Code
pub struct Buffer<'a> {
buf: &'a mut [u8],
}
pub fn foo<'a>(buffer: &mut Buffer<'a>) {
buffer.buf = &mut buffer.buf[..];
}Current output
error[E0621]: explicit lifetime required in the type of `buffer`
--> src/lib.rs:6:5
|
6 | buffer.buf = &mut buffer.buf[..];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime `'a` required
|
help: add explicit lifetime `'a` to the type of `buffer`
|
5 | pub fn foo<'a>(buffer: &'a mut Buffer<'a>) {
| ++Desired output
Ideally, a detailed explanation with a better recommendation:
error: cannot produce a reference with lifetime `'a` using a reborrow
--> src/lib.rs:6:5
|
6 | buffer.buf = &mut buffer.buf[..];
| ^^^^^^^^^^ ^^^^ ^^^^^^^^^^ cannot move out of `buffer.buf` because it is behind an `&mut` reference
| | | therefore, this is a reborrow with a shorter lifetime than `'a`
| | lifetime `'a` required to assign to field `buf`
|
help: to modify an `&mut` reference, the original must be moved rather than reborrowed
|
6 | buffer.buf = &mut std::mem::take(&mut buffer.buf)[..];
| +++++++++++++++++++ +
Less ideally but easier to implement: simply remove the “help” subdiagnostic, whenever the suggested lifetime already appears in the type being altered. But that’s not very thorough, because “explicit lifetime required” might prompt users to do the same wrong thing on their own initiative.
Rationale and extra context
In almost all cases, &'a mut Foo<'a> is not what the user needs, because the Foo borrows itself forever, and thus can only be used once after this reference is created. The compiler should not recommend creating this type.
Other cases
This incorrect help does not appear if the function parameter is a self parameter:
impl<'a> Buffer<'a> {
pub fn foo(self: &mut Buffer<'a>) {
self.buf = &mut self.buf[..];
}
}Rust Version
1.95.0, playground
Anything else?
@rustbot label +D-confusing
Code
Current output
Desired output
Ideally, a detailed explanation with a better recommendation:
Less ideally but easier to implement: simply remove the “help” subdiagnostic, whenever the suggested lifetime already appears in the type being altered. But that’s not very thorough, because “explicit lifetime required” might prompt users to do the same wrong thing on their own initiative.
Rationale and extra context
In almost all cases,
&'a mut Foo<'a>is not what the user needs, because theFooborrows itself forever, and thus can only be used once after this reference is created. The compiler should not recommend creating this type.Other cases
This incorrect help does not appear if the function parameter is a
selfparameter:Rust Version
1.95.0, playground
Anything else?
@rustbot label +D-confusing