Provide better suggestions for inference errors on `.collect()?`
When encountering an inference error when calling `.collect()` followed by `?`, detect the return type of the enclosing function and suggest `.collect::<Result<_, _>>()?` instead of `.collect::<Vec<_>>()?`:
```
error[E0283]: type annotations needed
--> $DIR/question-mark-type-infer.rs:10:21
|
LL | l.iter().map(f).collect()?
| ^^^^^^^ cannot infer type of the type parameter `B` declared on the method `collect`
|
= note: cannot satisfy `_: FromIterator<Result<i32, ()>>`
note: required by a bound in `collect`
--> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
help: consider specifying the generic argument
|
LL | l.iter().map(f).collect::<Result<_, _>>()?
| ++++++++++++++++
```
On inference error in let binding, account for `?`:
```
error[E0282]: type annotations needed
--> $DIR/question-mark-type-inference-in-chain.rs:31:9
|
LL | let mut tags = lines.iter().map(|e| parse(e)).collect()?;
| ^^^^^^^^
...
LL | tags.sort();
| ---- type must be known at this point
|
help: consider giving `tags` an explicit type
|
LL | let mut tags: Vec<_> = lines.iter().map(|e| parse(e)).collect()?;
| ++++++++
```
Address rust-lang/rust#129269.
Stuart Cook
7514258efd