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.

src/base.rs

@@ -785,7 +785,7 @@ fn codegen_stmt<'tcx>(
                             let variant_dest = lval.downcast_variant(fx, variant_index);
                             (variant_index, variant_dest, active_field_index)
                         }
-                        _ => (VariantIdx::from_u32(0), lval, None),
+                        _ => (FIRST_VARIANT, lval, None),
                     };
                     if active_field_index.is_some() {
                         assert_eq!(operands.len(), 1);

src/lib.rs

@@ -86,7 +86,7 @@ mod prelude {
         self, FloatTy, Instance, InstanceDef, IntTy, ParamEnv, Ty, TyCtxt, TypeAndMut,
         TypeFoldable, TypeVisitableExt, UintTy,
     };
-    pub(crate) use rustc_target::abi::{Abi, Scalar, Size, VariantIdx};
+    pub(crate) use rustc_target::abi::{Abi, Scalar, Size, VariantIdx, FIRST_VARIANT};
 
     pub(crate) use rustc_data_structures::fx::FxHashMap;
 

src/unsize.rs

@@ -146,7 +146,7 @@ pub(crate) fn coerce_unsized_into<'tcx>(
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
             assert_eq!(def_a, def_b);
 
-            for i in 0..def_a.variant(VariantIdx::new(0)).fields.len() {
+            for i in 0..def_a.variant(FIRST_VARIANT).fields.len() {
                 let src_f = src.value_field(fx, mir::Field::new(i));
                 let dst_f = dst.place_field(fx, mir::Field::new(i));