`unnecessary_cast` false negative: parameter vs. local

Summary

unnecessary_cast appears to trigger for a parameter, but not for a local.

Lint Name

unnecessary_cast

Reproducer

I tried this code:

fn g() -> i16 { 42 }

pub fn f1() -> i32 {
    let x = g();
    x as i32 as i32
}

pub fn f2(x: i16) -> i32 {
    x as i32 as i32
}

I expected to see this happen: two lints, one on each function.

Instead, this happened: one lint, only on f2.

Version

rustc 1.83.0-nightly (18b1161ec 2024-10-02)
binary: rustc
commit-hash: 18b1161ec9eeab8927f91405bca0ddf59a4a26c9
commit-date: 2024-10-02
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

Start by running the provided Rust reproducer with the unnecessary_cast lint enabled and compare the diagnostics for f1 and f2. Trace the lint's handling of the local and parameter expressions, then ensure the reproducer produces two lint warnings without changing the intended lint behavior.