Unhelpful missing try type annotation help suggestion

[CAD97]

Given the following code: [playground]

#![feature(try_blocks)]

fn main() {
    let _ = try {
        Err::<(), _>("lol")?
    };
}

The current output is:

error[[E0284]](https://doc.rust-lang.org/nightly/error-index.html#E0284): type annotations needed
 --> src/main.rs:4:9
  |
4 |     let _ = try {
  |         ^
5 |         Err::<(), _>("lol")?
  |         -------------------- type must be known at this point
  |
  = note: cannot satisfy `<_ as Try>::Residual == _`
help: consider giving this pattern a type
  |
4 |     let _: _ = try {
  |          +++

... alright, I'll try it

error[[E0282]](https://doc.rust-lang.org/nightly/error-index.html#E0282): type annotations needed
 --> src/main.rs:4:12
  |
4 |     let _: _ = try {
  |            ^ cannot infer type

The same <_ as Try>::Residual == _ error and : _ help occurs with just Err("lol")?, but the example here gives the Ok type to give the compiler maximal information to present a better error. Unless I misunderstand the try desugaring (likely), the error should at least be <_ as Try>::Residual == Result<Infallible, &str>?

The suggestion is perhaps fine, since it's (probably?) not possible to give a better suggestion, but if the suggestion is kept, at a minimum the follow-up error after applying the hint shouldn't lose information; ideally it should be possible to notice that this is a try block type ascription and provide a hint as to what the compiler does know, and why it needs help.

The minimum amount of type information that I could get to compile is let _: Result<_, &str> = try { Err("lol")? };.

This will likely get better "for free" if try is changed to be less general. (I recall a discussed possibility about requiring the Residual type to match, in order to make type inference work in more cases, of which I think this is one that's ideally supposed to work without further annotation.)

@rustbot label +F-try_blocks +requires-nightly +D-invalid-suggestion

(rather, D-unhelpful-suggestion)

[estebank]

We no longer emit a suggestion:

error[[E0284]](https://doc.rust-lang.org/nightly/error_codes/E0284.html): type annotations needed: cannot satisfy `_ -> _`
 --> src/main.rs:4:13
  |
4 |       let _ = try {
  |  _____________^
5 | |         Err::<(), _>("lol")?
6 | |     };
  | |_____^ cannot satisfy `_ -> _`