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Make RustString an extern type to avoid improper_ctypes warnings #132549

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Make RustString an extern type to avoid improper_ctypes warnings #132549

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@Zalathar Zalathar commented Nov 3, 2024

Currently, any FFI function that uses &RustString needs to also add #[ignore(improper_ctypes)] to silence a warning.

The warning is not completely bogus, because RustString contains Vec<u8> and therefore does not have a guaranteed layout. But we have no way of telling the lint that this doesn't matter, because the C++ code only uses that pointer opaquely and never relies on its underlying layout.

Ideally there would be some way to silence improper_ctypes at the type-definition site. But because there isn't, casting to and from a separate extern type is better than having to annotate every single use site.

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rustbot commented Nov 3, 2024

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 3, 2024
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Zalathar commented Nov 3, 2024

I also want to rename RustString to something else, because it's a raw byte buffer rather than a UTF-8 sequence, but that would be too much scope creep for this PR.

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Zalathar commented Nov 3, 2024

See also #116963 for general dissatisfaction with improper_ctypes.

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Zalathar commented Nov 3, 2024

I'm not 100% confident that casting between normal references and extern type references like this is OK, but it seems reasonable?

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Zalathar commented Nov 3, 2024

See 47e932b for why RustString is in rustc_llvm.

@Zalathar Zalathar force-pushed the rust-string branch 4 times, most recently from a1dcb02 to 730d088 Compare November 3, 2024 22:31
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Zalathar commented Nov 3, 2024

Rebased to remove another ignore added by #132514 (diff).

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bors commented Nov 8, 2024

☔ The latest upstream changes (presumably #132762) made this pull request unmergeable. Please resolve the merge conflicts.

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Zalathar commented Nov 8, 2024

Rebased to resolve conflicts and remove more ignore/expect added by (my) other PRs.

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Zalathar commented Nov 8, 2024
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compiler/rustc_llvm/src/lib.rs
#![feature(rustdoc_internals)]
#![warn(unreachable_pub)]
// tidy-alphabetical-end

// NOTE: This crate only exists to allow linking on mingw targets.
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I'm not sure what this was originally referring to, but it's pretty clearly no longer true of the present crate.

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cuviper requested changes Nov 8, 2024
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Overall it looks like a nice improvement! I just have a couple quibbles.

compiler/rustc_llvm/src/lib.rs Outdated Show resolved Hide resolved
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@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 8, 2024
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Zalathar commented Nov 9, 2024

Addressed review feedback (diff).

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Zalathar commented Nov 9, 2024

@rustbot ready

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 9, 2024
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cuviper commented Nov 9, 2024

@bors r+

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bors commented Nov 9, 2024

📌 Commit 89d7efa has been approved by cuviper

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 9, 2024
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