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

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.

…)` cast

rustbot · 2025-11-10T15:38:47Z

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.

GuillaumeGomez · 2025-11-10T15:39:28Z

Applied @Urgau's suggestions.

…s ever working for function pointers

Urgau · 2025-11-10T19:53:34Z

r=me once CI is green

Urgau · 2025-11-10T21:23:58Z

@bors r+

bors · 2025-11-10T21:24:01Z

📌 Commit 66b4e93 has been approved by Urgau

It is now in the queue for this repository.

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.

…nteger, r=urgau Add new `function_casts_as_integer` lint The `function_casts_as_integer` lint detects cases where users cast a function pointer into an integer. *warn-by-default* ### Example ```rust 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 rust-lang#81686 and https://stackoverflow.com/questions/68701177/whats-the-meaning-of-casting-a-rust-enum-variant-to-a-numeric-data-type r? `@urgau`

…nteger, r=urgau Add new `function_casts_as_integer` lint The `function_casts_as_integer` lint detects cases where users cast a function pointer into an integer. *warn-by-default* ### Example ```rust 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 rust-lang#81686 and https://stackoverflow.com/questions/68701177/whats-the-meaning-of-casting-a-rust-enum-variant-to-a-numeric-data-type r? ``@urgau``

…nteger, r=urgau Add new `function_casts_as_integer` lint The `function_casts_as_integer` lint detects cases where users cast a function pointer into an integer. *warn-by-default* ### Example ```rust 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 rust-lang#81686 and https://stackoverflow.com/questions/68701177/whats-the-meaning-of-casting-a-rust-enum-variant-to-a-numeric-data-type r? ```@urgau```

Rollup of 16 pull requests Successful merges: - #141470 (Add new `function_casts_as_integer` lint) - #143619 (`c_variadic`: Add future-incompatibility warning for `...` arguments without a pattern outside of `extern` blocks) - #146495 (rustdoc: Erase `#![doc(document_private_items)]`) - #147771 (Rename `*exact_{div,shr,shl}` to `*{div,shr,shl}_exact`) - #147833 (rustdoc-json: move `target` to `json::conversions`) - #147952 (Add a timeout to the `remote-test-client` connection) - #147955 (compiletest: Migrate `TestProps` directive handling to a system of named handlers) - #148480 (Add `Steal::risky_hack_borrow_mut`) - #148506 (Special case detecting `'static` lifetime requirement coming from `-> Box<dyn Trait>`) - #148508 (Provide more context when mutably borrowing an imutably borrowed value) - #148530 (update the bootstrap readme) - #148608 (Add test for --test-builder success path) - #148636 (bootstrap: respect `build.python` on macOS) - #148639 (test(rustdoc): move tests into jump-to-def) - #148647 (Check unsafety for non-macro attributes in `validate_attr`) - #148667 (a few small clippy fixes) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 15 pull requests Successful merges: - #141470 (Add new `function_casts_as_integer` lint) - #143619 (`c_variadic`: Add future-incompatibility warning for `...` arguments without a pattern outside of `extern` blocks) - #146495 (rustdoc: Erase `#![doc(document_private_items)]`) - #147771 (Rename `*exact_{div,shr,shl}` to `*{div,shr,shl}_exact`) - #147833 (rustdoc-json: move `target` to `json::conversions`) - #147955 (compiletest: Migrate `TestProps` directive handling to a system of named handlers) - #148480 (Add `Steal::risky_hack_borrow_mut`) - #148506 (Special case detecting `'static` lifetime requirement coming from `-> Box<dyn Trait>`) - #148508 (Provide more context when mutably borrowing an imutably borrowed value) - #148530 (update the bootstrap readme) - #148608 (Add test for --test-builder success path) - #148636 (bootstrap: respect `build.python` on macOS) - #148639 (test(rustdoc): move tests into jump-to-def) - #148647 (Check unsafety for non-macro attributes in `validate_attr`) - #148667 (a few small clippy fixes) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #141470 - GuillaumeGomez:function_casts_as_integer, r=urgau Add new `function_casts_as_integer` lint The `function_casts_as_integer` lint detects cases where users cast a function pointer into an integer. *warn-by-default* ### Example ```rust 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 r? ````@urgau````

rustbot assigned Urgau May 23, 2025