Rollup merge of #109621 - scottmcm:update-variantidx, r=compiler-errors

Refactor: `VariantIdx::from_u32(0)` -> `FIRST_VARIANT`

Since structs are always `VariantIdx(0)`, there's a bunch of files where the only reason they had `VariantIdx` or `vec::Idx` imported at all was to get the first variant.

So this uses a constant for that, and adds some doc-comments to `VariantIdx` while I'm there, since [it doesn't have any today](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/abi/struct.VariantIdx.html).

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index 66fc1c07e20..2be385d551e 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -17,7 +17,7 @@ use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeVisitableExt
 use rustc_mir_dataflow::impls::MaybeStorageLive;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
-use rustc_target::abi::{Size, VariantIdx};
+use rustc_target::abi::{Size, FIRST_VARIANT};
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 enum EdgeKind {
@@ -359,7 +359,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         check_equal(self, location, *f_ty);
                     }
                     ty::Adt(adt_def, substs) => {
-                        let var = parent_ty.variant_index.unwrap_or(VariantIdx::from_u32(0));
+                        let var = parent_ty.variant_index.unwrap_or(FIRST_VARIANT);
                         let Some(field) = adt_def.variant(var).fields.get(f.as_usize()) else {
                             fail_out_of_bounds(self, location);
                             return;