Add a lint to catch clashing extern fn declarations.

[jumbatm]

Closes#69390.

Adds lint clashing_extern_decl to detect when, within a single crate, an extern function of the same name is declared with different types. Because two symbols of the same name cannot be resolved to two different functions at link time, and one function cannot possibly have two types, a clashing extern declaration is almost certainly a mistake.

This lint does not run between crates because a project may have dependencies which both rely on the same extern function, but declare it in a different (but valid) way. For example, they may both declare an opaque type for one or more of the arguments (which would end up distinct types), or use types that are valid conversions in the language the extern fn is defined in. In these cases, we can't say that the clashing declaration is incorrect.

r? @eddyb

[jumbatm]

Just as a reminder, per #69390 (comment), this PR should go through a crater-check run before merging (which should also let us see if changing the lint to be Deny from the get-go would break many current crates).

[eddyb]

r? @nagisa cc @hanna-kruppe (LGTM at a glance though)

[nagisa]

This is missing a test (and I think lint will fail to correctly handle) for two two not clashing declarations that use independently declared types. That is:

mod one {#[repr(C)]structBanana{weight:u64}extern"C"{fnweigh_banana(count:*constBanana) -> u64;}}mod two {#[repr(C)]structBanana{weight:u64}// note: distinct type// For a weirder corner case (may still be valid depending on how C code is written):// #[repr(C)] struct Banana { weight: u64, some_optional_field: u64 }extern"C"{fnweigh_banana(count:*constBanana) -> u64;}}

[bjorn3]

Should this also check across multiple crates?

[jumbatm]

Current thinking is no, because two crates may bind to the same extern function with signatures that are different, but compatible under that language or ABI.

[bors]

☔ The latest upstream changes (presumably #71566) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #72036) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #72276) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-highfive]

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error[E0531]: cannot find tuple struct or tuple variant `UnnormalizedProjection` in this scope
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2195 | | (UnnormalizedProjection(..), UnnormalizedProjection(..))
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--> src/librustc_lint/builtin.rs:2195:48
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2195 | | (UnnormalizedProjection(..), UnnormalizedProjection(..))
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0531`.
For more information about this error, try `rustc --explain E0531`.
error: could not compile `rustc_lint`.
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[jumbatm]

Hey @nagisa, this is ready for re-review.

[nagisa]

This overall LGTM now. I think it does get the balance right.

Can you please clean up the commits/history a little? Squashing some together, especially those that refer to rustfmt, etc would be ideal.

r=me once that's done.

[nagisa]

@bors r+

[bors]

📌 Commit 337abf1 has been approved by nagisa

[bors]

🌲 The tree is currently closed for pull requests below priority 9000, this pull request will be tested once the tree is reopened

[jumbatm]

Should we do a crater-check run first? Given this fixes a soundness issue, I'd like to make this lint Deny by default, but want to see if it'd break any existing crates.

[jumbatm]

Hey @nagisa, can I get another review? Fingers crossed this can get merged soon.

[nagisa]

@bors r+

[bors]

📌 Commit 556b7ba has been approved by nagisa

[bors]

⌛ Testing commit 556b7ba with merge 4399d4f09c61ea4b3a9070ddc591ab4d72a563d9...

[bors]

💥 Test timed out

[Dylan-DPC-zz]

going to retry again

@bors retry

[bors]

⌛ Testing commit 556b7ba with merge 228a0ed...

[bors]

☀️ Test successful - checks-azure
Approved by: nagisa
Pushing 228a0ed to master...

[petrochenkov]

The lint doesn't follow the lint naming conventions - https://github.com/rust-lang/rfcs/blob/master/text/0344-conventions-galore.md#lints.

It should preferably be renamed clashing_extern_decl -> clashing_extern_declarations until it's on stable.