Implement special-cased projection error message for some common traits

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

@@ -1315,6 +1315,13 @@ trait InferCtxtPrivExt<'hir, 'tcx> {
         error: &MismatchedProjectionTypes<'tcx>,
     );
 
+    fn maybe_detailed_projection_msg(
+        &self,
+        pred: ty::ProjectionPredicate<'tcx>,
+        normalized_ty: ty::Term<'tcx>,
+        expected_ty: ty::Term<'tcx>,
+    ) -> Option<String>;
+
     fn fuzzy_match_tys(
         &self,
         a: Ty<'tcx>,
@@ -1542,23 +1549,19 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                     normalized_ty,
                     data.term,
                 ) {
-                    values = Some(infer::ValuePairs::Terms(ExpectedFound::new(
-                        is_normalized_ty_expected,
-                        normalized_ty,
-                        data.term,
-                    )));
+                    values = Some((data, is_normalized_ty_expected, normalized_ty, data.term));
                     err_buf = error;
                     err = &err_buf;
                 }
             }
 
-            let mut diag = struct_span_err!(
-                self.tcx.sess,
-                obligation.cause.span,
-                E0271,
-                "type mismatch resolving `{}`",
-                predicate
-            );
+            let msg = values
+                .and_then(|(predicate, _, normalized_ty, expected_ty)| {
+                    self.maybe_detailed_projection_msg(predicate, normalized_ty, expected_ty)
+                })
+                .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
+            let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
+
             let secondary_span = match predicate.kind().skip_binder() {
                 ty::PredicateKind::Projection(proj) => self
                     .tcx
@@ -1596,7 +1599,13 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 &mut diag,
                 &obligation.cause,
                 secondary_span,
-                values,
+                values.map(|(_, is_normalized_ty_expected, normalized_ty, term)| {
+                    infer::ValuePairs::Terms(ExpectedFound::new(
+                        is_normalized_ty_expected,
+                        normalized_ty,
+                        term,
+                    ))
+                }),
                 err,
                 true,
                 false,
@@ -1606,6 +1615,33 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         });
     }
 
+    fn maybe_detailed_projection_msg(
+        &self,
+        pred: ty::ProjectionPredicate<'tcx>,
+        normalized_ty: ty::Term<'tcx>,
+        expected_ty: ty::Term<'tcx>,
+    ) -> Option<String> {
+        let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
+        let self_ty = pred.projection_ty.self_ty();
+
+        if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
+            Some(format!(
+                "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
+                fn_kind = self_ty.prefix_string(self.tcx)
+            ))
+        } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
+            Some(format!(
+                "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
+            ))
+        } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
+            Some(format!(
+                "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
+            ))
+        } else {
+            None
+        }
+    }
+
     fn fuzzy_match_tys(
         &self,
         mut a: Ty<'tcx>,

src/test/ui/associated-types/associated-types-overridden-binding-2.rs

@@ -4,5 +4,5 @@ trait I32Iterator = Iterator<Item = i32>;
 
 fn main() {
     let _: &dyn I32Iterator<Item = u32> = &vec![42].into_iter();
-    //~^ ERROR type mismatch
+    //~^ ERROR expected `std::vec::IntoIter<u32>` to be an iterator that yields `i32`, but it yields `u32`
 }

src/test/ui/associated-types/associated-types-overridden-binding-2.stderr

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<std::vec::IntoIter<u32> as Iterator>::Item == i32`
+error[E0271]: expected `std::vec::IntoIter<u32>` to be an iterator that yields `i32`, but it yields `u32`
   --> $DIR/associated-types-overridden-binding-2.rs:6:43
    |
 LL |     let _: &dyn I32Iterator<Item = u32> = &vec![42].into_iter();

src/test/ui/async-await/async-block-control-flow-static-semantics.rs

@@ -15,7 +15,7 @@ fn return_targets_async_block_not_fn() -> u8 {
         return 0u8;
     };
     let _: &dyn Future<Output = ()> = &block;
-    //~^ ERROR type mismatch
+    //~^ ERROR expected `impl Future<Output = u8>` to be a future that resolves to `()`, but it resolves to `u8`
 }
 
 async fn return_targets_async_block_not_async_fn() -> u8 {
@@ -24,7 +24,7 @@ async fn return_targets_async_block_not_async_fn() -> u8 {
         return 0u8;
     };
     let _: &dyn Future<Output = ()> = &block;
-    //~^ ERROR type mismatch resolving `<impl Future<Output = u8> as Future>::Output == ()`
+    //~^ ERROR expected `impl Future<Output = u8>` to be a future that resolves to `()`, but it resolves to `u8`
 }
 
 fn no_break_in_async_block() {
@@ -42,7 +42,9 @@ fn no_break_in_async_block_even_with_outer_loop() {
 }
 
 struct MyErr;
-fn err() -> Result<u8, MyErr> { Err(MyErr) }
+fn err() -> Result<u8, MyErr> {
+    Err(MyErr)
+}
 
 fn rethrow_targets_async_block_not_fn() -> Result<u8, MyErr> {
     //~^ ERROR mismatched types

src/test/ui/async-await/async-block-control-flow-static-semantics.stderr

@@ -31,7 +31,7 @@ LL | |
 LL | | }
    | |_^ expected `u8`, found `()`
 
-error[E0271]: type mismatch resolving `<impl Future<Output = u8> as Future>::Output == ()`
+error[E0271]: expected `impl Future<Output = u8>` to be a future that resolves to `()`, but it resolves to `u8`
   --> $DIR/async-block-control-flow-static-semantics.rs:26:39
    |
 LL |     let _: &dyn Future<Output = ()> = &block;
@@ -47,7 +47,7 @@ LL | fn return_targets_async_block_not_fn() -> u8 {
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
-error[E0271]: type mismatch resolving `<impl Future<Output = u8> as Future>::Output == ()`
+error[E0271]: expected `impl Future<Output = u8>` to be a future that resolves to `()`, but it resolves to `u8`
   --> $DIR/async-block-control-flow-static-semantics.rs:17:39
    |
 LL |     let _: &dyn Future<Output = ()> = &block;
@@ -56,7 +56,7 @@ LL |     let _: &dyn Future<Output = ()> = &block;
    = note: required for the cast from `impl Future<Output = u8>` to the object type `dyn Future<Output = ()>`
 
 error[E0308]: mismatched types
-  --> $DIR/async-block-control-flow-static-semantics.rs:47:44
+  --> $DIR/async-block-control-flow-static-semantics.rs:49:44
    |
 LL | fn rethrow_targets_async_block_not_fn() -> Result<u8, MyErr> {
    |    ----------------------------------      ^^^^^^^^^^^^^^^^^ expected enum `Result`, found `()`
@@ -67,7 +67,7 @@ LL | fn rethrow_targets_async_block_not_fn() -> Result<u8, MyErr> {
            found unit type `()`
 
 error[E0308]: mismatched types
-  --> $DIR/async-block-control-flow-static-semantics.rs:56:50
+  --> $DIR/async-block-control-flow-static-semantics.rs:58:50
    |
 LL | fn rethrow_targets_async_block_not_async_fn() -> Result<u8, MyErr> {
    |    ----------------------------------------      ^^^^^^^^^^^^^^^^^ expected enum `Result`, found `()`

src/test/ui/hrtb/issue-62203-hrtb-ice.rs

@@ -38,9 +38,13 @@ fn main() {
     let v = Unit2.m(
         //~^ ERROR type mismatch
         L {
-        //~^ ERROR type mismatch
-            f : |x| { drop(x); Unit4 }
-        });
+            //~^ ERROR to be a closure that returns `Unit3`, but it returns `Unit4`
+            f: |x| {
+                drop(x);
+                Unit4
+            },
+        },
+    );
 }
 
 impl<'a> Ty<'a> for Unit2 {

src/test/ui/hrtb/issue-62203-hrtb-ice.stderr

@@ -1,8 +1,8 @@
-error[E0271]: type mismatch resolving `for<'r> <L<[closure@$DIR/issue-62203-hrtb-ice.rs:42:17: 42:20]> as T0<'r, (&'r u8,)>>::O == <_ as Ty<'r>>::V`
+error[E0271]: type mismatch resolving `for<'r> <L<[closure@$DIR/issue-62203-hrtb-ice.rs:42:16: 42:19]> as T0<'r, (&'r u8,)>>::O == <_ as Ty<'r>>::V`
   --> $DIR/issue-62203-hrtb-ice.rs:38:19
    |
 LL |     let v = Unit2.m(
-   |                   ^ type mismatch resolving `for<'r> <L<[closure@$DIR/issue-62203-hrtb-ice.rs:42:17: 42:20]> as T0<'r, (&'r u8,)>>::O == <_ as Ty<'r>>::V`
+   |                   ^ type mismatch resolving `for<'r> <L<[closure@$DIR/issue-62203-hrtb-ice.rs:42:16: 42:19]> as T0<'r, (&'r u8,)>>::O == <_ as Ty<'r>>::V`
    |
 note: expected this to be `<_ as Ty<'_>>::V`
   --> $DIR/issue-62203-hrtb-ice.rs:21:14
@@ -22,19 +22,22 @@ LL |     where
 LL |         F: for<'r> T0<'r, (<Self as Ty<'r>>::V,), O = <B as Ty<'r>>::V>,
    |                                                   ^^^^^^^^^^^^^^^^^^^^ required by this bound in `T1::m`
 
-error[E0271]: type mismatch resolving `for<'r> <[closure@$DIR/issue-62203-hrtb-ice.rs:42:17: 42:20] as FnOnce<((&'r u8,),)>>::Output == Unit3`
+error[E0271]: expected `[closure@$DIR/issue-62203-hrtb-ice.rs:42:16: 42:19]` to be a closure that returns `Unit3`, but it returns `Unit4`
   --> $DIR/issue-62203-hrtb-ice.rs:40:9
    |
 LL |       let v = Unit2.m(
    |                     - required by a bound introduced by this call
 LL |
 LL | /         L {
 LL | |
-LL | |             f : |x| { drop(x); Unit4 }
-LL | |         });
+LL | |             f: |x| {
+LL | |                 drop(x);
+LL | |                 Unit4
+LL | |             },
+LL | |         },
    | |_________^ expected struct `Unit3`, found struct `Unit4`
    |
-note: required because of the requirements on the impl of `for<'r> T0<'r, (&'r u8,)>` for `L<[closure@$DIR/issue-62203-hrtb-ice.rs:42:17: 42:20]>`
+note: required because of the requirements on the impl of `for<'r> T0<'r, (&'r u8,)>` for `L<[closure@$DIR/issue-62203-hrtb-ice.rs:42:16: 42:19]>`
   --> $DIR/issue-62203-hrtb-ice.rs:17:16
    |
 LL | impl<'a, A, T> T0<'a, A> for L<T>

src/test/ui/impl-trait/issues/issue-78722.rs

@@ -7,7 +7,7 @@ type F = impl core::future::Future<Output = u8>;
 struct Bug {
     V1: [(); {
         fn concrete_use() -> F {
-            //~^ ERROR type mismatch
+            //~^ ERROR expected `impl Future<Output = ()>` to be a future that resolves to `u8`, but it resolves to `()`
             async {}
         }
         let f: F = async { 1 };

src/test/ui/impl-trait/issues/issue-78722.stderr

@@ -16,7 +16,7 @@ LL |         let f: F = async { 1 };
 LL |     }],
    |     - value is dropped here
 
-error[E0271]: type mismatch resolving `<impl Future<Output = ()> as Future>::Output == u8`
+error[E0271]: expected `impl Future<Output = ()>` to be a future that resolves to `u8`, but it resolves to `()`
   --> $DIR/issue-78722.rs:9:30
    |
 LL |         fn concrete_use() -> F {

src/test/ui/intrinsics/const-eval-select-bad.rs

@@ -27,7 +27,7 @@ fn baz(n: bool) -> i32 {
 
 const fn return_ty_mismatch() {
     const_eval_select((1,), foo, bar);
-    //~^ ERROR type mismatch
+    //~^ ERROR expected `fn(i32) -> bool {bar}` to be a fn item that returns `i32`, but it returns `bool`
 }
 
 const fn args_ty_mismatch() {

src/test/ui/intrinsics/const-eval-select-bad.stderr

@@ -51,7 +51,7 @@ note: required by a bound in `const_eval_select`
 LL |     G: FnOnce<ARG, Output = RET> + ~const Destruct,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `const_eval_select`
 
-error[E0271]: type mismatch resolving `<fn(i32) -> bool {bar} as FnOnce<(i32,)>>::Output == i32`
+error[E0271]: expected `fn(i32) -> bool {bar}` to be a fn item that returns `i32`, but it returns `bool`
   --> $DIR/const-eval-select-bad.rs:29:5
    |
 LL |     const_eval_select((1,), foo, bar);

src/test/ui/issues/issue-31173.rs

@@ -3,12 +3,13 @@ use std::vec::IntoIter;
 pub fn get_tok(it: &mut IntoIter<u8>) {
     let mut found_e = false;
 
-    let temp: Vec<u8> = it.take_while(|&x| {
-        found_e = true;
-        false
-    })
+    let temp: Vec<u8> = it
+        .take_while(|&x| {
+            found_e = true;
+            false
+        })
         .cloned()
-        //~^ ERROR type mismatch resolving
+        //~^ ERROR to be an iterator that yields `&_`, but it yields `u8`
         .collect(); //~ ERROR the method
 }
 

src/test/ui/issues/issue-31173.stderr

@@ -1,5 +1,5 @@
-error[E0271]: type mismatch resolving `<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]> as Iterator>::Item == &_`
-  --> $DIR/issue-31173.rs:10:10
+error[E0271]: expected `TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:7:21: 7:25]>` to be an iterator that yields `&_`, but it yields `u8`
+  --> $DIR/issue-31173.rs:11:10
    |
 LL |         .cloned()
    |          ^^^^^^ expected reference, found `u8`
@@ -12,11 +12,11 @@ note: required by a bound in `cloned`
 LL |         Self: Sized + Iterator<Item = &'a T>,
    |                                ^^^^^^^^^^^^ required by this bound in `cloned`
 
-error[E0599]: the method `collect` exists for struct `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]>>`, but its trait bounds were not satisfied
-  --> $DIR/issue-31173.rs:12:10
+error[E0599]: the method `collect` exists for struct `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:7:21: 7:25]>>`, but its trait bounds were not satisfied
+  --> $DIR/issue-31173.rs:13:10
    |
 LL |         .collect();
-   |          ^^^^^^^ method cannot be called on `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]>>` due to unsatisfied trait bounds
+   |          ^^^^^^^ method cannot be called on `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:7:21: 7:25]>>` due to unsatisfied trait bounds
    |
   ::: $SRC_DIR/core/src/iter/adapters/cloned.rs:LL:COL
    |
@@ -29,10 +29,10 @@ LL | pub struct TakeWhile<I, P> {
    | -------------------------- doesn't satisfy `<_ as Iterator>::Item = &_`
    |
    = note: the following trait bounds were not satisfied:
-           `<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]> as Iterator>::Item = &_`
-           which is required by `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]>>: Iterator`
-           `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]>>: Iterator`
-           which is required by `&mut Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]>>: Iterator`
+           `<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:7:21: 7:25]> as Iterator>::Item = &_`
+           which is required by `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:7:21: 7:25]>>: Iterator`
+           `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:7:21: 7:25]>>: Iterator`
+           which is required by `&mut Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:7:21: 7:25]>>: Iterator`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-33941.rs

@@ -3,7 +3,7 @@
 use std::collections::HashMap;
 
 fn main() {
-    for _ in HashMap::new().iter().cloned() {} //~ ERROR type mismatch
-    //~^ ERROR type mismatch
-    //~| ERROR type mismatch
+    for _ in HashMap::new().iter().cloned() {} //~ ERROR expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
+    //~^ ERROR expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
+    //~| ERROR expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
 }

src/test/ui/issues/issue-33941.stderr

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _, _> as Iterator>::Item == &_`
+error[E0271]: expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
   --> $DIR/issue-33941.rs:6:36
    |
 LL |     for _ in HashMap::new().iter().cloned() {}
@@ -12,7 +12,7 @@ note: required by a bound in `cloned`
 LL |         Self: Sized + Iterator<Item = &'a T>,
    |                                ^^^^^^^^^^^^ required by this bound in `cloned`
 
-error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _, _> as Iterator>::Item == &_`
+error[E0271]: expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
   --> $DIR/issue-33941.rs:6:14
    |
 LL |     for _ in HashMap::new().iter().cloned() {}
@@ -23,7 +23,7 @@ LL |     for _ in HashMap::new().iter().cloned() {}
    = note: required because of the requirements on the impl of `Iterator` for `Cloned<std::collections::hash_map::Iter<'_, _, _>>`
    = note: required because of the requirements on the impl of `IntoIterator` for `Cloned<std::collections::hash_map::Iter<'_, _, _>>`
 
-error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _, _> as Iterator>::Item == &_`
+error[E0271]: expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
   --> $DIR/issue-33941.rs:6:14
    |
 LL |     for _ in HashMap::new().iter().cloned() {}

src/test/ui/never_type/fallback-closure-wrap.fallback.stderr

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<[closure@$DIR/fallback-closure-wrap.rs:18:40: 18:47] as FnOnce<()>>::Output == ()`
+error[E0271]: expected `[closure@$DIR/fallback-closure-wrap.rs:18:40: 18:47]` to be a closure that returns `()`, but it returns `!`
   --> $DIR/fallback-closure-wrap.rs:18:31
    |
 LL |       let error = Closure::wrap(Box::new(move || {

src/test/ui/never_type/fallback-closure-wrap.rs

@@ -16,7 +16,7 @@ use std::marker::PhantomData;
 
 fn main() {
     let error = Closure::wrap(Box::new(move || {
-        //[fallback]~^ ERROR type mismatch resolving
+        //[fallback]~^ to be a closure that returns `()`, but it returns `!`
         panic!("Can't connect to server.");
     }) as Box<dyn FnMut()>);
 }

src/test/ui/traits/assoc-type-in-superbad.rs

@@ -4,13 +4,13 @@
 
 use std::vec::IntoIter;
 
-pub trait Foo: Iterator<Item=<Self as Foo>::Key> {
+pub trait Foo: Iterator<Item = <Self as Foo>::Key> {
     type Key;
 }
 
 impl Foo for IntoIter<i32> {
-    type Key = u32; //~ ERROR type mismatch
+    type Key = u32;
+    //~^ ERROR expected `std::vec::IntoIter<i32>` to be an iterator that yields `u32`, but it yields `i32`
 }
 
-fn main() {
-}
+fn main() {}