Suggest correct syntax when writing type arg instead of assoc type

[estebank]

• When confusing an associated type with a type argument, suggest the appropriate syntax. Given Iterator<isize>, suggest Iterator<Item = isize>.
• When encountering multiple missing associated types, emit only one diagnostic.
• Point at associated type def span for context.
• Point at each extra type argument.

Follow up to #48288, fix #20977.

[rust-highfive]

r? @petrochenkov

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[estebank]

cc @nikomatsakis

[estebank]

The output can still be improved, IMO

error[E0107]: wrong number of type arguments: expected 0, found 1
 --> file2.rs:1:34
  |
1 | pub struct Foo { i: Box<Iterator<isize>> }
  |                                  ^^^^^ unexpected type argument

error[E0191]: the value of the associated type `Item` (from the trait `std::iter::Iterator`) must be specified
   --> file2.rs:1:25
    |
1   | pub struct Foo { i: Box<Iterator<isize>> }
    |                         ^^^^^^^^^^^^^^^ missing associated type `Item` value
    |
   ::: /Users/ekuber/personal/rust/src/libcore/iter/iterator.rs:104:5
    |
104 |     type Item;
    |     ---------- `Item` defined here
help: if you meant to assign the missing associated type, use the name
    |
1   | pub struct Foo { i: Box<Iterator<Item = isize>> }
    |                                  ^^^^^^^^^^^^

error: aborting due to 2 previous errors

The span pointing at the associated type definition span seems superfluous (at least without the corresponding trait's def span) and the wording could do with some copy editing.

[durka]

"type value" is not good IMO. How about

associated type std::iter::Iterator::Item must be specified

and

to specify Item, write

[petrochenkov]

@bors r+

[bors]

📌 Commit 5f569e2b43b455b7c4bdf6469ecb05829dba1461 has been approved by petrochenkov

[bors]

☔ The latest upstream changes (presumably #55859) made this pull request unmergeable. Please resolve the merge conflicts.

[estebank]

@bors r=petrochenkov

[bors]

📌 Commit 5cbd6e3df7cbd2c613af6d8dc0faf0663bbe2e36 has been approved by petrochenkov

[bors]

⌛ Testing commit 5cbd6e3df7cbd2c613af6d8dc0faf0663bbe2e36 with merge 42741ddb5182fac9cb0afb1f8425ae963b0b9db4...

[bors]

💔 Test failed - status-travis

[estebank]

@bors r=petrochenkov

[bors]

📌 Commit 510f836 has been approved by petrochenkov

[bors]

⌛ Testing commit 510f836 with merge aecbcd1...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: petrochenkov
Pushing aecbcd1 to master...