Readonly check for tuple

Reproducer

I tried this code:

fn main() {
    let a = 1;
    let b = 2;
    (a, b) = (b, a);
}

Does the code make use of any (1.49) nightly feature ?

• Nightly

Godbolt link

No response

Actual behavior

It does not emit an error

Expected behavior

I expected to see...

error[E0384]: cannot assign twice to immutable variable `a`
  --> <source>:15:6
   |
13 |     let a = 1;
   |         - first assignment to `a`
14 |     let b = 2;
15 |     (a, b) = (b, a);
   |      ^ cannot assign twice to immutable variable
   |
help: consider making this binding mutable
   |
13 |     let mut a = 1;
   |         +++

error[E0384]: cannot assign twice to immutable variable `b`
  --> <source>:15:9
   |
14 |     let b = 2;
   |         - first assignment to `b`
15 |     (a, b) = (b, a);
   |         ^ cannot assign twice to immutable variable
   |
help: consider making this binding mutable
   |
14 |     let mut b = 2;
   |         +++

GCC Version

508b195