Some errors could be better handled by detecting a pattern

[Canop]

Exemple Use Case:

It's possibly common enough for people using several languages to apply the wrong, easy to recognize, construct.

Recognizing this construct in case an error is detected would avoid a sequence of suboptimal hints.

Here's the exemple of typing for let thing of some_iterator (JavaScript inspired):

A more relevant hint would suggest the typical Rust iteration.

Triage note:

@estebank suggested I create an issue here

[compiler-errors]

I think this issue could be phrase a bit more specifically, i.e. "parse JS style for-loop iteration and suggest fixing it", unless you're suggesting that we try to parse all sorts of different for loop concepts from a diverse set of languages, which I think is going to be significantly harder.

[compiler-errors]

Alternatively, this could be fixed with a few different error recoveries. First, we should recover from this:

for let ident in collection { .. }
//  ^^^ suggest removing this

Then we could also recover from:

for pattern of collection { .. }
         // ^^ suggest replacing this with `in`

[estebank]

Detecting let pattern after for is indeed one recovery. I believe we already have a of -> in check, but if not we can add it. Finally, I think it'd be nice if we could detect that we don't know for sure and "just" swallow the rest of the expression (find the next ; accounting for nested braces) or the end of the item (easier).

[BGR360]

@rustbot label A-diagnostics T-compiler

[veera-sivarajan]

Output on rustc 1.81.0 without warnings:

error: expected pattern, found `let`
 --> <source>:3:9
  |
3 |     for let thing of collection {
  |         ^^^
  |
help: remove the unnecessary `let` keyword
  |
3 -     for let thing of collection {
3 +     for  thing of collection {
  |

error: missing `in` in `for` loop
 --> <source>:3:19
  |
3 |     for let thing of collection {
  |                   ^^
  |
help: try using `in` here instead
  |
3 |     for let thing in collection {
  |                   ~~

error: aborting due to 2 previous errors; 1 warning emitted

[estebank]

Given the current output, I feel like we can close this ticket, while also acknowledging that the space of possible parses of mistaken code hasn't been exhausted yet.