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Add new function_casts_as_integer lint #141470

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@GuillaumeGomez GuillaumeGomez commented May 23, 2025

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

@rustbot rustbot added O-unix Operating system: Unix-like 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. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 23, 2025
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@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch 2 times, most recently from 07f2c3c to 4978962 Compare May 23, 2025 20:54
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@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch 2 times, most recently from 3db3153 to d8b1955 Compare May 24, 2025 10:10
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@Urgau Urgau added T-lang Relevant to the language team, which will review and decide on the PR/issue. and removed T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 24, 2025
Comment on lines 291 to 295
lint_function_casts_as_integer = casting a function into an integer implicitly
.cast_as_fn = add `{$cast_from_ty} as {$cast_to_ty}`
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Suggested change
lint_function_casts_as_integer = casting a function into an integer implicitly
.cast_as_fn = add `{$cast_from_ty} as {$cast_to_ty}`
lint_function_casts_as_integer = direct cast of function item into an integer
.cast_as_fn = first cast to a function pointer `{$cast_from_ty}`

how about this wording? it's not really implicit, it may just be unexpected

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Sounds good (better) to me!

@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch from d8b1955 to 45984df Compare May 24, 2025 18:45
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@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch from 45984df to a6107b4 Compare May 24, 2025 19:09
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@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch from a6107b4 to 24d757e Compare May 24, 2025 22:47
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rustbot commented May 24, 2025

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

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@Urgau Urgau 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 26, 2025
@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch from 24d757e to 3529162 Compare May 27, 2025 14:12
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rustbot commented May 27, 2025

The Miri subtree was changed

cc @rust-lang/miri

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/// 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,
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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 🤔

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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.

@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch from 4f843ad to 1645fd5 Compare May 27, 2025 18:47
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@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch from 1645fd5 to bc71088 Compare May 27, 2025 19:40
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rustbot commented May 27, 2025

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

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bors commented May 28, 2025

☔ The latest upstream changes (presumably #141668) made this pull request unmergeable. Please resolve the merge conflicts.

@GuillaumeGomez GuillaumeGomez force-pushed the function_casts_as_integer branch from bc71088 to 1adb4d1 Compare May 28, 2025 09:34
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CI finally passed. \o/

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