false positive for if-let-rescope on enums when the pattern exhaustively matches all cases with significant-drop fields

[steffahn]

Coming from #133167 (comment) and from this new forum thread, my takeaway of some remaining clear false positives is:

• if the if let matches on an enum and only some enum variant(s) have fields with potentially-significant destructor, but those are exhaustively matched in the non-else case of the if let, then there can be no actual change in behavior, because the else case is only reached in case the drop is not significant

• in the case of cargo-edit, this was if let Some(…irrefutable…) = expr on an Option<T>

• in the urlo thread, I’ve identified the case of if let Err(…irrefutable…) on a Result<(), E>

Here’s a simple repro for an Option case:

Code

#![allow(unused)]
#![warn(if_let_rescope)]

struct Struct;
impl Drop for Struct {
    fn drop(&mut self) {}
}

fn f(option_s: Option<Struct>) {
    if let Some(s) = option_s {
        // …
    } else {
        // …
    }
}

Here’s the code in the playground.

Current output

warning: `if let` assigns a shorter lifetime since Edition 2024
  --> src/lib.rs:10:8
   |
10 |     if let Some(s) = option_s {
   |        ^^^^^^^^^^^^^^--------
   |                      |
   |                      this value has a significant drop implementation which may observe a major change in drop order and requires your discretion
   |
   = warning: this changes meaning in Rust 2024
   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/temporary-if-let-scope.html>
help: the value is now dropped here in Edition 2024
  --> src/lib.rs:12:5
   |
12 |     } else {
   |     ^
note: the lint level is defined here
  --> src/lib.rs:2:9
   |
2  | #![warn(if_let_rescope)]
   |         ^^^^^^^^^^^^^^
help: a `match` with a single arm can preserve the drop order up to Edition 2021
   |
10 ~     match option_s { Some(s) => {
11 |         // …
12 ~     } _ => {
13 |         // …
14 ~     }}
   |

@rustbot label A-edition-2024, A-lints, L-if_let_rescope, C-enhancement