Detect assignment operators that do nothing (e.g. `+= 0`)

What it does

Currently, identity_op can detect expressions that contain useless operators:

warning: this operation has no effect
 --> src/lib.rs:3:15
  |
3 |     counter = counter + 0;
  |               ^^^^^^^^^^^ help: consider reducing it to: `counter`

However, nothing detects a useless assignment version of the same operation: counter += 0 produces no lint at all. Detecting this seems to me to be a straightforward extension of what identity_op is already doing — though perhaps it should be a separate lint, since identity_op is sometimes a style negative, as in #13813, and this case is much more likely to be an outright bug.

Advantage

Detects code that is unnecessary and may be a bug.

Drawbacks

As with identity_op, the author might want code that completes a regular pattern even if one case is a no-op.

Example

pub fn example() -> i32 {
    let mut counter = 0;
    counter += 0;
    counter
}

Could be written as:

pub fn example() -> i32 {
    let mut counter = 0;
    counter
}

But the diagnostic should focus on the hypothesis that this is a bug, not that the code can be simplified while preserving its behavior.

Comparison with existing lints

No response

Additional Context

No response

Start by locating the existing identity_op lint and its tests, then trace how assignment operators are represented and diagnosed. Add coverage for compound assignments such as counter += 0; done means the useless assignment is reported with a bug-focused diagnostic while preserving the existing behavior for ordinary identity operations.