Sometimes RA reports errors that rustc doesn't agree exist

[Lokathor]

Lately, in various places in my project I get errors like

parsing.rs
expected 2 arguments, found 1 [Ln 176, Col 6]

but the thing is: there's not a wrong number of arguments. the project builds with cargo build and runs with cargo run and even shows correct output. So why the heck is RA telling me about fake errors. I thought it was something to do with like, bad cached data or something, but even after deleting the target/ directory the same problem happened a few days later.

• rust-analyzer version: rust-analyzer version: 0.3.2188-standalone (ba56d9b 2024-11-17) [c:\Users\Daniel.vscode\extensions\rust-lang.rust-analyzer-0.3.2188-win32-x64\server\rust-analyzer.exe]
• rustc version: rustc 1.82.0-nightly (6de928dce 2024-08-18)
• editor or extension: VSCode rust-analyzer extension
• relevant settings: ??? I've no clue what could be related here.
• repository link (if public, optional): https://github.com/Lokathor/yagbas/tree/1e2e182548b144c1d47de03126d3fc49d8f9d89b

[lnicola]

Can you post a screenshot of the error? r-a has its own diagnostics, which sometimes have false positives. It happens with Into, for example.

[Lokathor]

[lnicola]

Yeah, given the bounds on that function, I'm not really surprised. Adding explicit type annotations usually works around this.

[Lokathor]

uh, where? to the closure?

[lnicola]

Maybe, or to what fn_body returns (splitting the chain at that point).

[Lokathor]

the type of fn_body is...

let fn_body: Labelled<NestedIn<Collect<SeparatedBy<MapWith<impl Parser<'_, I, Statement, Full<Rich<'_, TokenTree, FileSpan>, (), ()>> + 
Clone, Statement, impl Fn(Statement, &mut MapExtra<'_, '_, I, Full<Rich<'_, TokenTree, FileSpan>, (), ()>>) -> FileSpanned<Statement>>, 
Repeated<impl Parser<'_, I, (), Full<Rich<'_, TokenTree, FileSpan>, (), ()>> + Clone, (), I, Full<Rich<'_, TokenTree, FileSpan>, (), ()>>, 
FileSpanned<Statement>, (), I, Full<Rich<'_, TokenTree, FileSpan>, (), ()>>, FileSpanned<Statement>, {unknown}>, impl Parser<'_, I, I, 
Full<Rich<'_, TokenTree, FileSpan>, (), ()>> + Clone, I, Full<Rich<'_, TokenTree, FileSpan>, (), ()>, {unknown}, Full<Rich<'_, TokenTree, 
FileSpan>, (), ()>>, &str>

[lnicola]

Try Go to definitions on those methods in the chain, one will probably be from the wrong trait.

[Lokathor]

I didn't write the chumsky library i'm using here, but all the steps in the chain seem to go to the right traits. (using Ctrl+clicking)

[lnicola]

Ouch, we're so slow on that file. The problem is on map, we pick the wrong one:

Explicitly choosing the trait fixes the false positive:

  let x = Parser::map(
    kw_fn_p().ignore_then(name).then(args).then(fn_body),
    |((name, arguments), statements)| Function { name, arguments, statements },
  )
  .labelled("function")
  .as_context();