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32 changes: 26 additions & 6 deletions clippy_lints/src/transmute/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -527,24 +527,44 @@ declare_clippy_lint! {
/// Checks if transmute calls have all generics specified.
///
/// ### Why is this bad?
/// If not set, some unexpected output type could be retrieved instead of the expected one,
/// potentially leading to invalid code.
/// If not, one or more unexpected types could be used during `transmute()`, potentially leading
/// to Undefined Behavior or other problems.
///
/// This is particularly dangerous in case a seemingly innocent/unrelated change can cause type
/// inference to start inferring a different type. E.g. the transmute is the tail expression of
/// an `if` branch, and a different branches type changes, causing the transmute to silently
/// have a different type, instead of a proper error.
/// This is particularly dangerous in case a seemingly innocent/unrelated change causes type
/// inference to result in a different type. For example, if `transmute()` is the tail
/// expression of an `if`-branch, and the `else`-branch type changes, the compiler may silently
/// infer a different type to be returned by `transmute()`. That is because the compiler is
/// free to change the inference of a type as long as that inference is technically correct,
/// regardless of the programmer's unknown expectation.
///
/// Both type-parameters, the input- and the output-type, to any `transmute()` should
/// be given explicitly: Setting the input-type explicitly avoids confusion about what the
/// argument's type actually is. Setting the output-type explicitly avoids type-inference
/// to infer a technically correct yet unexpected type.
///
/// ### Example
/// ```no_run
/// # unsafe {
/// // Avoid "naked" calls to `transmute()`!
/// let x: i32 = std::mem::transmute([1u16, 2u16]);
///
/// // `first_answers` is intended to transmute a slice of bool to a slice of u8.
/// // But the programmer forgot to index the first element of the outer slice,
/// // so we are actually transmuting from "pointers to slices" instead of
/// // transmuting from "a slice of bool", causing a nonsensical result.
/// let the_answers: &[&[bool]] = &[&[true, false, true]];
/// let first_answers: &[u8] = std::mem::transmute(the_answers);
/// # }
/// ```
/// Use instead:
/// ```no_run
/// # unsafe {
/// let x = std::mem::transmute::<[u16; 2], i32>([1u16, 2u16]);
///
/// // The explicit type parameters on `transmute()` makes the intention clear,
/// // and cause a type-error if the actual types don't match our expectation.
/// let the_answers: &[&[bool]] = &[&[true, false, true]];
/// let first_answers: &[u8] = std::mem::transmute::<&[bool], &[u8]>(the_answers[0]);
/// # }
/// ```
#[clippy::version = "1.79.0"]
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