clippy::missing_transmute_annotations: Checks if transmute calls have all generics specified

Does it matter?

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 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.

Code that is most likely wrong or useless. Warns by default.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

let mut x: i32 = 0;
// Avoid "naked" calls to `transmute()`!
x = 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);

Write this instead

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]);

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

#[allow(clippy::missing_transmute_annotations)]
fn my_function() { /* ... */ }

[lints.clippy]
missing_transmute_annotations = "allow"

cargo clippy -- -A clippy::missing-transmute-annotations

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.