fix #108495, postfix decrement and prefix decrement has no warning

[nx2k3]

Fixes #108495

[rustbot]

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[Nilstrieb]

Thank you for the PR, but we cannot do this recovery this way.

let _ = i--i;

and

let _ = --i;

compile today (double negation). The only case we would be able to handle is the case of something like i--+i, which is not allowed today. You'd have to be careful to make sure that whatever comes after the second - is not allowed to start an expression (there should be a helper function for that). I think it should work then, but I'm not entirely sure.

I see your issues mentions a "warning". I think emitting a warning on double negation is a great idea!

[fmease]

I see your issues mentions a "warning". I think emitting a warning on double negation is a great idea!

Fyi, Clippy already emits a warning for this by default:

warning: `--x` could be misinterpreted as pre-decrement by C programmers, is usually a no-op
 --> e.rs:3:9
  |
3 | let _ = --i;
  |         ^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#double_neg
  = note: `#[warn(clippy::double_neg)]` on by default

Not sure if it's worth moving this Clippy lint into rustc (this sometimes does happen though, although I don't know what the process is usually like).

[WaffleLapkin]

this sometimes does happen though, although I don't know what the process is usually like

The process is more or less "ask T-compiler for approval, if they give a green light, make a PR that adds (to rustc) and deprecates (in clippy) the lint". See #99272 and #99696 as an example.

Although I don't think it's worth it in the case of clippy::double_neg lint.

[WaffleLapkin]

LGTM, thanks!
(I've committed some small fmt changes so that I don't bother you with them)

[WaffleLapkin]

@bors r+ rollup

[bors]

📌 Commit 031206b has been approved by WaffleLapkin

It is now in the queue for this repository.