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 actually 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 yields `{expected_ty}`, but it actually yields `{normalized_ty}`"
+            ))
+        } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
+            Some(format!(
+                "expected `{self_ty}` to be an iterator of `{expected_ty}`, but it actually returns items of `{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 of `i32`, but it actually returns items of `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 of `i32`, but it actually returns items of `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 yields `()`, but it actually yields `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 yields `()`, but it actually yields `u8`
 }
 
 fn no_break_in_async_block() {

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 yields `()`, but it actually yields `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 yields `()`, but it actually yields `u8`
   --> $DIR/async-block-control-flow-static-semantics.rs:17:39
    |
 LL |     let _: &dyn Future<Output = ()> = &block;

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

@@ -38,7 +38,7 @@ fn main() {
     let v = Unit2.m(
         //~^ ERROR type mismatch
         L {
-        //~^ ERROR type mismatch
+        //~^ ERROR to be a closure that returns `Unit3`, but it actually returns `Unit4`
             f : |x| { drop(x); Unit4 }
         });
 }

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

@@ -22,7 +22,7 @@ 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:42:17: 42:20]` to be a closure that returns `Unit3`, but it actually returns `Unit4`
   --> $DIR/issue-62203-hrtb-ice.rs:40:9
    |
 LL |       let v = Unit2.m(

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 yields `u8`, but it actually yields `()`
             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 yields `u8`, but it actually yields `()`
   --> $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 actually 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 actually returns `bool`
   --> $DIR/const-eval-select-bad.rs:29:5
    |
 LL |     const_eval_select((1,), foo, bar);

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

@@ -8,7 +8,7 @@ pub fn get_tok(it: &mut IntoIter<u8>) {
         false
     })
         .cloned()
-        //~^ ERROR type mismatch resolving
+        //~^ ERROR to be an iterator of `&_`, but it actually returns items of `u8`
         .collect(); //~ ERROR the method
 }
 

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

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]> as Iterator>::Item == &_`
+error[E0271]: expected `TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 6:43]>` to be an iterator of `&_`, but it actually returns items of `u8`
   --> $DIR/issue-31173.rs:10:10
    |
 LL |         .cloned()

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 of `&_`, but it actually returns items of `(&_, &_)`
+    //~^ ERROR expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator of `&_`, but it actually returns items of `(&_, &_)`
+    //~| ERROR expected `std::collections::hash_map::Iter<'_, _, _>` to be an iterator of `&_`, but it actually returns items of `(&_, &_)`
 }

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 of `&_`, but it actually returns items of `(&_, &_)`
   --> $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 of `&_`, but it actually returns items of `(&_, &_)`
   --> $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 of `&_`, but it actually returns items of `(&_, &_)`
   --> $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 actually 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 actually returns `!`
         panic!("Can't connect to server.");
     }) as Box<dyn FnMut()>);
 }

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

@@ -9,7 +9,8 @@ pub trait Foo: Iterator<Item=<Self as Foo>::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 of `u32`, but it actually returns items of `i32`
 }
 
 fn main() {

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

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<std::vec::IntoIter<i32> as Iterator>::Item == u32`
+error[E0271]: expected `std::vec::IntoIter<i32>` to be an iterator of `u32`, but it actually returns items of `i32`
   --> $DIR/assoc-type-in-superbad.rs:12:16
    |
 LL |     type Key = u32;

src/test/ui/type-alias-impl-trait/issue-57961.rs

@@ -8,7 +8,7 @@ trait Foo {
 
 impl Foo for () {
     type Bar = std::vec::IntoIter<u32>;
-    //~^ ERROR type mismatch resolving `<std::vec::IntoIter<u32> as Iterator>::Item == X
+    //~^ ERROR expected `std::vec::IntoIter<u32>` to be an iterator of `X`, but it actually returns items of `u32`
 }
 
 fn incoherent() {

src/test/ui/type-alias-impl-trait/issue-57961.stderr

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<std::vec::IntoIter<u32> as Iterator>::Item == X`
+error[E0271]: expected `std::vec::IntoIter<u32>` to be an iterator of `X`, but it actually returns items of `u32`
   --> $DIR/issue-57961.rs:10:16
    |
 LL | type X = impl Sized;

src/test/ui/type-alias-impl-trait/issue-98604.rs

@@ -9,5 +9,5 @@ async fn test() {}
 #[allow(unused_must_use)]
 fn main() {
     Box::new(test) as AsyncFnPtr;
-    //~^ ERROR type mismatch
+    //~^ ERROR expected `fn() -> impl Future<Output = ()> {test}` to be a fn item that returns `Pin<Box<(dyn Future<Output = ()> + 'static)>>`, but it actually returns `impl Future<Output = ()>`
 }

src/test/ui/type-alias-impl-trait/issue-98604.stderr

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<fn() -> impl Future<Output = ()> {test} as FnOnce<()>>::Output == Pin<Box<(dyn Future<Output = ()> + 'static)>>`
+error[E0271]: expected `fn() -> impl Future<Output = ()> {test}` to be a fn item that returns `Pin<Box<(dyn Future<Output = ()> + 'static)>>`, but it actually returns `impl Future<Output = ()>`
   --> $DIR/issue-98604.rs:11:5
    |
 LL |     Box::new(test) as AsyncFnPtr;

src/test/ui/type-alias-impl-trait/issue-98608.rs

@@ -2,7 +2,7 @@ fn hi() -> impl Sized { std::ptr::null::<u8>() }
 
 fn main() {
     let b: Box<dyn Fn() -> Box<u8>> = Box::new(hi);
-    //~^ ERROR type mismatch resolving `<fn() -> impl Sized {hi} as FnOnce<()>>::Output == Box<u8>`
+    //~^ ERROR expected `fn() -> impl Sized {hi}` to be a fn item that returns `Box<u8>`, but it actually returns `impl Sized`
     let boxed = b();
     let null = *boxed;
     println!("{null:?}");

src/test/ui/type-alias-impl-trait/issue-98608.stderr

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<fn() -> impl Sized {hi} as FnOnce<()>>::Output == Box<u8>`
+error[E0271]: expected `fn() -> impl Sized {hi}` to be a fn item that returns `Box<u8>`, but it actually returns `impl Sized`
   --> $DIR/issue-98608.rs:4:39
    |
 LL | fn hi() -> impl Sized { std::ptr::null::<u8>() }