Attribute for types that should always be passed by value

[ecstatic-morse]

Inspired by #76914. Currently, we have a lint in rustc for types that should always be passed by value. Usually, these types are thin wrappers around references, so there is no benefit to an extra layer of indirection. However, this lint uses a hard-coded list of (diagnostic) names (currently Ty and TyCtxt). We should use an attribute (e.g. #[rustc_pass_by_value]) to control this. Note that Ty is a type alias, not a struct.

This is worthwhile because there are more widely used types that are merely a wrapper around a reference (e.g. ty::Predicate). One could also imagine crates in the ecosystem that also do a lot of interning might take advantage of this.

cc @lcnr

[ecstatic-morse]

Clippy has a more expansive, allow-by-default version of this BTW.

[mdibaiee]

@rustbot claim

[mdibaiee]

@ecstatic-morse I just tinkered around with this today a little bit.

I wasn't clear on one part: Do we want this lint to be internal to rustc, or just a builtin for everyone? I was confused because at some point you say:

One could also imagine crates in the ecosystem that also do a lot of interning might take advantage of this.

[ecstatic-morse]

@mdibaiee Internal to rustc for now. For the broader ecosystem, a clippy lint is probably more appropriate.