Lint variable only ever assigned a single value

[oli-obk]

In both the rust source and the rustfmt source I encountered code of the sort

let first = false;
for x in y {
    if first {
        first = false;
        // do some logic
    }
    // do some logic
}

If a variable never gets a mutable reference taken to and all assignments assign the same value, we should have a correctness lint tell the user they are doing something that makes no sense in any program.

This is probably easiest implemented as an MIR lint

[oxalica]

Just ran into a similar typo bug. oxalica/nil#92