SE-0345: `if let` shorthand for shadowing an existing optional variable

[tera]

@tera

1) shadowing let x = y     <--->  let x = y // x was defined in an outer block
2) shadowing let x         <--->  let x = x
3) shadowing let x = 2*x   <--->  let x = 2*x
4) let x = x                      // ❌ error: use explicit shadowing
5) let x = 2*x                    // ❌ error: add explicit shadowing
6) shadowing let x = x            // ❌ error or warning: remove "= x"
7) let yourSurname = yourSurname  // ❌ error: use explicit shadowing
8) let yоurSurname = yourSurname  // ok (homograph spoofing "о" vs "o")

ditto for if / guard and var.

I'm slowly gravitating towards this and coming to a conclusion that this is superior to the "if let" shortcut being discussed on the following fronts:

• handles situation more generally, including cases outside of if / guard
• makes shadowing intent explicit (prohibits the current implicit shadowing "let x = x" in all contexts).
• prohibits repeating the same name (shadowing let someName = someName) thus freeing user's brainpower from parsing the string and checking if the two names match or don't match.
• as a result makes homograph spoofing more obvious (a minor bonus point)

I appreciate that the cost of adding a new keyword is high, but in this case I'd consider doing this.

With that reasoning in mind I sadly have to change my +1 to -1 for the proposal being discussed.