let accepts any irrefutable pattern.
a @ b @ c @ ref mut d is an irrefutable pattern that says:
- matches a mutable, copyable place
- copy the value to variables
a,b,c - put a mutable reference to the place in variable
d
Since you're not actually matching the pattern, those 4 variables are uninitialized.
This is a pattern that says:
- matches a mutable, copyable place
- copy the value to variables
a,b - put a mutable reference to the place in variables
c,d
Since creating two mutable reference to the same place would be invalid, such a pattern is just declared invalid upfront.
The fact that you aren't matching the pattern is irrelevant because the pattern itself is declared invalid.
Here is another interesting example:
let 0u8..=255u8;