Code
struct MyVec<T>(T);
impl<T> MyVec<T> {
fn replace_with<U>(&self, mut f: impl FnMut() -> U) -> MyVec<U> {
// reports correct error E0596,
// but with odd explanation in the rustc output: -------- ""change this to return `FnMut` instead of `Fn`""
self.map(|_| f())
}
fn map<U>(&self, _: impl Fn(&T) -> U) -> MyVec<U> {
todo!()
}
}Current output
error[E0596]: cannot borrow `f` as mutable, as it is a captured variable in a `Fn` closure
--> src/lib.rs:7:22
|
4 | fn replace_with<U>(&self, mut f: impl FnMut() -> U) -> MyVec<U> {
| ------------ -------- change this to return `FnMut` instead of `Fn`
...
7 | self.map(|_| f())
| --- ^ cannot borrow as mutable
| |
| in this closure
For more information about this error, try `rustc --explain E0596`.Desired output
error[E0596]: cannot borrow `f` as mutable, as it is a captured variable in a `Fn` closure
--> src/lib.rs:7:22
|
7 | self.map(|_| f())
| -------- --- ^ cannot borrow as mutable
| | |
| | in this closure
| expects `Fn` instead of `FnMut`
8 | }
9 | fn map<U>(&self, _: impl Fn(&T) -> U) -> MyVec<U> {
| ---------------- change this to accept `FnMut` instead of `Fn`
For more information about this error, try `rustc --explain E0596`.Rationale and extra context
Affected playground, with correct error but misleading explanation.
Removing impl block and self parameters shows the correct/expected diagnostic explanation,
demonstrated in this playground
Other cases
No response
Rust Version
Duplicated on playground with both "Stable 1.75.0", and on playground selecting nightly:
Build using the Nightly version: 1.77.0-nightly
(2024-01-13 d78329b92e8d141d1950)
Anything else?
Overall, not a huge issue - the actual error indication is correct.
The small oddity is where it points to the function return type as the location of the Fn. This took me a bit to identify the mismatched Fn/FnMut 🙂
Code
Current output
Desired output
Rationale and extra context
Affected playground, with correct error but misleading explanation.
Removing
implblock andselfparameters shows the correct/expected diagnostic explanation,demonstrated in this playground
Other cases
No response
Rust Version
Duplicated on playground with both "Stable 1.75.0", and on playground selecting nightly:
Anything else?
Overall, not a huge issue - the actual error indication is correct.
The small oddity is where it points to the function return type as the location of the
Fn. This took me a bit to identify the mismatchedFn/FnMut🙂