Move special treatment of derive(Copy, PartialEq, Eq) from expansion infrastructure to elsewhere

[petrochenkov]

As described in #62086 (comment).

Reminder:

• derive(PartialEq, Eq) makes the type it applied to a "structural match" type, so constants of this type can be used in patterns (and const generics in the future).
• derive(Copy) notifies other derives that the type it applied to implements Copy, so derive(Clone) can generate optimized code and other derives can generate code working with packed types and types with rustc_layout_scalar_valid_range attributes.

First, the special behavior is now enabled after properly resolving the derives, rather than after textually comparing them with "Copy", "PartialEq" and "Eq" in fn add_derived_markers.

The markers are no longer kept as attributes in AST since derives cannot modify items and previously did it through hacks in the expansion infra.
Instead, the markers are now kept in a "global context" available from all the necessary places, namely - resolver.

For derive(PartialEq, Eq) the markers are created by the derive macros themselves and then consumed during HIR lowering to add the #[structural_match] attribute in HIR.
This is still a hack, but now it's a hack local to two specific macros rather than affecting the whole expansion infra.
Ideally we should find the way to put #[structural_match] on the impls rather than on the original item, and then consume it in rustc_mir, then no hacks in expansion and lowering will be required.
(I'll make an issue about this for someone else to solve, after this PR lands.)

The marker for derive(Copy) cannot be emitted by the Copy macro itself because we need to know it before the Copy macro is expanded for expanding other macros.
So we have to do it in resolve and block expansion of any derives in a derive(...) container until we know for sure whether this container has Copy in it or not.
Nasty stuff.

r? @eddyb or @matthewjasper

[Centril]

I would think the better way to do this would be a perma unstable trait:

#[lang = "structural_match"]
unsafe trait Structural {}

for which implementations are generated.

[petrochenkov]

Yeah, something like that would also work.
PartialEq produces impl Structural, rustc_mir somehow checks for T: Structural + Eq.

[Centril]

cc also @varkor

This could be useful for const generics (#60286), e.g. fn foo<T: Structural, const A: T>();. (But that would require #[derive(PartialEq, Eq)] -> Structural, not just PartialEq.)

[petrochenkov]

r? @matthewjasper
eddyb is temporarily away and has too many assigned PRs already

[matthewjasper]

@bors r+

[bors]

📌 Commit 2a9b752 has been approved by matthewjasper

[bors]

⌛ Testing commit 2a9b752 with merge e1d7e4a...

[bors]

☀️ Test successful - checks-azure
Approved by: matthewjasper
Pushing e1d7e4a to master...