Do not suggest to add type annotations for unnameable types

[FabianWolff]

Consider this example:

const A = || 42;

struct S<T> { t: T }
const B: _ = S { t: || 42 };

This currently produces the following output:

error: missing type for `const` item
 --> src/lib.rs:1:7
  |
1 | const A = || 42;
  |       ^ help: provide a type for the item: `A: [closure@src/lib.rs:1:11: 1:16]`

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/lib.rs:4:10
  |
4 | const B: _ = S { t: || 42 };
  |          ^
  |          |
  |          not allowed in type signatures
  |          help: replace `_` with the correct type: `S<[closure@src/lib.rs:4:21: 4:26]>`

error: aborting due to 2 previous errors

However, these suggestions are obviously useless, because the suggested types cannot be written down. With my changes, the suggestion is replaced with a note, because there is no simple fix:

error: missing type for `const` item
 --> test.rs:1:7
  |
1 | const A = || 42;
  |       ^
  |
note: however, the inferred type `[[email protected]:1:11: 1:16]` cannot be named
 --> test.rs:1:11
  |
1 | const A = || 42;
  |           ^^^^^

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> test.rs:4:10
  |
4 | const B: _ = S { t: || 42 };
  |          ^ not allowed in type signatures
  |
note: however, the inferred type `S<[[email protected]:4:21: 4:26]>` cannot be named
 --> test.rs:4:14
  |
4 | const B: _ = S { t: || 42 };
  |              ^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

[rust-highfive]

r? @jackh726

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[jackh726]

@bors delegate+

r=me with or without @fee1-dead's suggestion on suggesting a fn ptr instead of a fn def (I'd like to see it here, but also okay with that as a followup)

[bors]

✌️ @FabianWolff can now approve this pull request

[jackh726]

@bors r+

[bors]

📌 Commit 79dc9a7 has been approved by jackh726

[bors]

⌛ Testing commit 79dc9a7 with merge 60f1a2f...

[bors]

☀️ Test successful - checks-actions
Approved by: jackh726
Pushing 60f1a2f to master...