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).

2 files changed, 5 insertions, 8 deletions

diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 9cba8870f23..7136e42189a 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -10,7 +10,7 @@ use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Instance, InstanceDef, ParamEnv, Ty, TyCtxt};
 use rustc_session::config::OptLevel;
 use rustc_span::{hygiene::ExpnKind, ExpnData, LocalExpnId, Span};
-use rustc_target::abi::VariantIdx;
+use rustc_target::abi::FIRST_VARIANT;
 use rustc_target::spec::abi::Abi;
 
 use crate::simplify::{remove_dead_blocks, CfgSimplifier};
@@ -911,7 +911,7 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                     check_equal(self, *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 {
                         self.validation = Err("malformed MIR");
                         return;
diff --git a/compiler/rustc_mir_transform/src/shim.rs b/compiler/rustc_mir_transform/src/shim.rs
index ebe63d6cb7e..970a0a8d4bf 100644
--- a/compiler/rustc_mir_transform/src/shim.rs
+++ b/compiler/rustc_mir_transform/src/shim.rs
@@ -5,7 +5,7 @@ use rustc_middle::mir::*;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::InternalSubsts;
 use rustc_middle::ty::{self, EarlyBinder, GeneratorSubsts, Ty, TyCtxt};
-use rustc_target::abi::VariantIdx;
+use rustc_target::abi::{VariantIdx, FIRST_VARIANT};
 
 use rustc_index::vec::{Idx, IndexVec};
 
@@ -816,11 +816,8 @@ pub fn build_adt_ctor(tcx: TyCtxt<'_>, ctor_id: DefId) -> Body<'_> {
 
     let source_info = SourceInfo::outermost(span);
 
-    let variant_index = if adt_def.is_enum() {
-        adt_def.variant_index_with_ctor_id(ctor_id)
-    } else {
-        VariantIdx::new(0)
-    };
+    let variant_index =
+        if adt_def.is_enum() { adt_def.variant_index_with_ctor_id(ctor_id) } else { FIRST_VARIANT };
 
     // Generate the following MIR:
     //