Skipping borrowck because of trivial const

[chenyukang]

r? @saethlin

the assertion is added in PR: #148040

I think we also need to skip trvial const in mir_borrowck.

[saethlin]

I'm okay with this change, but I would like @lcnr's opinion on whether this sort of whack-a-mole approach is sustainable. (I don't want to have an ever-growing number of is_trivial_const checks that eventually upsets the types team)

[lcnr]

What are the requirements for something to be a trivial const, are they written down somewhere? E.g. is const FOO: Tait = 1; a trivial const?

I personally feel quite... unhappy about this change. @BoxyUwU has been changing lowering of some constants for const generics to never actually get lowered to HIR expression but instead eagerly convert them into type system constants. That means we don't have a body for them and don't even try to call any of the typeck/mir_borrowck queries.

Having both of these impls seems fairly bad to me. It seems like ideally there should only be one way to get constants without corresponding MIR body.

I don't have the time to engage with that area myself right now :x I think we should not have both systems and ideally you'd chat with @BoxyUwU to figure out whether you can unify the two somehow

[saethlin]

Well, that's why I asked.

I agree that Boxy's approach seems better.

If you wanted to know the semantics I implemented, they're all in here: https://github.com/rust-lang/rust/blob/2fb805367dd3012ce5c50865d03c86e70bf10f0f/compiler/rustc_mir_transform/src/trivial_const.rs

[lcnr]

E.g. is const FOO: Tait = 1; a trivial const?

so this ends up as a trivial const? we never check that the type of the const is not an opaque. This code would start to fail/misbehave with this PR because MIR borrowck doesn't return the defining use anymore, would it not (at least with the new solver)?