Let-else source expression move behavior · Issue #89688 · rust-lang/rust (original) (raw)

The move behavior on non-matching of expressions by feature(let_else) seems currently inconsistent with behavior of, for example, if let.

An if-let construct does not move from the matched expression if the pattern doesn't match:

fn example_if_let(value: Option) { if let Some(inner) = value { println!("inner: {}", inner); } else { println!("other: {:?}", value); } }

fn main() { example_if_let(Some("foo".into())); example_if_let(None); }

compiles and prints

However, let-else seems to unconditionally move from the matched expression:

#![feature(let_else)]

fn example_let_else(value: Option) { let Some(inner) = value else { println!("other: {:?}", value); return; }; println!("inner: {}", inner); }

fn main() { example_let_else(Some("foo".into())); example_let_else(None); }

fails to compile:

error[E0382]: borrow of moved value: `value`
 --> src/main.rs:5:33
  |
3 | fn example_let_else(value: Option<String>) {
  |                     ----- move occurs because `value` has type `Option<String>`, which does not implement the `Copy` trait
4 |     let Some(inner) = value else {
  |                       ----- value moved here
5 |         println!("other: {:?}", value);
  |                                 ^^^^^ value borrowed here after move

I'm wondering if this is intentional, as it would seem to make it impossible to use the original unmatched value in an panic message (or diverge otherwise with the value or any value based on it).

rustc 1.57.0-nightly (485ced56b 2021-10-07)
binary: rustc
commit-hash: 485ced56b8753ec86936903f2a8c95e9be8996a1
commit-date: 2021-10-07
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0

Proposed Label: F-let-else