E0381 incorrectly reports potentially uninitialized usage

[XSpielinbox]

Code

fn main() {
    let v1: Vec<i32> = vec![1, 2, 3];
    let mut iter = v1.iter();

    let mut first_encounter = true;
    let mut token: i32;
    let mut following_token: i32;
    loop {
        if first_encounter {
            if let Some(token) = iter.next() {
                match iter.next() {
                    Some(temp) => {
                        following_token = *temp;
                        first_encounter = false;
                        println!("{} followed by {}", token, following_token);
                    }
                    _ => {
                        println!("{} is last", token);
                    }
                }
            } else {
                break;
            }
        } else {
            token = following_token;
            first_encounter = true;
            match iter.next() {
                Some(temp) => {
                    following_token = *temp;
                    first_encounter = false;
                    println!("{} followed by {}", token, following_token);
                }
                _ => {
                    println!("{} is last", token);
                }
            }
        }
    }
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0381]: used binding `following_token` isn't initialized
  --> src/main.rs:25:21
   |
 7 |     let mut following_token: i32;
   |         ------------------- binding declared here but left uninitialized
...
17 |                     _ => {
   |                     - if this pattern is matched, `following_token` is not initialized
...
21 |             } else {
   |              ------ if the `if` condition is `false` and this `else` arm is executed, `following_token` is not initialized
...
25 |             token = following_token;
   |                     ^^^^^^^^^^^^^^^ `following_token` used here but it isn't initialized
   |
help: consider assigning a value
   |
 7 |     let mut following_token: i32 = 42;
   |                                  ++++

For more information about this error, try `rustc --explain E0381`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

The code compiles and running the binary produces:

1 followed by 2
2 followed by 3
3 is last

Rationale and extra context

As the usage flagged by rustc can only be reached when first_encounter is false, following_token cannot be uninitialized as it has been set at the point first_encounter is set to false in the other branch. It cannot be reached in the first loop iteration.

My actual code is way more complicated where I iterate over complex structs where I cannot define a meaningful default value.

Other cases

Rust Version

rustc 1.95.0 (59807616e 2026-04-14)
binary: rustc
commit-hash: 59807616e1fa2540724bfbac14d7976d7e4a3860
commit-date: 2026-04-14
host: x86_64-unknown-linux-gnu
release: 1.95.0
LLVM version: 22.1.2

Anything else?

Is code is also available on the Rust Playground.

This may be related to #121733.

[kpreid]

As a general principle, the compiler intentionally never uses the condition of an if, or any other value of an expression or variable, as part of deciding whether to accept or reject a program. So, it does not matter that first_encounter is only false when following_token is initialized.

However, the diagnostic could be improved: since it is already talking about things like “if the if condition is false”, it could reasonably include a “note:” specifying that the compiler is not actually analyzing whether that condition could ever be relevantly false, just describing a hypothetical situation.

[eval-exec]

@rustbot claim