Auto merge of #50309 - samWson:issue-49938, r=eddyb

Issue 49938: Reference tagged unions discr(iminant) as tag

Here the changes reference the Tagged type _discriminant_ as _tag_ instead. This is the correct terminology when referencing how tagged unions are represented in memory.

src/librustc/ty/layout.rs

@@ -1057,7 +1057,7 @@ impl<'a, 'tcx> LayoutCx<'tcx, TyCtxt<'a, 'tcx, 'tcx>> {
                 }
                 tcx.intern_layout(LayoutDetails {
                     variants: Variants::Tagged {
-                        discr: tag,
+                        tag,
                         variants: layout_variants,
                     },
                     fields: FieldPlacement::Arbitrary {
@@ -1218,7 +1218,7 @@ impl<'a, 'tcx> LayoutCx<'tcx, TyCtxt<'a, 'tcx, 'tcx>> {
                     })
                     .collect();
                 record(adt_kind.into(), adt_packed, match layout.variants {
-                    Variants::Tagged { ref discr, .. } => Some(discr.value.size(self)),
+                    Variants::Tagged { ref tag, .. } => Some(tag.value.size(self)),
                     _ => None
                 }, variant_infos);
             }
@@ -1622,7 +1622,7 @@ impl<'a, 'tcx, C> TyLayoutMethods<'tcx, C> for Ty<'tcx>
                     }
 
                     // Discriminant field for enums (where applicable).
-                    Variants::Tagged { ref discr, .. } |
+                    Variants::Tagged { tag: ref discr, .. } |
                     Variants::NicheFilling { niche: ref discr, .. } => {
                         assert_eq!(i, 0);
                         let layout = LayoutDetails::scalar(tcx, discr.clone());
@@ -1736,10 +1736,10 @@ impl<'a> HashStable<StableHashingContext<'a>> for Variants {
                 index.hash_stable(hcx, hasher);
             }
             Tagged {
-                ref discr,
+                ref tag,
                 ref variants,
             } => {
-                discr.hash_stable(hcx, hasher);
+                tag.hash_stable(hcx, hasher);
                 variants.hash_stable(hcx, hasher);
             }
             NicheFilling {

src/librustc_lint/types.rs

@@ -820,8 +820,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for VariantSizeDifferences {
                     bug!("failed to get layout for `{}`: {}", t, e)
                 });
 
-                if let layout::Variants::Tagged { ref variants, ref discr, .. } = layout.variants {
-                    let discr_size = discr.value.size(cx.tcx).bytes();
+                if let layout::Variants::Tagged { ref variants, ref tag, .. } = layout.variants {
+                    let discr_size = tag.value.size(cx.tcx).bytes();
 
                     debug!("enum `{}` is {} bytes large with layout:\n{:#?}",
                       t, layout.size.bytes(), layout);

src/librustc_mir/interpret/eval_context.rs

@@ -960,20 +960,20 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
                                layout::Abi::Uninhabited);
                 }
             }
-            layout::Variants::Tagged { ref discr, .. } => {
+            layout::Variants::Tagged { ref tag, .. } => {
                 let discr_val = dest_ty.ty_adt_def().unwrap()
                     .discriminant_for_variant(*self.tcx, variant_index)
                     .val;
 
                 // raw discriminants for enums are isize or bigger during
                 // their computation, but the in-memory tag is the smallest possible
                 // representation
-                let size = discr.value.size(self.tcx.tcx).bits();
+                let size = tag.value.size(self.tcx.tcx).bits();
                 let amt = 128 - size;
                 let discr_val = (discr_val << amt) >> amt;
 
-                let (discr_dest, discr) = self.place_field(dest, mir::Field::new(0), layout)?;
-                self.write_primval(discr_dest, PrimVal::Bytes(discr_val), discr.ty)?;
+                let (discr_dest, tag) = self.place_field(dest, mir::Field::new(0), layout)?;
+                self.write_primval(discr_dest, PrimVal::Bytes(discr_val), tag.ty)?;
             }
             layout::Variants::NicheFilling {
                 dataful_variant,

src/librustc_target/abi/mod.rs

@@ -716,10 +716,10 @@ pub enum Variants {
     },
 
     /// General-case enums: for each case there is a struct, and they all have
-    /// all space reserved for the discriminant, and their first field starts
-    /// at a non-0 offset, after where the discriminant would go.
+    /// all space reserved for the tag, and their first field starts
+    /// at a non-0 offset, after where the tag would go.
     Tagged {
-        discr: Scalar,
+        tag: Scalar,
         variants: Vec<LayoutDetails>,
     },
 

src/librustc_trans/debuginfo/metadata.rs

@@ -1429,8 +1429,8 @@ fn prepare_enum_metadata<'a, 'tcx>(cx: &CodegenCx<'a, 'tcx>,
     let discriminant_type_metadata = match layout.variants {
         layout::Variants::Single { .. } |
         layout::Variants::NicheFilling { .. } => None,
-        layout::Variants::Tagged { ref discr, .. } => {
-            Some(discriminant_type_metadata(discr.value))
+        layout::Variants::Tagged { ref tag, .. } => {
+            Some(discriminant_type_metadata(tag.value))
         }
     };
 

src/librustc_trans/mir/place.rs

@@ -273,8 +273,8 @@ impl<'a, 'tcx> PlaceRef<'tcx> {
         let lldiscr = discr.load(bx).immediate();
         match self.layout.variants {
             layout::Variants::Single { .. } => bug!(),
-            layout::Variants::Tagged { ref discr, .. } => {
-                let signed = match discr.value {
+            layout::Variants::Tagged { ref tag, .. } => {
+                let signed = match tag.value {
                     layout::Int(_, signed) => signed,
                     _ => false
                 };