Fix evaluating trivial drop glue in constants

[oli-obk]

struct A;
impl Drop for A {
    fn drop(&mut self) {}
}

const FOO: Option<A> = None;

const BAR: () = (FOO, ()).1;

was erroring with

error: any use of this value will cause an error
 --> src/lib.rs:9:1
  |
9 | const BAR: () = (FOO, ()).1;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^-^
  |                           |
  |                           calling non-const function `std::ptr::real_drop_in_place::<(std::option::Option<A>, ())> - shim(Some((std::option::Option<A>, ())))`
  |
  = note: #[deny(const_err)] on by default

error: aborting due to previous error

before this PR. According to godbolt this last compiled successfully in 1.27

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[pnkfelix]

Can you add a ui test for the error case showing that const FOO: Option<A> = Some(A); will continue be rejected under this change?

I skimmed over the existing tests and I didn't immediately see anything covering that (where we have the glue for Option around the type implementing Drop).

• I don't count
  

  rust/src/test/ui/consts/const-eval/const_let.rs

  Line 17 in 6bba352

  const Z: () = { let mut x = None; x = Some(FakeNeedsDrop); };

  because that seems like a slightly different path...

[pnkfelix]

r=me with aforementioned test added.

[oli-obk]

We do have

rust/src/test/ui/static/static-drop-scope.rs

Lines 17 to 21 in 79d8a0f

	static EARLY_DROP_S: i32 = (WithDtor, 0).1;
	//~^ ERROR destructors cannot be evaluated at compile-time
	const EARLY_DROP_C: i32 = (WithDtor, 0).1;
	//~^ ERROR destructors cannot be evaluated at compile-time

and

rust/src/test/ui/span/E0493.rs

Line 17 in 79d8a0f

const F : Foo = (Foo { a : 0 }, Foo { a : 1 }).1;

but neither of these actually use an Option so I added some tests to static-drop-scope.rs

@bors r=pnkfelix

[bors]

📌 Commit 39aa89b has been approved by pnkfelix

[bors]

🌲 The tree is currently closed for pull requests below priority 1000, this pull request will be tested once the tree is reopened

[Centril]

Failed in rollup, #57874 (comment).
@bors r-

(please please --bless your tests with --compare-mode=nll :) )

[oli-obk]

@bors r=pnkfelix

[bors]

📌 Commit 506393e has been approved by pnkfelix