Lint useless shadowing

What it does

Shadowing let x and then let mut x bindings with no intermediate uses of x could be combined to a single statement.

Advantage

• Removes unneeded binding
• Perhaps optimizes stores/loads if it happens across function boundaries/crates
• Increases robustness of refactoring

Drawbacks

No response

Example

fn foo() -> i32 {
    let x = 6;
    let mut x = x;
    x += 1;
    x
}

Could be written as:

fn foo() -> i32 {
    let mut x = 6;
    x += 1;
    x
}

The issue names no files, tests, or entry points. Start by locating Clippy's existing lint implementation and test patterns for shadowing, then use the supplied Rust example to define the no-intermediate-use case and verify that the lint suggestion preserves behavior.