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Auto merge of #130540 - veera-sivarajan:fix-87525, r=estebank

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Add a Lint for Pointer to Integer Transmutes in Consts

Fixes #87525

This PR adds a MirLint for pointer to integer transmutes in const functions and associated consts. The implementation closely follows this comment: https://github.com/rust-lang/rust/pull/85769#issuecomment-880969112. More details about the implementation can be found in the comments.

Note: This could break some sound code as mentioned by RalfJung in https://github.com/rust-lang/rust/pull/85769#issuecomment-886491680:

> ... technically const-code could transmute/cast an int to a ptr and then transmute it back and that would be correct -- so the lint will deny some sound code. Does not seem terribly likely though.

References:
1. https://doc.rust-lang.org/std/mem/fn.transmute.html
2. https://doc.rust-lang.org/reference/items/associated-items.html#associated-constants
This commit is contained in:
bors 2024-10-06 02:39:23 +00:00
commit daebce4247
10 changed files with 262 additions and 6 deletions
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35
compiler/rustc_lint_defs/src/builtin.rs
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@ -81,6 +81,7 @@ declare_lint_pass! {
PRIVATE_INTERFACES,
PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
PTR_CAST_ADD_AUTO_TO_OBJECT,
PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
PUB_USE_OF_PRIVATE_EXTERN_CRATE,
REDUNDANT_IMPORTS,
REDUNDANT_LIFETIMES,
@ -4998,3 +4999,37 @@ declare_lint! {
reference: "issue #124535 <https://github.com/rust-lang/rust/issues/124535>",
};
}
declare_lint! {
/// The `ptr_to_integer_transmute_in_consts` lint detects pointer to integer
/// transmute in const functions and associated constants.
///
/// ### Example
///
/// ```rust
/// const fn foo(ptr: *const u8) -> usize {
/// unsafe {
/// std::mem::transmute::<*const u8, usize>(ptr)
/// }
/// }
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Transmuting pointers to integers in a `const` context is undefined behavior.
/// Any attempt to use the resulting integer will abort const-evaluation.
///
/// But sometimes the compiler might not emit an error for pointer to integer transmutes
/// inside const functions and associated consts because they are evaluated only when referenced.
/// Therefore, this lint serves as an extra layer of defense to prevent any undefined behavior
/// from compiling without any warnings or errors.
///
/// See [std::mem::transmute] in the reference for more details.
///
/// [std::mem::transmute]: https://doc.rust-lang.org/std/mem/fn.transmute.html
pub PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
Warn,
"detects pointer to integer transmutes in const functions and associated constants",
}