single-use-lifetimes may remove attributes of wrong lifetime? #105031
Description
Given the following code:
#![feature(dropck_eyepatch)]
fn main() {
// Since the second param to `D1` is may_dangle, it is legal for
// the region of that parameter to end before the drop code for D1
// is executed.
(D1(&S1("ex1"), &S1("dang1"))).0;
}
#[derive(Debug)]
struct S1(&'static str);
#[derive(Debug)]
struct D1<'a, 'b>(&'a S1, &'b S1);
// The `#[may_dangle]` means that references of type `&'b _` may be
// invalid during the execution of this destructor; i.e. in this case
// the destructor code is not allowed to read or write `*self.1`, while
// it can read/write `*self.0`.
unsafe impl<'a, #[may_dangle] 'b> Drop for D1<'a, 'b> {
fn drop(&mut self) {
println!("D1({:?}, _)", self.0);
}
}The current output is:
warning: lifetime parameter `'a` only used once
--> src/main.rs:20:13
|
20 | unsafe impl<'a, #[may_dangle] 'b> Drop for D1<'a, 'b> {
| ^^ this lifetime... -- ...is used only here
|
= note: requested on the command line with `-W single-use-lifetimes`
help: elide the single-use lifetime
|
20 - unsafe impl<'a, #[may_dangle] 'b> Drop for D1<'a, 'b> {
20 + unsafe impl<'b> Drop for D1<'_, 'b> {
|
warning: lifetime parameter `'b` only used once
--> src/main.rs:20:31
|
20 | unsafe impl<'a, #[may_dangle] 'b> Drop for D1<'a, 'b> {
| ^^ this lifetime... -- ...is used only here
|
help: elide the single-use lifetime
|
20 - unsafe impl<'a, #[may_dangle] 'b> Drop for D1<'a, 'b> {
20 + unsafe impl<'a> Drop for D1<'a, '_> {
It's trying to remove the attribute of lifetime 'b when removing lifetime 'a
and causes a compiler error all in all:
The following errors were reported:
warning: lifetime parameter `'a` only used once
--> src/main.rs:20:13
|
20 | unsafe impl<'a> Drop for D1<'a, '_> {
| ^^ -- ...is used only here
| |
| this lifetime...
|
= note: requested on the command line with `-W single-use-lifetimes`
help: elide the single-use lifetime
|
20 - unsafe impl<'a> Drop for D1<'a, '_> {
20 + unsafe impl Drop for D1<'_, '_> {
|
error[E0199]: implementing the trait `std::ops::Drop` is not unsafe
--> src/main.rs:20:1
|
20 | / unsafe impl<'a> Drop for D1<'a, '_> {
21 | | fn drop(&mut self) {
22 | | println!("D1({:?}, _)", self.0);
23 | | }
24 | | }
| |_^
|
help: remove `unsafe` from this trait implementation
|
20 - unsafe impl<'a> Drop for D1<'a, '_> {
20 + impl<'a> Drop for D1<'a, '_> {
|
error: aborting due to previous error; 1 warning emitted
For more information about this error, try `rustc --explain E0199`.
Original diagnostics will follow.