Add new `function_casts_as_integer` lint #141470

GuillaumeGomez · 2025-05-23T20:38:37Z

The function_casts_as_integer lint detects cases where users cast a function pointer into an integer.

warn-by-default

Example

fn foo() {}
let x = foo as usize;

warning: casting a function into an integer implicitly
  --> $DIR/function_casts_as_integer.rs:9:17
   |
LL |     let x = foo as usize;
   |                 ^^^^^^^^
   |
help: add `fn() as usize`
   |
LL |     let x = foo as fn() as usize;
   |                 +++++++

Explanation

You should never cast a function directly into an integer but go through a cast as fn first to make it obvious what's going on. It also allows to prevent confusion with (associated) constants.

Related to #81686 and https://stackoverflow.com/questions/68701177/whats-the-meaning-of-casting-a-rust-enum-variant-to-a-numeric-data-type

Also Discussed here, reported as a CVE here and in firefox's bugzilla here.

r? @Urgau

compiler/rustc_lint/messages.ftl

rustbot · 2025-05-24T22:47:37Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

rustbot · 2025-05-27T14:12:39Z

The Miri subtree was changed

cc @rust-lang/miri

flip1995 · 2025-05-27T16:04:52Z

compiler/rustc_lint/src/function_cast_as_integer.rs

+    /// a cast as `fn` first to make it obvious what's going on. It also allows
+    /// to prevent confusion with (associated) constants.
+    pub FUNCTION_CASTS_AS_INTEGER,
+    Warn,


Clippy has a few lints for fn to integer casts. But they are all restriction or style lints in Clippy. Adding a warn-by-default lint about this to rustc might be a bit aggressive 🤔

I know, I implemented one myself. 😉 I think it highlights the fact that this is a big issue and that the compiler should warn about it and eventually even forbid this fn to integer cast (you need to cast to an fn pointer first).

But in any case, it's up to the lang team.

Agreed 👍 Just want to add this information as "prior art" for the lang team to make this decision. Even though it might've sounded like it, I'm not against adding this lint to rustc.

Clippy question: Do you think if this lint gets added to rustc, we can (partially) deprecate Clippy lints?

Hard to say. For example confusing_method_to_numeric_cast provides extra information about what (likely) went wrong. But with the current lint, they likely would already have seen the problem and fixed it. So by default I'd say yes. But we could eventually uplift part of them to add the extra context clippy has that this lint doesn't provide. Would make it much more interesting and even more useful.

Yeah, a partial uplift might be good then, should this be accepted.

GuillaumeGomez · 2025-11-10T21:26:00Z

@bors r=urgau rollup

bors · 2025-11-10T21:26:03Z

💡 This pull request was already approved, no need to approve it again.

There's another pull request that is currently being tested, blocking this pull request: Implement &pin patterns and ref pin binding modes #148397

bors · 2025-11-10T21:26:05Z

📌 Commit 66b4e93 has been approved by urgau

It is now in the queue for this repository.

RalfJung · 2025-11-12T07:02:16Z

src/tools/miri/src/shims/native_lib/trace/parent.rs


    // Move the instr ptr into the deprotection code.
-    #[expect(clippy::as_conversions)]
+    #[allow(unknown_lints)]


When you add things that are needed only for bootstraping, please mark them with cfg_attr(bootstrap) so that they don't unnecessarily stick around longer than they have to.

Wasn't sure it was used for tools outside compiler folder. Noted for next time.

RalfJung · 2025-11-12T07:08:31Z

src/tools/miri/tests/pass/backtrace/backtrace-api-v1.rs

@@ -1,5 +1,7 @@
 //@normalize-stderr-test: "::<.*>" -> ""

+#![allow(function_casts_as_integer)]


Why did you allow the lint here? I can't find an as-int cast in this file and on the playground the file seems fine even without the allow.

rustbot assigned Urgau May 23, 2025