Error is not the best with mistyping Struct instead of struct

[passcod]

Given the following code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=0c755cbfa505e42fb58dd902968675e5

Struct Foo {
   hello: String,
}

The current output is:

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `Foo`
 --> src/main.rs:2:8
  |
2 | Struct Foo {
  |        ^^^ expected one of 8 possible tokens

Identical on stable and nightly (per playground).

It would be friendler to show some help about the proper casing of struct.

[estebank]

@TaKO8Ki while you're at it, we probably should 1) create a kw::* to "concept string" map, and 2) do a levenshtein distance check between current and previous token against that map to see what you might have meant. That would make the report be better for more than just this one case.

[veera-sivarajan]

@estebank I'm interested in implementing a fix for this but I don't know what's a "concept string" in this context.

Could you please explain more about it?

[passcod]

I think Esteban meant, expanded:

1. create a mapping between the types in rustc_span::symbols::kw to static strings of these symbols (e.g. kw::Struct -> "struct") — I imagine this involves adding to the macro that generates this mod in the first place.
2. when rustc encounters a token error:
   1. check the raw token (here, Struct) against the mapping, with a fuzzy-match algorithm like levenshtein distance
   2. if there's a close enough match (defined against a threshold that would probably need manual tuning to be maximally helpful), suggest that symbol
   3. otherwise do the current thing

That would not only catch Struct -> struct, but also all other miscasings, as well as other typos like strucc or ipml; you'd need to do some tuning so it doesn't suggest move when you type mode and meant mod, or maybe suggest both.

[veera-sivarajan]

Thank you for the detailed explanation.

@rustbot claim

[veera-sivarajan]

Hi,

I thought of a slightly better but more complicated approach. In case you have any suggestions: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Tracking.20Expected.20Keywords.20when.20Parsing/near/463517139