bjoernager/rust
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Add more comments to discriminant calculations.

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This commit is contained in:
Scott McMurray 2025-03-13 00:39:18 -07:00
parent 143f39362a
commit 2b15dd1ddd
1 changed files with 9 additions and 0 deletions
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9
compiler/rustc_codegen_ssa/src/mir/operand.rs
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@ -428,9 +428,14 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
let dl = &bx.tcx().data_layout; let dl = &bx.tcx().data_layout;
let cast_to_layout = bx.cx().layout_of(cast_to); let cast_to_layout = bx.cx().layout_of(cast_to);
let cast_to = bx.cx().immediate_backend_type(cast_to_layout); let cast_to = bx.cx().immediate_backend_type(cast_to_layout);
// We check uninhabitedness separately because a type like
// `enum Foo { Bar(i32, !) }` is still reported as `Variants::Single`,
// *not* as `Variants::Empty`.
if self.layout.is_uninhabited() { if self.layout.is_uninhabited() {
return bx.cx().const_poison(cast_to); return bx.cx().const_poison(cast_to);
} }
let (tag_scalar, tag_encoding, tag_field) = match self.layout.variants { let (tag_scalar, tag_encoding, tag_field) = match self.layout.variants {
Variants::Empty => unreachable!("we already handled uninhabited types"), Variants::Empty => unreachable!("we already handled uninhabited types"),
Variants::Single { index } => { Variants::Single { index } => {
@ -438,7 +443,11 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
if let Some(discr) = self.layout.ty.discriminant_for_variant(bx.tcx(), index) { if let Some(discr) = self.layout.ty.discriminant_for_variant(bx.tcx(), index) {
discr.val discr.val
} else { } else {
// This arm is for types which are neither enums nor coroutines,
// and thus for which the only possible "variant" should be the first one.
assert_eq!(index, FIRST_VARIANT); assert_eq!(index, FIRST_VARIANT);
// There's thus no actual discriminant to return, so we return
// what it would have been if this was a single-variant enum.
0 0
}; };
return bx.cx().const_uint_big(cast_to, discr_val); return bx.cx().const_uint_big(cast_to, discr_val);