rustc since 1.36 accepts invalid code after return

Code

I tried this code:

fn foo() -> u8 {
    return 0;
    let a: u8 = 1;
    a = 2; // error: second write to an immutable variable
    return a;
}
fn main() -> () {
    println!("{}", foo());
}

rustc-1.35 rejects the code and says:

error[E0384]: cannot assign twice to immutable variable `a`
 --> <source>:4:5
  |
3 |     let a: u8 = 1;
  |         - first assignment to `a`
4 |     a = 2;
  |     ^^^^^ cannot assign twice to immutable variable

Current rustc accepts the code

I acknowledge that the invalid write happens in a dead region, but still report it as all rustc versions reject the following code:

fn foo() -> u8 {
    if true {
        return 0;
    }
    let a: u8 = 1;
    a = 2; // error: second write to an immutable variable
    return a;
}
fn main() -> () {
    println!("{}", foo());
}

Start by compiling the two Rust reproductions with rustc 1.35 and a current compiler, comparing diagnostics for the unreachable assignment. Trace the compiler's unreachable-code and immutable-assignment checks to find where the regression was introduced. Done means the invalid write is rejected again, with a regression test covering both examples.