clippy::option_if_let_else recommends code that fails borrow check.

[harrysarson]

I tried this code:

#![warn(clippy::pedantic)]

struct B(String);

fn f(optional: Option<B>) -> String {
    let b1 = B("!!".to_string());
    if let Some(b2) = optional {
        b1.0 + &b2.0
    } else {
        b1.0
    }
}

// fn f_as_recommended(optional: Option<B>) -> String {
//     let b1 = B("!!".to_string());
//     optional.map_or(b1.0, |b2|  b1.0 + &b2.0)
// }

fn main() {
    println!("{}", f(None));
}

Which gives the following cargo clippy output

    Checking playground v0.0.1 (/playground)
warning: use Option::map_or instead of an if let/else
  --> src/main.rs:7:5
   |
7  | /     if let Some(b2) = optional {
8  | |         b1.0 + &b2.0
9  | |     } else {
10 | |         b1.0
11 | |     }
   | |_____^ help: try: `optional.map_or(b1.0, |b2| b1.0 + &b2.0)`
   |
note: the lint level is defined here
  --> src/main.rs:1:9
   |
1  | #![warn(clippy::pedantic)]
   |         ^^^^^^^^^^^^^^^^
   = note: `#[warn(clippy::option_if_let_else)]` implied by `#[warn(clippy::pedantic)]`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#option_if_let_else

warning: 1 warning emitted

    Finished dev [unoptimized + debuginfo] target(s) in 0.32s

But when I apply clippy's suggestion, borrow checking rightly rejects by code because both closures try to move the value of b1 (and the compiler cannot know that only one closure will ever be called`.

Meta

• clippy 0.0.212 (2020-09-16 285fc7d): (from playground)
• rustc 1.46.0 stable or 1.48.0-nightly (2020-09-16 285fc7d704fcdd7b2a37): (from playground)

[harrysarson]

Oops, duplicate of #5822