Add basic checks for well-formedness of fn/fn_mut lang items

[FabianWolff]

This pull request fixes #83471. Lang items are never actually checked for well-formedness (#9307). This means that one can get an ICE quite easily, e.g. as follows:

#![feature(lang_items)]
#[lang = "fn"]
trait MyFn {
    const call: i32 = 42;
}

fn main() {
    (|| 42)();
}

or this:

#![feature(lang_items)]
#[lang = "fn"]
trait MyFn {
    fn call(i: i32, j: i32);
}

fn main() {
    (|| 42)();
}

Ideally, there should probably be a more comprehensive strategy for checking lang items for well-formedness, but for the time being, I have added some rudimentary well-formedness checks that prevent #83471 and similar issues.

[rust-highfive]

r? @matthewjasper

(rust-highfive has picked a reviewer for you, use r? to override)

[bors]

☔ The latest upstream changes (presumably #87963) made this pull request unmergeable. Please resolve the merge conflicts.

[wesleywiser]

Hey @pnkfelix! This came up during the compiler team triage meeting. If you have a chance, could you please review or if you don't have time, we can re-roll to get another reviewer. Thanks!

[estebank]

@bors try @rust-timer queue

@FabianWolff can you address the change for a more targeted message and rebase on top of a recent master?

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 28585bf0f5b25e84500627bb38edfcaaebee3efd with merge 3777fcbdcee493db60907aa73da15ddf58397697...

[bors]

☀️ Try build successful - checks-actions
Build commit: 3777fcbdcee493db60907aa73da15ddf58397697 (3777fcbdcee493db60907aa73da15ddf58397697)

[rust-timer]

Queued 3777fcbdcee493db60907aa73da15ddf58397697 with parent 15d9ba0, future comparison URL.

[rust-timer]

Finished benchmarking commit (3777fcbdcee493db60907aa73da15ddf58397697): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

[estebank]

Left one last nitpick, but after that this is r=me.

[estebank]

@bors r+

[bors]

📌 Commit cb6c139 has been approved by estebank

[bors]

⌛ Testing commit cb6c139 with merge 043972f...

[bors]

☀️ Test successful - checks-actions
Approved by: estebank
Pushing 043972f to master...