Don't show no_effect warning on unit structs implementing fn_once

[F3real]

Fixes #7792

changelog: Don't show [no_effect] or [unecessary_operation] warning for unit struct implementing FnOnce

[rust-highfive]

r? @camsteffen

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[camsteffen]

I don't think this is the right fix. Perhaps try inspecting tcx.type_dependent_def(callee.hir_id).

Add a test with

struct GreetStruct(u32);

impl FnOnce<(&str,)> for GreetStruct {
    type Output = ();

    extern "rust-call" fn call_once(self, (who,): (&str,)) -> Self::Output {
        println!("hello {}", who);
    }
}

fn main() {
    GreetStruct(1)("world");
}

[F3real]

@camsteffen Thanks for suggestion, I am not that familiar with the rustc API.
I looked into it briefly but it gives none as result, I think that it requires MethodCall and not Call as ExprKind.

[camsteffen]

Hmm yeah I haven't seen it work with Call either, it was just a guess...

[camsteffen]

Ah did you try expr.hir_id?

[F3real]

@camsteffen I've tried it with expr.hir_id and it works. So one option would be to add something like this:

fn is_fn_once(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> bool {
    if_chain! {
        if let Some(def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
        if match_def_path(cx, def_id, &paths::FN_ONCE_CALL_ONCE);
        then {
            return true;
        }
    }
    false
}

Hardcoded path is used as it seems FnOnce trait has no corresponding Symbol. But I've noticed we also have false positive for FnMut and probably Fn case, so those should also be added.

Other way that we could go is more similar to first solution I had, if we use:
Res::Def(DefKind::Variant | DefKind::Ctor(_, CtorKind::Fn | CtorKind::Fictive), ..) in match instead of Res::Def(DefKind::Struct | DefKind::Variant | DefKind::Ctor(..), ..) we avoid these cases. I think original intention was to cover construction of touples (as far as I can see from tests) but implementation seems to be too broad.

[camsteffen]

I think you can just do this

ExprKind::Call(callee, args) => {
    if cx.typeck_results().type_dependent_def(expr.hir_id).is_some() {
        // type-dependent function call like `impl FnOnce for X`
        return false;
    }
    ..

[camsteffen]

Thanks!

@bors r+

[bors]

📌 Commit e9bf5ec has been approved by camsteffen

[bors]

⌛ Testing commit e9bf5ec with merge 94ca94f...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: camsteffen
Pushing 94ca94f to master...