Is TypeFlags::NOMINAL_FLAGS unnecessary now?

[eddyb]

This seems to contain all the flags in TypeFlags. If I'm guessing right, the flags that used to not be in here, all turned into queries? So they're now cached as queries, instead of by mutating Ty itself?

rust/src/librustc_middle/ty/mod.rs

Lines 603 to 624 in af89eb5

	/// Flags representing the nominal content of a type,
	/// computed by FlagsComputation. If you add a new nominal
	/// flag, it should be added here too.
	const NOMINAL_FLAGS = TypeFlags::HAS_TY_PARAM.bits
	| TypeFlags::HAS_RE_PARAM.bits
	| TypeFlags::HAS_CT_PARAM.bits
	| TypeFlags::HAS_TY_INFER.bits
	| TypeFlags::HAS_RE_INFER.bits
	| TypeFlags::HAS_CT_INFER.bits
	| TypeFlags::HAS_TY_PLACEHOLDER.bits
	| TypeFlags::HAS_RE_PLACEHOLDER.bits
	| TypeFlags::HAS_CT_PLACEHOLDER.bits
	| TypeFlags::HAS_FREE_LOCAL_REGIONS.bits
	| TypeFlags::HAS_TY_PROJECTION.bits
	| TypeFlags::HAS_TY_OPAQUE.bits
	| TypeFlags::HAS_CT_PROJECTION.bits
	| TypeFlags::KEEP_IN_LOCAL_TCX.bits
	| TypeFlags::HAS_TY_ERR.bits
	| TypeFlags::HAS_FREE_REGIONS.bits
	| TypeFlags::HAS_RE_LATE_BOUND.bits
	| TypeFlags::HAS_RE_ERASED.bits
	| TypeFlags::STILL_FURTHER_SPECIALIZABLE.bits;

cc @nikomatsakis @Zoxc

[nikomatsakis]

If it contains all the flags, then it seems to have no purpose. I'm honestly not sure what the name "nominal" is even meant to refer to here exactly -- I'm sure I gave it that name, but it's not very clear. I guess the point is "flags that should be inherited by containing types; ie., if the type T has some flag F, and F is "nominal", then Vec<T> should also have F".

Are there any flags that don't meet that definition anymore?

[eddyb]

@nikomatsakis I think there were "nominal flags" and "cached properties" (using interior mutability), but now the latter is completely gone.

[nikomatsakis]

that sounds right. I'd be in favor of removing NOMINAL_FLAGS then.

[tsandstr]

I would love to help with this one. A quick search shows that NOMINAL_FLAGS is only used once in the entire code base, here:

rust/src/librustc_middle/ty/flags.rs

Lines 30 to 32 in 537ccdf

	fn add_flags(&mut self, flags: TypeFlags) {
	self.flags = self.flags | (flags & TypeFlags::NOMINAL_FLAGS);
	}

Looks like this could just be changed to
self.flags = self.flags | flags;

Is that all it will take?

[eddyb]

Sounds correct, yeah.

[tsandstr]

Cool great

[tsandstr]

Oops, I'm still learning git. Sorry for the spam there. Only 4cdb206 is relevant.