rustc thinks that two non-intersecting borrows intersect. And fails compilation

[USSURATONCACHI]

Here is a minimal example

fn main() {
    let mut data = "hello".to_owned();
    let out = process_data(&mut data);
    println!("{out:?}"); // Expected output: `Some("bye")`
}

/// Process data and return reference to its part.
fn process_data(data: &mut String) -> Option<&str> {
    loop {
        // Modify data in any way
        data.replace_range(0..5, "bye");

        // Access data via immutable reference, and try to reference with inherited lifetime.
        match find_word(data) {
            None => continue,
            Some(word) => return Some(word),
        }
    }
}

// You can use any function that takes a reference, and returns a related reference
fn find_word(data: &str) -> Option<&str> {
    let pattern = "bye";
    data.find(pattern)
        .map(|start| &data[start..(start + pattern.len())])
}

I expected to see this happen:

$ cargo run
Some("bye")

Instead, this happened: I have got a compile error on a perfectly valid code.

[ussur issue]$ rustc src/main.rs
error[E0502]: cannot borrow `*data` as mutable because it is also borrowed as immutable
  --> src/main.rs:11:9
   |
8  | fn process_data(data: &mut String) -> Option<&str> {
   |                       - let's call the lifetime of this reference `'1`
...
11 |         data.replace_range(0..5, "bye");
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
...
14 |         match find_word(data) {
   |                         ---- immutable borrow occurs here
15 |             None => continue,
16 |             Some(word) => return Some(word),
   |                                  ---------- returning this value requires that `*data` is borrowed for `'1`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0502`.

The code being valid can be proved by the fact that if we remove the loop - the function compiles perfectly:

// YET this does compile correctly: exactly the same code without a loop.
fn process_data(data: &mut String) -> Option<&str> {
    data.replace_range(0..5, "bye");

    match find_word(data) {
        None => None,
        Some(word) => return Some(word),
    }
}

Moreover, breaking the loop by returning in no way breaks any borrowing rules. If reference is returned - then line that mutates data cannot be executed, and this error message makes no sense. If reference is not returned - this error message also make no sense.

UPD: Moreover, manually unwinding the loop also breaks compilation:

fn process_data_unnwinded(data: &mut String) -> Option<&str> {
    data.replace_range(0..5, "bye");

    match find_word(data) {
        None => {},
        Some(word) => return Some(word),
    };

    data.replace_range(0..5, "bye");

    match find_word(data) {
        None => {},
        Some(word) => return Some(word),
    };

    todo!()
}

Gives:

[ussur@ussur-maibookxseries issue]$ rustc src/main.rs 
error[E0502]: cannot borrow `*data` as mutable because it is also borrowed as immutable
  --> src/main.rs:37:5
   |
29 | fn process_data_unnwinded(data: &mut String) -> Option<&str> {
   |                                 - let's call the lifetime of this reference `'1`
...
32 |     match find_word(data) {
   |                     ---- immutable borrow occurs here
33 |         None => {},
34 |         Some(word) => return Some(word),
   |                              ---------- returning this value requires that `*data` is borrowed for `'1`
...
37 |     data.replace_range(0..5, "bye");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0502`.

Even though immutable borrow ends after match {} ends - rustc for some invalid reason thinks that returning a reference makes it borrowed "forever".

Meta

Bug works the same both on stable and nightly.
rustc --version --verbose of stable:

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-unknown-linux-gnu
release: 1.85.1
LLVM version: 19.1.7

of nightly:

rustc 1.87.0-nightly (a2e63569f 2025-03-26)
binary: rustc
commit-hash: a2e63569fd6702ac5dd027a80a9fdaadce73adae
commit-date: 2025-03-26
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.1

Backtrace does not exist since program does not compile.

[USSURATONCACHI]

Also checked on two following versions:

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7

rustc 1.88.0-nightly (38c560ae6 2025-04-15)
binary: rustc
commit-hash: 38c560ae681d5c0d3fd615eaedc537a282fb1086
commit-date: 2025-04-15
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

The issue is exactly the same.

[Urgau]

Fixed by Polonius.

@rustbot labels -needs-triage +fixed-by-polonius +A-borrow-checker