Handle tags better.

Currently, for the enums and comparison traits we always check the tag
for equality before doing anything else. This is a bit clumsy. This
commit changes things so that the tags are handled very much like a
zeroth field in the enum.

For `eq`/ne` this makes the code slightly cleaner.

For `partial_cmp` and `cmp` it's a more notable change: in the case
where the tags aren't equal, instead of having a tag equality check
followed by a tag comparison, it just does a single tag comparison.

The commit also improves how `Hash` works for enums: instead of having
duplicated code to hash the tag for every arm within the match, we do
it just once before the match.

All this required replacing the `EnumNonMatchingCollapsed` value with a
new `EnumTag` value.

For fieldless enums the new code is particularly improved. All the code
now produced is close to optimal, being very similar to what you'd write
by hand.

Author: nnethercote

Diff for: compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -148,7 +148,7 @@ fn cs_clone_simple(
             ),
         }
     }
-    BlockOrExpr::new_mixed(stmts, cx.expr_deref(trait_span, cx.expr_self(trait_span)))
+    BlockOrExpr::new_mixed(stmts, Some(cx.expr_deref(trait_span, cx.expr_self(trait_span))))
 }
 
 fn cs_clone(
@@ -177,9 +177,7 @@ fn cs_clone(
             all_fields = af;
             vdata = &variant.data;
         }
-        EnumNonMatchingCollapsed(..) => {
-            cx.span_bug(trait_span, &format!("non-matching enum variants in `derive({})`", name,))
-        }
+        EnumTag(..) => cx.span_bug(trait_span, &format!("enum tags in `derive({})`", name,)),
         StaticEnum(..) | StaticStruct(..) => {
             cx.span_bug(trait_span, &format!("associated function in `derive({})`", name))
         }

Diff for: compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs

@@ -73,16 +73,6 @@ pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> Bl
                 cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
             }
             CsFold::Fieldless => cx.expr_path(equal_path.clone()),
-            CsFold::EnumNonMatching(span, tag_tuple) => {
-                if tag_tuple.len() != 2 {
-                    cx.span_bug(span, "not exactly 2 arguments in `derive(Ord)`")
-                } else {
-                    let lft = cx.expr_addr_of(span, cx.expr_ident(span, tag_tuple[0]));
-                    let rgt = cx.expr_addr_of(span, cx.expr_ident(span, tag_tuple[1]));
-                    let fn_cmp_path = cx.std_path(&[sym::cmp, sym::Ord, sym::cmp]);
-                    cx.expr_call_global(span, fn_cmp_path, vec![lft, rgt])
-                }
-            }
         },
     );
     BlockOrExpr::new_expr(expr)

Diff for: compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs

@@ -51,7 +51,6 @@ pub fn expand_deriving_partial_eq(
                 }
                 CsFold::Combine(span, expr1, expr2) => cx.expr_binary(span, combiner, expr1, expr2),
                 CsFold::Fieldless => cx.expr_bool(span, base),
-                CsFold::EnumNonMatching(span, _tag_tuple) => cx.expr_bool(span, !base),
             },
         );
         BlockOrExpr::new_expr(expr)

Diff for: compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

@@ -82,17 +82,6 @@ pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_
                 cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
             }
             CsFold::Fieldless => cx.expr_some(span, cx.expr_path(equal_path.clone())),
-            CsFold::EnumNonMatching(span, tag_tuple) => {
-                if tag_tuple.len() != 2 {
-                    cx.span_bug(span, "not exactly 2 arguments in `derive(PartialOrd)`")
-                } else {
-                    let lft = cx.expr_addr_of(span, cx.expr_ident(span, tag_tuple[0]));
-                    let rgt = cx.expr_addr_of(span, cx.expr_ident(span, tag_tuple[1]));
-                    let fn_partial_cmp_path =
-                        cx.std_path(&[sym::cmp, sym::PartialOrd, sym::partial_cmp]);
-                    cx.expr_call_global(span, fn_partial_cmp_path, vec![lft, rgt])
-                }
-            }
         },
     );
     BlockOrExpr::new_expr(expr)

Diff for: compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -45,7 +45,7 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
     let (ident, vdata, fields) = match substr.fields {
         Struct(vdata, fields) => (substr.type_ident, *vdata, fields),
         EnumMatching(_, _, v, fields) => (v.ident, &v.data, fields),
-        EnumNonMatchingCollapsed(..) | StaticStruct(..) | StaticEnum(..) => {
+        EnumTag(..) | StaticStruct(..) | StaticEnum(..) => {
             cx.span_bug(span, "nonsensical .fields in `#[derive(Debug)]`")
         }
     };
@@ -176,6 +176,6 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
             stmts.push(names_let.unwrap());
         }
         stmts.push(values_let);
-        BlockOrExpr::new_mixed(stmts, expr)
+        BlockOrExpr::new_mixed(stmts, Some(expr))
     }
 }

Diff for: compiler/rustc_builtin_macros/src/deriving/encodable.rs

@@ -287,7 +287,7 @@ fn encodable_substructure(
                 fn_emit_enum_path,
                 vec![encoder, cx.expr_str(trait_span, substr.type_ident.name), blk],
             );
-            BlockOrExpr::new_mixed(vec![me], expr)
+            BlockOrExpr::new_mixed(vec![me], Some(expr))
         }
 
         _ => cx.bug("expected Struct or EnumMatching in derive(Encodable)"),