Closed
Description
For example, promotion succeeds in the example below, although compilation nevertheless fails due to checks for live drops:
pub const fn id<T>(x: T) -> T { x }
pub const C: () = {
let _: &'static _ = &String::new();
// Promotion failed, two errors:
// ERROR: destructors cannot be evaluated at compile-time
// ERROR: temporary value dropped while borrowed
let _: &'static _ = &id(&String::new());
// Promoted. Only one error:
// ERROR: destructors cannot be evaluated at compile-time
let _: &'static _ = &std::mem::ManuallyDrop::new(String::new());
// Promoted. No errors.
};With const_precise_live_drops, checking for live drops is delayed until after the promotion, so the following example compiles successfully:
#![feature(const_precise_live_drops)]
pub const fn id<T>(x: T) -> T { x }
pub const C: () = {
let _: &'static _ = &id(&String::new());
};The check for live drops can also be passed by providing a const drop implementation. The following example compiles successfully as well:
#![feature(const_fn_trait_bound)]
#![feature(const_mut_refs)]
#![feature(const_trait_impl)]
struct Panic;
impl const Drop for Panic { fn drop(&mut self) { panic!(); } }
pub const fn id<T>(x: T) -> T { x }
pub const C: () = {
let _: &'static _ = &id(&Panic);
};@rustbot label +A-const-eval