extend some comments and clarify some names around enum tag type computation

Author: RalfJung

compiler/rustc_abi/src/layout.rs

@@ -812,7 +812,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         let (max, min) = largest_niche
             // We might have no inhabited variants, so pretend there's at least one.
             .unwrap_or((0, 0));
-        let (min_ity, signed) = discr_range_of_repr(min, max); //Integer::repr_discr(tcx, ty, &repr, min, max);
+        let (min_ity, signed) = discr_range_of_repr(min, max); //Integer::discr_range_of_repr(tcx, ty, &repr, min, max);
 
         let mut align = dl.aggregate_align;
         let mut max_repr_align = repr.align;

compiler/rustc_abi/src/lib.rs

@@ -183,6 +183,11 @@ impl ReprOptions {
 
     /// Returns the discriminant type, given these `repr` options.
     /// This must only be called on enums!
+    ///
+    /// This is the "typeck type" of the discriminant, which is effectively the maximum size:
+    /// discriminant values will be wrapped to fit (with a lint). Layout can later decide to use a
+    /// smaller type for the tag that stores the discriminant at runtime and that will work just
+    /// fine, it just induces casts when getting/setting the discriminant.
     pub fn discr_type(&self) -> IntegerType {
         self.int.unwrap_or(IntegerType::Pointer(true))
     }

compiler/rustc_middle/src/ty/layout.rs

@@ -72,7 +72,10 @@ impl abi::Integer {
     /// signed discriminant range and `#[repr]` attribute.
     /// N.B.: `u128` values above `i128::MAX` will be treated as signed, but
     /// that shouldn't affect anything, other than maybe debuginfo.
-    fn repr_discr<'tcx>(
+    ///
+    /// This is the basis for computing the type of the *tag* of an enum (which can be smaller than
+    /// the type of the *discriminant*, which is determined by [`ReprOptions::discr_type`]).
+    fn discr_range_of_repr<'tcx>(
         tcx: TyCtxt<'tcx>,
         ty: Ty<'tcx>,
         repr: &ReprOptions,

compiler/rustc_ty_utils/src/layout.rs

@@ -639,8 +639,8 @@ fn layout_of_uncached<'tcx>(
             // UnsafeCell and UnsafePinned both disable niche optimizations
             let is_special_no_niche = def.is_unsafe_cell() || def.is_unsafe_pinned();
 
-            let get_discriminant_type =
-                |min, max| abi::Integer::repr_discr(tcx, ty, &def.repr(), min, max);
+            let discr_range_of_repr =
+                |min, max| abi::Integer::discr_range_of_repr(tcx, ty, &def.repr(), min, max);
 
             let discriminants_iter = || {
                 def.is_enum()
@@ -663,7 +663,7 @@ fn layout_of_uncached<'tcx>(
                     def.is_enum(),
                     is_special_no_niche,
                     tcx.layout_scalar_valid_range(def.did()),
-                    get_discriminant_type,
+                    discr_range_of_repr,
                     discriminants_iter(),
                     !maybe_unsized,
                 )
@@ -688,7 +688,7 @@ fn layout_of_uncached<'tcx>(
                     def.is_enum(),
                     is_special_no_niche,
                     tcx.layout_scalar_valid_range(def.did()),
-                    get_discriminant_type,
+                    discr_range_of_repr,
                     discriminants_iter(),
                     !maybe_unsized,
                 ) else {