Fix E0284 to not use incorrect wording

src/librustc_trait_selection/traits/error_reporting/mod.rs

@@ -1492,12 +1492,26 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
             ty::Predicate::Projection(ref data) => {
                 let trait_ref = data.to_poly_trait_ref(self.tcx);
                 let self_ty = trait_ref.self_ty();
+                let ty = data.skip_binder().ty;
                 if predicate.references_error() {
                     return;
                 }
-                let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
-                err.note(&format!("cannot satisfy `{}`", predicate));
-                err
+                if self_ty.needs_infer() && ty.needs_infer() {
+                    // We do this for the `foo.collect()?` case to produce a suggestion.
+                    let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
+                    err.note(&format!("cannot satisfy `{}`", predicate));
+                    err
+                } else {
+                    let mut err = struct_span_err!(
+                        self.tcx.sess,
+                        span,
+                        E0284,
+                        "type annotations needed: cannot satisfy `{}`",
+                        predicate,
+                    );
+                    err.span_label(span, &format!("cannot satisfy `{}`", predicate));
+                    err
+                }
             }
 
             _ => {

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

@@ -1,22 +1,18 @@
-error[E0284]: type annotations needed
+error[E0284]: type annotations needed: cannot satisfy `<Self as std::iter::Iterator>::Item == i32`
   --> $DIR/associated-types-overridden-binding.rs:4:12
    |
 LL | trait Foo: Iterator<Item = i32> {}
    |                     ---------- required by this bound in `Foo`
 LL | trait Bar: Foo<Item = u32> {}
-   |            ^^^^^^^^^^^^^^^ cannot infer type for type parameter `Self`
-   |
-   = note: cannot satisfy `<Self as std::iter::Iterator>::Item == i32`
+   |            ^^^^^^^^^^^^^^^ cannot satisfy `<Self as std::iter::Iterator>::Item == i32`
 
-error[E0284]: type annotations needed
+error[E0284]: type annotations needed: cannot satisfy `<Self as std::iter::Iterator>::Item == i32`
   --> $DIR/associated-types-overridden-binding.rs:7:21
    |
 LL | trait I32Iterator = Iterator<Item = i32>;
    |                              ---------- required by this bound in `I32Iterator`
 LL | trait U32Iterator = I32Iterator<Item = u32>;
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `Self`
-   |
-   = note: cannot satisfy `<Self as std::iter::Iterator>::Item == i32`
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `<Self as std::iter::Iterator>::Item == i32`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-12028.stderr

@@ -1,10 +1,8 @@
-error[E0284]: type annotations needed
+error[E0284]: type annotations needed: cannot satisfy `<_ as StreamHasher>::S == <H as StreamHasher>::S`
   --> $DIR/issue-12028.rs:27:14
    |
 LL |         self.input_stream(&mut stream);
-   |              ^^^^^^^^^^^^ cannot infer type for type parameter `H` declared on the trait `StreamHash`
-   |
-   = note: cannot satisfy `<_ as StreamHasher>::S == <H as StreamHasher>::S`
+   |              ^^^^^^^^^^^^ cannot satisfy `<_ as StreamHasher>::S == <H as StreamHasher>::S`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-69455.rs

@@ -26,5 +26,5 @@ impl Test<u64> for u64 {
 
 fn main() {
     let xs: Vec<u64> = vec![1, 2, 3];
-    println!("{}", 23u64.test(xs.iter().sum())); //~ ERROR: type annotations needed [E0284]
+    println!("{}", 23u64.test(xs.iter().sum())); //~ ERROR: type annotations needed
 }

src/test/ui/issues/issue-69455.stderr

@@ -1,16 +1,8 @@
-error[E0284]: type annotations needed
+error[E0284]: type annotations needed: cannot satisfy `<u64 as Test<_>>::Output == _`
   --> $DIR/issue-69455.rs:29:26
    |
-LL |     type Output;
-   |     ------------ `<Self as Test<Rhs>>::Output` defined here
-...
 LL |     println!("{}", 23u64.test(xs.iter().sum()));
-   |                    ------^^^^-----------------
-   |                    |     |
-   |                    |     cannot infer type for type `u64`
-   |                    this method call resolves to `<Self as Test<Rhs>>::Output`
-   |
-   = note: cannot satisfy `<u64 as Test<_>>::Output == _`
+   |                          ^^^^ cannot satisfy `<u64 as Test<_>>::Output == _`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-69683.rs

@@ -0,0 +1,32 @@
+pub trait Element<S> {
+    type Array;
+}
+
+impl<T> Element<()> for T {
+    type Array = T;
+}
+
+impl<T: Element<S>, S> Element<[S; 3]> for T {
+    type Array = [T::Array; 3];
+}
+
+trait Foo<I>
+where
+    u8: Element<I>,
+{
+    fn foo(self, x: <u8 as Element<I>>::Array);
+}
+
+impl<I> Foo<I> for u16
+where
+    u8: Element<I>,
+{
+    fn foo(self, _: <u8 as Element<I>>::Array) {}
+}
+
+fn main() {
+    let b: [u8; 3] = [0u8; 3];
+
+    0u16.foo(b); //~ ERROR type annotations needed
+    //<u16 as Foo<[(); 3]>>::foo(0u16, b);
+}

src/test/ui/issues/issue-69683.stderr

@@ -0,0 +1,9 @@
+error[E0284]: type annotations needed: cannot satisfy `<u8 as Element<_>>::Array == [u8; 3]`
+  --> $DIR/issue-69683.rs:30:10
+   |
+LL |     0u16.foo(b);
+   |          ^^^ cannot satisfy `<u8 as Element<_>>::Array == [u8; 3]`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0284`.

src/test/ui/issues/issue-71584.rs

@@ -0,0 +1,5 @@
+fn main() {
+    let n: u32 = 1;
+    let mut d: u64 = 2;
+    d = d % n.into(); //~ ERROR type annotations needed
+}

src/test/ui/issues/issue-71584.stderr

@@ -0,0 +1,9 @@
+error[E0284]: type annotations needed: cannot satisfy `<u64 as std::ops::Rem<_>>::Output == u64`
+  --> $DIR/issue-71584.rs:4:11
+   |
+LL |     d = d % n.into();
+   |           ^ cannot satisfy `<u64 as std::ops::Rem<_>>::Output == u64`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0284`.