This is effectively a rustdoc bug, the pattern is very much part of the body of the function, not the signature (and this split is explicitly represented in the compiler IRs, which rustdoc effectively undoes).
While it's not really how it works, the closest valid Rust that comes to mind is:
const foo: fn(u32) = |mut x| {};
What rustdoc should be doing is stripping binding modes (`ref` and/or `mut`), or even whole patterns - with more complex patterns, like `(a, mut b): (T, U)`, it can be beneficial to keep `(a, b)` as a way to label the two components, but the `mut` is superfluous for documentation.
While it's not really how it works, the closest valid Rust that comes to mind is:
What rustdoc should be doing is stripping binding modes (`ref` and/or `mut`), or even whole patterns - with more complex patterns, like `(a, mut b): (T, U)`, it can be beneficial to keep `(a, b)` as a way to label the two components, but the `mut` is superfluous for documentation.