`clippy::write_literal` should suggest using `write_str` for `fmt::Write`

Summary

The clippy::write_literal lint suggests changing code like write!(f, "{}", "{}") to write!(f, "{{}}"), but this can require a lot of changes to the string if it contains many { or } chars. If the first argument to write! implements fmt::Write, clippy should suggest changing it to f.write_str("{}") instead.

Reproducer

I tried this code:

use std::fmt::Write;
fn main() {
    let mut s = String::new();
    write!(s, "{}", "{}").unwrap();
}

I expected to see this happen:

warning: literal with an empty format string
 --> clippy-write-literal.rs:4:21
  |
4 |     write!(s, "{}", "{}").unwrap();
  |                     ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#write_literal
  = note: `#[warn(clippy::write_literal)]` on by default
help: try
  |
4 -     write!(s, "{}", "{}").unwrap();
4 +     s.write_str("{}").unwrap();
  |

warning: 1 warning emitted

Instead, this happened:

warning: literal with an empty format string
 --> clippy-write-literal.rs:4:21
  |
4 |     write!(s, "{}", "{}").unwrap();
  |                     ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#write_literal
  = note: `#[warn(clippy::write_literal)]` on by default
help: try
  |
4 -     write!(s, "{}", "{}").unwrap();
4 +     write!(s, "{{}}").unwrap();
  |

warning: 1 warning emitted

Version

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: aarch64-apple-darwin
release: 1.81.0
LLVM version: 18.1.7

Additional Labels

No response

Start by locating the write_literal lint and its handling of fmt::Write targets. Use the reproducer to compare the current suggestion with the requested s.write_str("{}").unwrap() form, then verify that the diagnostic changes while other write_literal cases remain correct.