FP map_identity: fix requires mutability

[matthiaskrgr]

Summary

.

Lint Name

map_identity

Reproducer

I tried this code:

// This is a reduction of a concrete test illustrating a case that was
// annoying to Rust developer stephaneyfx (see issue #46413).
//
// With resolving issue #54556, pnkfelix hopes that the new diagnostic
// output produced by NLL helps to *explain* the semantic significance
// of temp drop order, and thus why storing the result in `x` and then
// returning `x` works.

pub struct Statement;

pub struct Rows<'stmt>(&'stmt Statement);

impl<'stmt> Drop for Rows<'stmt> {
    fn drop(&mut self) {}
}

impl<'stmt> Iterator for Rows<'stmt> {
    type Item = String;

    fn next(&mut self) -> Option<Self::Item> {
        None
    }
}

fn get_names() -> Option<String> {
    let stmt = Statement;
    let rows = Rows(&stmt); 
    rows.map(|row| row).next() // boom

}

fn main() {}

I saw this happen:

warning: unnecessary map of the identity function
  --> src/main.rs:28:9
   |
28 |     rows.map(|row| row).next()
   |         ^^^^^^^^^^^^^^^ help: remove the call to `map`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_identity
   = note: `#[warn(clippy::map_identity)]` on by default

Applying the suggestion breaks the build

error[E0596]: cannot borrow `rows` as mutable, as it is not declared as mutable
  --> src/main.rs:28:5
   |
28 |     rows.next()
   |     ^^^^ cannot borrow as mutable
   |
help: consider changing this to be mutable
   |
27 |     let mut rows = Rows(&stmt);
   |         +++

For more information about this error, try `rustc --explain E0596`.

Version

rustc 1.85.0-nightly (8742e0556 2024-12-28)
binary: rustc
commit-hash: 8742e0556dee3c64f7144de2fb2e88936418865a
commit-date: 2024-12-28
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.6

Additional Labels

No response