Add interior-mutability suggestion to `static_mut_refs` by JohnTitor · Pull Request #151362 · rust-lang/rust

JohnTitor · 2026-01-19T09:29:01Z

I've skipped to expand catching below code as a mutable reference shouldn't be involved (maybe a new lint would be needed?):

static mut COUNTER: u64 = 0;
fn main() {
    unsafe { COUNTER = 1 };
}

estebank · 2026-03-02T01:20:30Z

+   = help: use a type that relies on "interior mutability" instead; to read more on this, visit <https://doc.rust-lang.org/reference/interior-mutability.html>
   = note: `#[warn(static_mut_refs)]` (part of `#[warn(rust_2024_compatibility)]`) on by default
+help: this type already provides "interior mutability", so its binding doesn't need to be declared as mutable
+   |
+LL -             static mut ONCE: Once = Once::new();
+LL +             static ONCE: Once = Once::new();
+   |


Should the note be presented given that we have the suggestion? We're telling people "use internal mutability type" and "you are already using internal mutability type". I think we can just make the bool false if the suggestion is Some.

Absolutely! Tweaked in 647dead (this PR)

estebank · 2026-03-02T01:28:25Z

+    let source_map = cx.sess().source_map();
+    let snippet = source_map.span_to_snippet(header_span).ok()?;
+
+    let (_static_start, static_end) = find_word(&snippet, "static", 0)?;
+    let (mut_start, mut_end) = find_word(&snippet, "mut", static_end)?;
+    let mut_end = extend_trailing_space(&snippet, mut_end);


Is there no way of attaining this from the HIR instead?

AFAIK we cannot get precise mut spans on HIR and we would still require a string trick like this. Mutability itself doesn't contain spans currently, e.g. https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/struct.StaticItem.html.

I'm still unhappy with having to build spans from text, but let's land the improvement. Could you add a // FIXME to these functions mentioning something along the way of "we want to replace this with operating on the HIR in some way"?

Added FIXME in ee8949e

estebank · 2026-03-02T01:30:22Z

+fn find_word(snippet: &str, word: &str, start: usize) -> Option<(usize, usize)> {
+    let bytes = snippet.as_bytes();
+    let word_bytes = word.as_bytes();
+    let mut search = start;
+    while search <= snippet.len() {
+        let found = snippet[search..].find(word)?;
+        let idx = search + found;
+        let end = idx + word_bytes.len();
+        let before_ok = idx == 0 || !is_ident_char(bytes[idx - 1]);
+        let after_ok = end >= bytes.len() || !is_ident_char(bytes[end]);
+        if before_ok && after_ok {
+            return Some((idx, end));
+        }
+        search = end;
+    }
+    None
+}
+
+fn is_ident_char(byte: u8) -> bool {
+    byte.is_ascii_alphanumeric() || byte == b'_'
+}
+
+fn extend_trailing_space(snippet: &str, mut end: usize) -> usize {
+    if let Some(ch) = snippet[end..].chars().next()
+        && (ch == ' ' || ch == '\t')
+    {
+        end += ch.len_utf8();
+    }
+    end
+}


I would prefer for us not to have to do all this :-/

But that'll only be possible if we can take the info from the HIR.

apiraino · 2026-04-30T13:34:35Z

@JohnTitor Seem there are a few comments pending, am I reading correctly? I'll switch the review flag to you to incorporate changes. Feel free to request a review with @rustbot ready, thanks!

@rustbot author

JohnTitor · 2026-04-30T22:27:19Z

@apiraino No, I areadly addressed them, waiting for another review.
@rustbot review

estebank

r=me after addressing the nitpciks.

View changes since this review

estebank · 2026-05-08T19:03:01Z

Can we add another test for the case where the structured suggestion wouldn't trigger but the new note would?

Added in 33d92f7.

estebank · 2026-05-08T19:04:42Z

+    let source_map = cx.sess().source_map();
+    let snippet = source_map.span_to_snippet(header_span).ok()?;
+
+    let (_static_start, static_end) = find_word(&snippet, "static", 0)?;
+    let (mut_start, mut_end) = find_word(&snippet, "mut", static_end)?;
+    let mut_end = extend_trailing_space(&snippet, mut_end);


I'm still unhappy with having to build spans from text, but let's land the improvement. Could you add a // FIXME to these functions mentioning something along the way of "we want to replace this with operating on the HIR in some way"?

rustbot · 2026-05-17T06:03:46Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

JohnTitor · 2026-05-17T10:53:35Z

Thanks for the review!
@bors r=estebank

rust-bors · 2026-05-17T10:53:38Z

📌 Commit ee8949e has been approved by estebank

It is now in the queue for this repository.

@estebank

…, r=estebank Add interior-mutability suggestion to `static_mut_refs` Closes rust-lang#151131 r? @estebank I've skipped to expand catching below code as a mutable _reference_ shouldn't be involved (maybe a new lint would be needed?): ```rs static mut COUNTER: u64 = 0; fn main() { unsafe { COUNTER = 1 }; } ```

…uwer Rollup of 14 pull requests Successful merges: - #151742 (Remove redundant information in `rustc_abi::Variants`) - #151362 (Add interior-mutability suggestion to `static_mut_refs`) - #156121 (compiler: suggest `.collect()` when `String` is expected and `Iterator` is found) - #156208 (Emit retags in codegen to support BorrowSanitizer (part 1)) - #156596 (Split `LintExpectationId`s) - #156607 (ci: Update FreeBSD version to FreeBSD 14) - #156376 (suggest hex escapes for C-style escapes) - #156577 (Test EII UI tests with prefer-dynamic) - #156585 (explicit tail calls: ignore some tests on unsupported LLVM targets) - #156598 (Avoid rustfix suggestions for macro-expanded unused imports) - #156616 (rustdoc: add test case for `-Drustdoc::` and `--cap-lints`) - #156633 (Add regression test for issue #41261) - #156635 (rename unexpected_try_recover function) - #156636 (minor `rustc_mir_transform` cleanup)

@estebank

Rollup merge of #151362 - JohnTitor:interior-mutability-sugg, r=estebank Add interior-mutability suggestion to `static_mut_refs` Closes #151131 r? @estebank I've skipped to expand catching below code as a mutable _reference_ shouldn't be involved (maybe a new lint would be needed?): ```rs static mut COUNTER: u64 = 0; fn main() { unsafe { COUNTER = 1 }; } ```

…uwer Rollup of 14 pull requests Successful merges: - rust-lang/rust#151742 (Remove redundant information in `rustc_abi::Variants`) - rust-lang/rust#151362 (Add interior-mutability suggestion to `static_mut_refs`) - rust-lang/rust#156121 (compiler: suggest `.collect()` when `String` is expected and `Iterator` is found) - rust-lang/rust#156208 (Emit retags in codegen to support BorrowSanitizer (part 1)) - rust-lang/rust#156596 (Split `LintExpectationId`s) - rust-lang/rust#156607 (ci: Update FreeBSD version to FreeBSD 14) - rust-lang/rust#156376 (suggest hex escapes for C-style escapes) - rust-lang/rust#156577 (Test EII UI tests with prefer-dynamic) - rust-lang/rust#156585 (explicit tail calls: ignore some tests on unsupported LLVM targets) - rust-lang/rust#156598 (Avoid rustfix suggestions for macro-expanded unused imports) - rust-lang/rust#156616 (rustdoc: add test case for `-Drustdoc::` and `--cap-lints`) - rust-lang/rust#156633 (Add regression test for issue rust-lang/rust#41261) - rust-lang/rust#156635 (rename unexpected_try_recover function) - rust-lang/rust#156636 (minor `rustc_mir_transform` cleanup)

rustbot assigned estebank Jan 19, 2026

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 Jan 19, 2026

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JohnTitor force-pushed the interior-mutability-sugg branch from 41e2b83 to b9914a8 Compare January 19, 2026 09:56

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JohnTitor force-pushed the interior-mutability-sugg branch from b9914a8 to 7c76d7e Compare February 8, 2026 07:54

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estebank reviewed Mar 2, 2026

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JohnTitor force-pushed the interior-mutability-sugg branch from 7c76d7e to 647dead Compare March 6, 2026 12:44

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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 Apr 30, 2026

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 Apr 30, 2026

estebank approved these changes May 8, 2026

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estebank 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 May 8, 2026

JohnTitor added 4 commits May 17, 2026 14:09

Add interior-mutability suggestion to static_mut_refs

2071c66

Tweak note condition

cbaee5d

Add test for the case span is unavailable

33d92f7

Add FIXME for interior_mutability_suggestion

ee8949e

JohnTitor force-pushed the interior-mutability-sugg branch from 647dead to ee8949e Compare May 17, 2026 06:03

rust-bors Bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 17, 2026

This was referenced May 17, 2026

Rollup of 9 pull requests #156663

Closed

Rollup of 14 pull requests #156670

Merged

rust-bors Bot merged commit 9de8fa9 into rust-lang:main May 18, 2026
11 checks passed

rustbot added this to the 1.97.0 milestone May 18, 2026

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apiraino commented Apr 30, 2026

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