inaccurate const drop error doesn't mention #![feature(const_precise_live_drops)]

[meithecatte]

struct Test<T>(T);

impl<T> Test<T> {
    const fn unpack(self) -> T {
        self.0
    }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0493]: destructors cannot be evaluated at compile-time
 --> src/lib.rs:4:21
  |
4 |     const fn unpack(self) -> T {
  |                     ^^^^ constant functions cannot evaluate destructors
5 |         self.0
6 |     }
  |     - value is dropped here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0493`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

However, no destructors are actually ran, as can be seen in the MIR output:

fn <impl at src/lib.rs:3:1: 7:2>::unpack(_1: Test<T>) -> T {
    debug self => _1;                    // in scope 0 at src/lib.rs:4:15: 4:19
    let mut _0: T;                       // return place in scope 0 at src/lib.rs:4:24: 4:25

    bb0: {
        _0 = move (_1.0: T);             // scope 0 at src/lib.rs:5:9: 5:15
        return;                          // scope 0 at src/lib.rs:6:6: 6:6
    }
}

The same effect can be observed if the field access is replaced by pattern matching, either in let or match.

[meithecatte]

@rustbot modify labels: A-const-fn C-bug T-compiler

[meithecatte]

After a source code dive (!) I found out that this is governed by the const_precise_live_drops feature flag (#73255). Perhaps it would be useful for the error to mention this feature when relevant?

[meithecatte]

@rustbot modify labels: -C-bug A-diagnostics