generic_const_args: fix unreachable

[khyperia]

I'm really dumb, missed something obvious, and wasn't any test in the test suite to catch it.

Fixes #162923

Introduced in #162760

r? @BoxyUwU or @matthiaskrgr if you'd like

[rustbot]

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[khyperia]

wait no I haven't had my coffee yet what am I doing, this isn't the right fix, one moment... (I mean, it worked, but, yeah, actual fix pushed now)

[BoxyUwU]

haha when I was reviewing that PR I thought about how I hate precedence on these things and can never tell where the parens go 😆

View changes since the review

[BoxyUwU]

what does the THIR look like for this test, where does a NamedConst come from with a defid of an anon 🤔

[khyperia]

what does the THIR look like for this test, where does a NamedConst come from with a defid of an anon 🤔

comes from here

rust/compiler/rustc_mir_build/src/thir/cx/expr.rs

Line 374 in 50d5409

let kind = ExprKind::NamedConst { def_id: did, args, user_ty: None };

for reference this is what the code looks like:

enum T<const N: u8 = { T::<0>::B as u8 }> {
    A = 2,
    B,
}

the anon const for the default in enum T<const N: u8 = { this one here }> is called T::{constant#0}. the anon const for the A = { this one here }, is called T::A::{constant#0}. lowering a reference to T::<0>::B takes the discr_did (T::A::{constant#0}) and adds discr_offset (1) to it to get T::<0>::B. this THIR expr is then used, casted to u8, and that's the body of the anon const of the default generic param.

T::{constant#0} = (T::A::{constant#0} + 1) as u8;