i32`, found `()`
 note: the method call chain might not have had the expected associated types
   --> $DIR/invalid-iterator-chain.rs:44:15
    |

tests/ui/never_type/never-value-fallback-issue-66757.nofallback.stderr

@@ -5,6 +5,7 @@ LL |     <E as From<_>>::from(never);
    |      ^ the trait `From<()>` is not implemented for `E`
    |
    = help: the trait `From<!>` is implemented for `E`
+   = help: for that trait implementation, expected `!`, found `()`
 
 error: aborting due to previous error
 

tests/ui/on-unimplemented/impl-substs.stderr

@@ -8,6 +8,7 @@ LL |     Foo::<usize>::foo((1i32, 1i32, 1i32));
    |
    = help: the trait `Foo<usize>` is not implemented for `(i32, i32, i32)`
    = help: the trait `Foo<i32>` is implemented for `(i32, i32, i32)`
+   = help: for that trait implementation, expected `i32`, found `usize`
 
 error: aborting due to previous error
 

tests/ui/on-unimplemented/on-impl.stderr

@@ -8,6 +8,7 @@ LL |     Index::<u32>::index(&[1, 2, 3] as &[i32], 2u32);
    |
    = help: the trait `Index<u32>` is not implemented for `[i32]`
    = help: the trait `Index<usize>` is implemented for `[i32]`
+   = help: for that trait implementation, expected `usize`, found `u32`
 
 error[E0277]: the trait bound `[i32]: Index<u32>` is not satisfied
   --> $DIR/on-impl.rs:22:5
@@ -17,6 +18,7 @@ LL |     Index::<u32>::index(&[1, 2, 3] as &[i32], 2u32);
    |
    = help: the trait `Index<u32>` is not implemented for `[i32]`
    = help: the trait `Index<usize>` is implemented for `[i32]`
+   = help: for that trait implementation, expected `usize`, found `u32`
 
 error[E0277]: the trait bound `[i32]: Index<u32>` is not satisfied
   --> $DIR/on-impl.rs:22:5
@@ -26,6 +28,7 @@ LL |     Index::<u32>::index(&[1, 2, 3] as &[i32], 2u32);
    |
    = help: the trait `Index<u32>` is not implemented for `[i32]`
    = help: the trait `Index<usize>` is implemented for `[i32]`
+   = help: for that trait implementation, expected `usize`, found `u32`
 
 error: aborting due to 3 previous errors
 

tests/ui/on-unimplemented/slice-index.stderr

@@ -6,6 +6,7 @@ LL |     x[1i32];
    |
    = help: the trait `SliceIndex<[i32]>` is not implemented for `i32`
    = help: the trait `SliceIndex<[i32]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `i32`
    = note: required for `[i32]` to implement `Index<i32>`
 
 error[E0277]: the type `[i32]` cannot be indexed by `RangeTo<i32>`

tests/ui/str/str-idx.stderr

@@ -8,6 +8,7 @@ LL |     let _: u8 = s[4];
    = note: you can use `.chars().nth()` or `.bytes().nth()`
            for more information, see chapter 8 in The Book: <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
    = help: the trait `SliceIndex<[_]>` is implemented for `usize`
+   = help: for that trait implementation, expected `[_]`, found `str`
    = note: required for `str` to implement `Index<{integer}>`
 
 error[E0277]: the type `str` cannot be indexed by `{integer}`
@@ -22,6 +23,7 @@ LL |     let _ = s.get(4);
    = note: you can use `.chars().nth()` or `.bytes().nth()`
            for more information, see chapter 8 in The Book: <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
    = help: the trait `SliceIndex<[_]>` is implemented for `usize`
+   = help: for that trait implementation, expected `[_]`, found `str`
 note: required by a bound in `core::str::<impl str>::get`
   --> $SRC_DIR/core/src/str/mod.rs:LL:COL
 
@@ -37,6 +39,7 @@ LL |     let _ = s.get_unchecked(4);
    = note: you can use `.chars().nth()` or `.bytes().nth()`
            for more information, see chapter 8 in The Book: <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
    = help: the trait `SliceIndex<[_]>` is implemented for `usize`
+   = help: for that trait implementation, expected `[_]`, found `str`
 note: required by a bound in `core::str::<impl str>::get_unchecked`
   --> $SRC_DIR/core/src/str/mod.rs:LL:COL
 

tests/ui/str/str-mut-idx.stderr

@@ -32,6 +32,7 @@ LL |     s[1usize] = bot();
    |
    = help: the trait `SliceIndex<str>` is not implemented for `usize`
    = help: the trait `SliceIndex<[_]>` is implemented for `usize`
+   = help: for that trait implementation, expected `[_]`, found `str`
    = note: required for `str` to implement `Index<usize>`
 
 error[E0277]: the type `str` cannot be indexed by `{integer}`
@@ -46,6 +47,7 @@ LL |     s.get_mut(1);
    = note: you can use `.chars().nth()` or `.bytes().nth()`
            for more information, see chapter 8 in The Book: <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
    = help: the trait `SliceIndex<[_]>` is implemented for `usize`
+   = help: for that trait implementation, expected `[_]`, found `str`
 note: required by a bound in `core::str::<impl str>::get_mut`
   --> $SRC_DIR/core/src/str/mod.rs:LL:COL
 
@@ -61,6 +63,7 @@ LL |     s.get_unchecked_mut(1);
    = note: you can use `.chars().nth()` or `.bytes().nth()`
            for more information, see chapter 8 in The Book: <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
    = help: the trait `SliceIndex<[_]>` is implemented for `usize`
+   = help: for that trait implementation, expected `[_]`, found `str`
 note: required by a bound in `core::str::<impl str>::get_unchecked_mut`
   --> $SRC_DIR/core/src/str/mod.rs:LL:COL
 

tests/ui/suggestions/issue-101623.stderr

@@ -8,6 +8,7 @@ LL |     Trait::do_stuff({ fun(&mut *inner) });
    |     required by a bound introduced by this call
    |
    = help: the trait `Trait<'_>` is implemented for `()`
+   = help: for that trait implementation, expected `()`, found `*mut ()`
 
 error: aborting due to previous error
 

tests/ui/suggestions/suggest-dereferencing-index.stderr

@@ -6,6 +6,7 @@ LL |     let one_item_please: i32 = [1, 2, 3][i];
    |
    = help: the trait `SliceIndex<[{integer}]>` is not implemented for `&usize`
    = help: the trait `SliceIndex<[{integer}]>` is implemented for `usize`
+   = help: for that trait implementation, expected `usize`, found `&usize`
    = note: required for `[{integer}]` to implement `Index<&usize>`
 help: dereference this index
    |

tests/ui/traits/coercion-generic-bad.stderr

@@ -5,6 +5,7 @@ LL |     let s: Box<dyn Trait<isize>> = Box::new(Struct { person: "Fred" });
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<isize>` is not implemented for `Struct`
    |
    = help: the trait `Trait<&'static str>` is implemented for `Struct`
+   = help: for that trait implementation, expected `&'static str`, found `isize`
    = note: required for the cast from `Box<Struct>` to `Box<dyn Trait<isize>>`
 
 error: aborting due to previous error

tests/ui/try-block/try-block-bad-type.stderr

@@ -6,6 +6,7 @@ LL |         Err("")?;
    |
    = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
    = help: the trait `From<Infallible>` is implemented for `TryFromSliceError`
+   = help: for that trait implementation, expected `Infallible`, found `&str`
    = note: required for `Result<u32, TryFromSliceError>` to implement `FromResidual<Result<Infallible, &str>>`
 
 error[E0271]: type mismatch resolving `<Result<i32, i32> as Try>::Output == &str`

tests/ui/try-trait/bad-interconversion.stderr

@@ -74,6 +74,7 @@ LL |     ControlFlow::Continue(Err("hello")?)
    |
    = help: the trait `FromResidual<Result<Infallible, &str>>` is not implemented for `ControlFlow<String>`
    = help: the trait `FromResidual<ControlFlow<String, Infallible>>` is implemented for `ControlFlow<String>`
+   = help: for that trait implementation, expected `ControlFlow<String, Infallible>`, found `Result<Infallible, &str>`
 
 error[E0277]: the `?` operator can only be used on `ControlFlow`s in a function that returns `ControlFlow`
   --> $DIR/bad-interconversion.rs:37:12
@@ -85,6 +86,7 @@ LL |     Some(3)?;
    |
    = help: the trait `FromResidual<Option<Infallible>>` is not implemented for `ControlFlow<u64>`
    = help: the trait `FromResidual<ControlFlow<u64, Infallible>>` is implemented for `ControlFlow<u64>`
+   = help: for that trait implementation, expected `ControlFlow<u64, Infallible>`, found `Option<Infallible>`
 
 error[E0277]: the `?` operator in a function that returns `ControlFlow<B, _>` can only be used on other `ControlFlow<B, _>`s (with the same Break type)
   --> $DIR/bad-interconversion.rs:43:29
@@ -97,6 +99,7 @@ LL |     ControlFlow::Break(4_u8)?;
    = help: the trait `FromResidual<ControlFlow<u8, Infallible>>` is not implemented for `ControlFlow<i64>`
    = note: unlike `Result`, there's no `From`-conversion performed for `ControlFlow`
    = help: the trait `FromResidual<ControlFlow<i64, Infallible>>` is implemented for `ControlFlow<i64>`
+   = help: for that trait implementation, expected `i64`, found `u8`
 
 error: aborting due to 8 previous errors