Add `undocumented_may_panic_call` lint by hcbarker · Pull Request #15969 · rust-lang/rust-clippy

hcbarker · 2025-10-28T08:01:04Z

Checks for calls to functions marked with #[clippy::may_panic] or configured in may-panic-functions that lack a // Panic: comment documenting why the panic is acceptable.

fixes #15861

changelog: new_lint: [undocumented_may_panic_call]

rustbot · 2025-10-28T08:01:09Z

r? @Jarcho

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hcbarker · 2025-10-28T08:01:23Z

r? blyxyas

rustbot · 2025-10-28T08:01:27Z

blyxyas is not on the review rotation at the moment.
They may take a while to respond.

blyxyas · 2025-10-31T01:41:21Z

+    /// #[clippy::may_panic]
+    /// fn my_panicable_func(n: u32) {
+    ///     if n % 2 == 0 {
+    ///         panic!("even number not allowed")


Little typo in the documentation =^w^=

Suggested change

/// panic!("even number not allowed")

/// panic!("even numbers are not allowed")

blyxyas · 2025-10-31T01:42:10Z

+    /// #[clippy::may_panic]
+    /// fn my_panicable_func(n: u32) {
+    ///     if n % 2 == 0 {
+    ///         panic!("even number not allowed")


Suggested change

/// panic!("even number not allowed")

/// panic!("even numbers are not allowed")

blyxyas · 2025-11-11T17:20:48Z

+        if get_unique_attr(cx.sess(), cx.tcx.get_all_attrs(def_id), sym::may_panic).is_some() {
+            return true;
+        }
+
+        self.may_panic_def_ids.contains(&def_id)


Suggested change

if get_unique_attr(cx.sess(), cx.tcx.get_all_attrs(def_id), sym::may_panic).is_some() {

return true;

}

self.may_panic_def_ids.contains(&def_id)

get_unique_attr(cx.sess(), cx.tcx.get_all_attrs(def_id), sym::may_panic).is_some() || self.may_panic_def_ids.contains(&def_id)

blyxyas · 2025-11-11T20:47:15Z

+        && call_line.line > 0
+        && let Some(src) = call_line.sf.src.as_deref()


We need to handle macros (the most difficult portion in the art of lint-making), I've been thinking about this and I think that the best option is to permit this two scenarios:

The macro handles the comment, this is what's going on currently.

The callsite of the macro handles the comment.

So, for macro calls that are not justified in-macro, the user will need to write a PANIC comment.

So, we'll need to recurse through the HIR parents, check if we're in a macro, and if we're not, check for the comment.

The functions to check if we're in a macro are is_from_proc_macro, in_external_macro and from_expansion

blyxyas · 2025-11-11T23:07:59Z

+
+    // Panic: This is safe, just trust me.
+    let _ = t.trait_panic_method();
+}


Some more testing would be much appreciated, note that tests also test that every other lint is working correctly, so doing some extremely weird syntax (and, if it's too weird, explaining where it should lint) is very beneficial for our test suite.

Many of the bugs we receive are from extremely niche use cases, so testing for those before a lint goes into a release is very beneficial.

Suggested change

}

#[allow(clippy::needless_bool, clippy::unnecessary_operation)]

fn weird_exprs(t: &dyn MyTrait) {

// The Panic comment should be in the second If statement

if if if true { true } else { false } {

t.trait_panic_method() == 0

//~^ undocumented_may_panic_call

} else {

false

} {

true

} else {

false

};

if if if true { true } else { false } {

// PANIC: This is safe!

t.trait_panic_method() == 0

} else {

false

} {

true

} else {

false

};

#[clippy::may_panic]

fn panic<T>(val: T) -> T {val}

macro_rules! mac {

($t:tt) => ($t)

}

// Panic comment should go on the line before the panic() function

async {{};({(u8::max)(

// PANIC: Safe!

panic(1)+mac!(2),mac!(2))});

};

}

macro_rules! may_panic {

() => {dangerous(1)}

}

fn on_macro() {

// PANIC: Hi

may_panic!();

// REVIEWER: ^^^^^^^ This currently doesn't work! See other comments

}

blyxyas

Great first patch! Sorry for the delayed response, I took a vacation right after starting to review this (in fact, if you check the comment date for those first review comments, you'll see that I made them 2 weeks ago)

View changes since this review

hcbarker · 2025-11-20T16:58:11Z

@blyxyas Thanks for the review! It was my turn to be on vacation when I got it haha, but I'll be working on an update here over the next few days :)

hcbarker · 2025-11-25T08:48:05Z

@blyxyas I had to rebase and push --force-with-lease to fix some conflicts, and I responded to your comments and added handling and tests for macros!

blyxyas · 2026-01-06T18:58:58Z

+        get_builtin_attr(cx.sess(), cx.tcx.get_all_attrs(def_id), sym::may_panic).count() > 0
+            || self.may_panic_def_ids.contains(&def_id)


Even if less frequent, cheaper operations should go earlier ଲ( ⓛ ω ⓛ *)ଲ

Suggested change

get_builtin_attr(cx.sess(), cx.tcx.get_all_attrs(def_id), sym::may_panic).count() > 0

|| self.may_panic_def_ids.contains(&def_id)

self.may_panic_def_ids.contains(&def_id) ||

get_builtin_attr(cx.sess(), cx.tcx.get_all_attrs(def_id), sym::may_panic).count() > 0

blyxyas · 2026-01-06T19:16:56Z

+            hir::ExprKind::MethodCall(_path, _receiver, _args, _span) => {
+                cx.typeck_results().type_dependent_def_id(expr.hir_id)
+            },
+            _ => None,


Instead of None, what about a return?

Suggested change

_ => None,

_ => return,

blyxyas · 2026-01-06T19:26:49Z

+        let mut lint_span = None;
+        for macro_call in macro_backtrace(call_span) {
+            if has_panic_comment_above_span(cx, macro_call.span) {
+                return None;
+            }
+            lint_span = Some(macro_call.span);
+        }


Great way to handle macros!

blyxyas · 2026-02-25T19:52:26Z

@hcbarker How are you doing with this? I think that the lint would still be great to have

hcbarker · 2026-03-01T16:44:39Z

@blyxyas Ah! thanks for the bump, this totally fell off my radar. I'll get back to it and finish it up

Checks for calls to functions marked with `#[clippy::may_panic]` or configured in `may-panic-functions` that lack a `// Panic:` comment documenting why the panic is acceptable. changelog: new_lint: [`undocumented_may_panic_call`]

- Fixing typos - Code cleanup and more tests - Adding handling and tests for all kinds of macros

rustbot · 2026-03-23T05:45:22Z

This PR was rebased onto a different master 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.

hcbarker · 2026-03-23T05:45:49Z

r? blyxyas

thanks for the comments!

hcbarker · 2026-03-23T05:56:15Z

r? blyxyas

rustbot · 2026-03-28T18:00:55Z

☔ The latest upstream changes (possibly #16687) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned Jarcho Oct 28, 2025

rustbot added needs-fcp PRs that add, remove, or rename lints and need an FCP S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Oct 28, 2025

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rustbot assigned blyxyas and unassigned Jarcho Oct 28, 2025

hcbarker force-pushed the may-panic-attribute branch from bab87c9 to 4bb7fdc Compare October 28, 2025 08:02

blyxyas added the G-Rust-for-Linux Issues related to Rust-for-Linux wants and bugfixes label Oct 28, 2025

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blyxyas reviewed Nov 11, 2025

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blyxyas requested changes Nov 11, 2025

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rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Nov 11, 2025

hcbarker force-pushed the may-panic-attribute branch from 4bb7fdc to ac41ba9 Compare November 25, 2025 08:46

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blyxyas reviewed Jan 7, 2026

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ojeda mentioned this pull request Mar 10, 2026

Clippy wanted features & bugfixes Rust-for-Linux/linux#349

Open

79 tasks

hcbarker added 3 commits March 22, 2026 22:44

Add undocumented_may_panic_call lint

5b89b69

Checks for calls to functions marked with `#[clippy::may_panic]` or configured in `may-panic-functions` that lack a `// Panic:` comment documenting why the panic is acceptable. changelog: new_lint: [`undocumented_may_panic_call`]

Responding to PR comments

1a682f0

- Fixing typos - Code cleanup and more tests - Adding handling and tests for all kinds of macros

Cleanups from PR feedback

0c13765

hcbarker force-pushed the may-panic-attribute branch from ac41ba9 to 0c13765 Compare March 23, 2026 05:45

formatting, removing duplicate from rebase

9c88f19

hcbarker added 2 commits March 22, 2026 23:03

Adding allow(deprecated) for finding may_panic attr

c7fc806

Fix comment type in lib.rs

bbfea92

hcbarker requested a review from blyxyas March 23, 2026 06:28

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 from the author. (Use `@rustbot ready` to update this status) labels Mar 23, 2026

withjannisNLnetLabs mentioned this pull request Jun 12, 2026

Interesting Clippy lints NLnetLabs/domain#669

Open

	/// panic!("even number not allowed")
	/// panic!("even numbers are not allowed")

		&& call_line.line > 0
		&& let Some(src) = call_line.sf.src.as_deref()

-}
+#[allow(clippy::needless_bool, clippy::unnecessary_operation)]
+fn weird_exprs(t: &dyn MyTrait) {
+    // The Panic comment should be in the second If statement
+    if if if true { true } else { false } {
+        t.trait_panic_method() == 0
+    //~^ undocumented_may_panic_call
+    } else {
+        false
+    } {
+        true
+    } else {
+        false
+    };
+    if if if true { true } else { false } {
+        // PANIC: This is safe!
+        t.trait_panic_method() == 0
+    } else {
+        false
+    } {
+        true
+    } else {
+        false
+    };
+    #[clippy::may_panic]
+    fn panic<T>(val: T) -> T {val}
+    macro_rules! mac {
+        ($t:tt) => ($t)
+    }
+    // Panic comment should go on the line before the panic() function
+    async {{};({(u8::max)(
+        // PANIC: Safe!
+        panic(1)+mac!(2),mac!(2))});
+    };
+}
+macro_rules! may_panic {
+    () => {dangerous(1)}
+}
+fn on_macro() {
+    // PANIC: Hi
+    may_panic!();
+    // REVIEWER: ^^^^^^^ This currently doesn't work! See other comments
+}

		get_builtin_attr(cx.sess(), cx.tcx.get_all_attrs(def_id), sym::may_panic).count() > 0
		\|\| self.may_panic_def_ids.contains(&def_id)

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