Constify trait aliases

[oli-obk]

Allow const trait Foo = Bar + [const] Baz; trait alias declarations. Their rules are the same as with super traits of const traits. So [const] Baz or const Baz is only required for [const] Foo or const Foo bounds respectively.

tracking issue #41517 (part of the general trait alias feature gate, but I can split it out into a separate const trait alias feature gate. I just assumed that const traits would stabilize before trait aliases, and we'd want to stabilize trait aliases together with const trait aliases at the same time)

r? @compiler-errors @fee1-dead

[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

HIR ty lowering was modified

cc @fmease

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[nnethercote]

👍

[blyxyas]

Clippy changes are approved.

[bors]

☔ The latest upstream changes (presumably #143897) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #143843) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #143884) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

r? fee1-dead or re-roll perhaps

[fee1-dead]

quick review because i didn't get enough time

Didn't look at the solver parts yet, will do later

View changes since this review

[fee1-dead]

The commits should probably be squashed.

[oli-obk]

The commits should probably be squashed.

Hmm... I split them on purpose to only make atomic testable changes

[fee1-dead]

r=me after nit

View changes since this review

[fee1-dead]

The comment really applies to both of those lines, so

Suggested change

	ecx.add_goals(GoalSource::Misc, where_clause_bounds);
	let const_conditions = cx
	.const_conditions(goal.predicate.def_id().into())
	.iter_instantiated(cx, goal.predicate.trait_ref.args)
	.map(|bound_trait_ref| {
	goal.with(
	cx,
	bound_trait_ref.to_host_effect_clause(cx, goal.predicate.constness),
	)
	});
	// While you could think of trait aliases to have a single builtin impl
	// which uses its implied trait bounds as where-clauses, using
	// `GoalSource::ImplWhereClause` here would be incorrect, as we also
	// impl them, which means we're "stepping out of the impl constructor"
	// again. To handle this, we treat these cycles as ambiguous for now.
	ecx.add_goals(GoalSource::Misc, const_conditions);
	let const_conditions = cx
	.const_conditions(goal.predicate.def_id().into())
	.iter_instantiated(cx, goal.predicate.trait_ref.args)
	.map(|bound_trait_ref| {
	goal.with(
	cx,
	bound_trait_ref.to_host_effect_clause(cx, goal.predicate.constness),
	)
	});
	// While you could think of trait aliases to have a single builtin impl
	// which uses its implied trait bounds as where-clauses, using
	// `GoalSource::ImplWhereClause` here would be incorrect, as we also
	// impl them, which means we're "stepping out of the impl constructor"
	// again. To handle this, we treat these cycles as ambiguous for now.
	ecx.add_goals(GoalSource::Misc, where_clause_bounds);
	ecx.add_goals(GoalSource::Misc, const_conditions);

[bors]

☔ The latest upstream changes (presumably #147957) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...
525 |             ItemKind::Const(ident, ..)
    |                             ----- first introduced with type `rustc_span::Ident` here
...
531 |             | ItemKind::TraitAlias(ident, ..)
    |                                    ^^^^^ expected `Ident`, found `Constness`
    |
    = note: in the same arm, a binding must have the same type in all alternatives

For more information about this error, try `rustc --explain E0308`.
[RUSTC-TIMING] clippy_lints test:false 7.335