Description
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.