Improve display of error E0308

[joshleeb]

Ref. Forgetting to call a variant constructor causes a confusing error message #35241.

This PR modifies note_type_err to display a help message when a TyFnPtr or TyFnDef are found and the return type, of the function or function pointer, is the same as the type that is expected.

The output of compiling

struct Foo(u32);

fn test() -> Foo { Foo }

fn main() {}

is now

$ rustc src/test/ui/issue-35241.rs
error[E0308]: mismatched types
  --> src/test/ui/issue-35241.rs:13:20
   |
13 | fn test() -> Foo { Foo }
   |              ---   ^^^ expected struct `Foo`, found fn item
   |              |
   |              expected `Foo` because of return type
   |
   = help: did you mean `Foo { /* fields */ }`?
   = note: expected type `Foo`
              found type `fn(u32) -> Foo {Foo::{{constructor}}}`

error: aborting due to previous error

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

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[estebank]

The output when Foo is defined as struct Foo {a: u32} is:

error[E0423]: expected value, found struct `Foo`
 --> src/main.rs:3:20
  |
3 | fn test() -> Foo { Foo }
  |                    ^^^ did you mean `Foo { /* fields */ }`?

---

The help did you mean 'Foo { /* fields */ }'? in this PR is incorrect as it should be Foo(/* fields */) instead.

[estebank]

Last thing and this will be ready for merging (r=me), could you change diag.help(&message) here for span_label(cause.span, &message)?

That way the output will be:

error[E0308]: mismatched types
  --> $DIR/issue-35241.rs:13:20
   |
13 | fn test() -> Foo { Foo }
   |              ---   ^^^ 
   |              |     |
   |              |     expected struct `Foo`, found fn item
   |              |     did you mean `Foo(/* fields */)`?
   |              |
   |              expected `Foo` because of return type
   |
   = note: expected type `Foo`
              found type `fn(u32) -> Foo {Foo::{{constructor}}}`

---

Also, while you're at it, could you squash your PR into a single commit?

[estebank]

@bors r+

[bors]

📌 Commit 87c951d has been approved by estebank

[bors]

⌛ Testing commit 87c951d with merge a7d98c7...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: estebank
Pushing a7d98c7 to master...