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.

9 files changed, 245 insertions, 329 deletions

diff --git a/compiler/rustc_builtin_macros/src/deriving/clone.rs b/compiler/rustc_builtin_macros/src/deriving/clone.rs
index 72aa8e17d01..7755ff779c4 100644
--- a/compiler/rustc_builtin_macros/src/deriving/clone.rs
+++ b/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 --git a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
index aad0ab3f5bb..1612be86237 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
+++ b/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 --git a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
index 83534b62b46..0141b337726 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
+++ b/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 --git a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
index ce45b0e1965..2ebb01cc8a0 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
+++ b/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 --git a/compiler/rustc_builtin_macros/src/deriving/debug.rs b/compiler/rustc_builtin_macros/src/deriving/debug.rs
index 5de20520486..ceef893e862 100644
--- a/compiler/rustc_builtin_macros/src/deriving/debug.rs
+++ b/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 --git a/compiler/rustc_builtin_macros/src/deriving/encodable.rs b/compiler/rustc_builtin_macros/src/deriving/encodable.rs
index 49dbe51f762..70167cac68a 100644
--- a/compiler/rustc_builtin_macros/src/deriving/encodable.rs
+++ b/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)"),
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index c953cc36534..7ff75592a52 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -21,21 +21,14 @@
 //!   `struct T(i32, char)`).
 //! - `EnumMatching`, when `Self` is an enum and all the arguments are the
 //!   same variant of the enum (e.g., `Some(1)`, `Some(3)` and `Some(4)`)
-//! - `EnumNonMatchingCollapsed` when `Self` is an enum and the arguments
-//!   are not the same variant (e.g., `None`, `Some(1)` and `None`).
+//! - `EnumTag` when `Self` is an enum, for comparing the enum tags.
 //! - `StaticEnum` and `StaticStruct` for static methods, where the type
 //!   being derived upon is either an enum or struct respectively. (Any
 //!   argument with type Self is just grouped among the non-self
 //!   arguments.)
 //!
 //! In the first two cases, the values from the corresponding fields in
-//! all the arguments are grouped together. For `EnumNonMatchingCollapsed`
-//! this isn't possible (different variants have different fields), so the
-//! fields are inaccessible. (Previous versions of the deriving infrastructure
-//! had a way to expand into code that could access them, at the cost of
-//! generating exponential amounts of code; see issue #15375). There are no
-//! fields with values in the static cases, so these are treated entirely
-//! differently.
+//! all the arguments are grouped together.
 //!
 //! The non-static cases have `Option<ident>` in several places associated
 //! with field `expr`s. This represents the name of the field it is
@@ -142,21 +135,15 @@
 //!                }])
 //! ```
 //!
-//! For `C0(a)` and `C1 {x}` ,
+//! For the tags,
 //!
 //! ```{.text}
-//! EnumNonMatchingCollapsed(
-//!     &[<ident for self index value>, <ident of __arg1 index value>])
+//! EnumTag(
+//!     &[<ident of self tag>, <ident of other tag>], <expr to combine with>)
 //! ```
-//!
-//! It is the same for when the arguments are flipped to `C1 {x}` and
-//! `C0(a)`; the only difference is what the values of the identifiers
-//! <ident for self index value> and <ident of __arg1 index value> will
-//! be in the generated code.
-//!
-//! `EnumNonMatchingCollapsed` deliberately provides far less information
-//! than is generally available for a given pair of variants; see #15375
-//! for discussion.
+//! Note that this setup doesn't allow for the brute-force "match every variant
+//! against every other variant" approach, which is bad because it produces a
+//! quadratic amount of code (see #15375).
 //!
 //! ## Static
 //!
@@ -180,7 +167,7 @@ use std::iter;
 use std::vec;
 
 use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, BinOpKind, EnumDef, Expr, Generics, PatKind};
+use rustc_ast::{self as ast, EnumDef, Expr, Generics, PatKind};
 use rustc_ast::{GenericArg, GenericParamKind, VariantData};
 use rustc_attr as attr;
 use rustc_expand::base::{Annotatable, ExtCtxt};
@@ -235,6 +222,8 @@ pub struct MethodDef<'a> {
     pub attributes: Vec<ast::Attribute>,
 
     /// Can we combine fieldless variants for enums into a single match arm?
+    /// If true, indicates that the trait operation uses the enum tag in some
+    /// way.
     pub unify_fieldless_variants: bool,
 
     pub combine_substructure: RefCell<CombineSubstructureFunc<'a>>,
@@ -274,19 +263,22 @@ pub enum StaticFields {
 
 /// A summary of the possible sets of fields.
 pub enum SubstructureFields<'a> {
+    /// A non-static method with `Self` is a struct.
     Struct(&'a ast::VariantData, Vec<FieldInfo>),
+
     /// Matching variants of the enum: variant index, variant count, ast::Variant,
     /// fields: the field name is only non-`None` in the case of a struct
     /// variant.
     EnumMatching(usize, usize, &'a ast::Variant, Vec<FieldInfo>),
 
-    /// Non-matching variants of the enum, but with all state hidden from the
-    /// consequent code. The field is a list of `Ident`s bound to the variant
-    /// index values for each of the actual input `Self` arguments.
-    EnumNonMatchingCollapsed(&'a [Ident]),
+    /// The tag of an enum. The first field is a `FieldInfo` for the tags, as
+    /// if they were fields. The second field is the expression to combine the
+    /// tag expression with; it will be `None` if no match is necessary.
+    EnumTag(FieldInfo, Option<P<Expr>>),
 
     /// A static method where `Self` is a struct.
     StaticStruct(&'a ast::VariantData, StaticFields),
+
     /// A static method where `Self` is an enum.
     StaticEnum(&'a ast::EnumDef, Vec<(Ident, Span, StaticFields)>),
 }
@@ -324,8 +316,8 @@ impl BlockOrExpr {
         BlockOrExpr(vec![], Some(expr))
     }
 
-    pub fn new_mixed(stmts: Vec<ast::Stmt>, expr: P<Expr>) -> BlockOrExpr {
-        BlockOrExpr(stmts, Some(expr))
+    pub fn new_mixed(stmts: Vec<ast::Stmt>, expr: Option<P<Expr>>) -> BlockOrExpr {
+        BlockOrExpr(stmts, expr)
     }
 
     // Converts it into a block.
@@ -339,7 +331,6 @@ impl BlockOrExpr {
     // Converts it into an expression.
     fn into_expr(self, cx: &ExtCtxt<'_>, span: Span) -> P<Expr> {
         if self.0.is_empty() {
-            // No statements.
             match self.1 {
                 None => cx.expr_block(cx.block(span, vec![])),
                 Some(expr) => expr,
@@ -1135,44 +1126,34 @@ impl<'a> MethodDef<'a> {
     ///     fn eq(&self, other: &A) -> bool {
     ///         let __self_tag = ::core::intrinsics::discriminant_value(self);
     ///         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-    ///         if __self_tag == __arg1_tag {
+    ///         __self_tag == __arg1_tag &&
     ///             match (self, other) {
     ///                 (A::A2(__self_0), A::A2(__arg1_0)) =>
     ///                     *__self_0 == *__arg1_0,
     ///                 _ => true,
     ///             }
-    ///         } else {
-    ///             false // catch-all handler
-    ///         }
     ///     }
     /// }
     /// ```
-    /// Creates a match for a tuple of all `selflike_args`, where either all
-    /// variants match, or it falls into a catch-all for when one variant
-    /// does not match.
-    ///
-    /// There are N + 1 cases because is a case for each of the N
-    /// variants where all of the variants match, and one catch-all for
-    /// when one does not match.
-    ///
-    /// As an optimization we generate code which checks whether all variants
-    /// match first which makes llvm see that C-like enums can be compiled into
-    /// a simple equality check (for PartialEq).
-    ///
-    /// The catch-all handler is provided access the variant index values
-    /// for each of the selflike_args, carried in precomputed variables.
+    /// Creates a tag check combined with a match for a tuple of all
+    /// `selflike_args`, with an arm for each variant with fields, possibly an
+    /// arm for each fieldless variant (if `!unify_fieldless_variants` is not
+    /// true), and possibly a default arm.
     fn expand_enum_method_body<'b>(
         &self,
         cx: &mut ExtCtxt<'_>,
         trait_: &TraitDef<'b>,
         enum_def: &'b EnumDef,
         type_ident: Ident,
-        mut selflike_args: Vec<P<Expr>>,
+        selflike_args: Vec<P<Expr>>,
         nonselflike_args: &[P<Expr>],
     ) -> BlockOrExpr {
         let span = trait_.span;
         let variants = &enum_def.variants;
 
+        // Traits that unify fieldless variants always use the tag(s).
+        let uses_tags = self.unify_fieldless_variants;
+
         // There is no sensible code to be generated for *any* deriving on a
         // zero-variant enum. So we just generate a failing expression.
         if variants.is_empty() {
@@ -1189,27 +1170,82 @@ impl<'a> MethodDef<'a> {
             )
             .collect::<Vec<String>>();
 
-        // The `tag_idents` will be bound, solely in the catch-all, to
-        // a series of let statements mapping each selflike_arg to an int
-        // value corresponding to its discriminant.
-        let tag_idents = prefixes
-            .iter()
-            .map(|name| Ident::from_str_and_span(&format!("{}_tag", name), span))
-            .collect::<Vec<Ident>>();
+        // Build a series of let statements mapping each selflike_arg
+        // to its discriminant value.
+        //
+        // e.g. for `PartialEq::eq` builds two statements:
+        // ```
+        // let __self_tag = ::core::intrinsics::discriminant_value(self);
+        // let __arg1_tag = ::core::intrinsics::discriminant_value(other);
+        // ```
+        let get_tag_pieces = |cx: &ExtCtxt<'_>| {
+            let tag_idents: Vec<_> = prefixes
+                .iter()
+                .map(|name| Ident::from_str_and_span(&format!("{}_tag", name), span))
+                .collect();
 
-        // Builds, via callback to call_substructure_method, the
-        // delegated expression that handles the catch-all case,
-        // using `__variants_tuple` to drive logic if necessary.
-        let catch_all_substructure = EnumNonMatchingCollapsed(&tag_idents);
+            let mut tag_exprs: Vec<_> = tag_idents
+                .iter()
+                .map(|&ident| cx.expr_addr_of(span, cx.expr_ident(span, ident)))
+                .collect();
 
-        let first_fieldless = variants.iter().find(|v| v.data.fields().is_empty());
+            let self_expr = tag_exprs.remove(0);
+            let other_selflike_exprs = tag_exprs;
+            let tag_field = FieldInfo { span, name: None, self_expr, other_selflike_exprs };
+
+            let tag_let_stmts: Vec<_> = iter::zip(&tag_idents, &selflike_args)
+                .map(|(&ident, selflike_arg)| {
+                    let variant_value = deriving::call_intrinsic(
+                        cx,
+                        span,
+                        sym::discriminant_value,
+                        vec![selflike_arg.clone()],
+                    );
+                    cx.stmt_let(span, false, ident, variant_value)
+                })
+                .collect();
+
+            (tag_field, tag_let_stmts)
+        };
+
+        // There are some special cases involving fieldless enums where no
+        // match is necessary.
+        let all_fieldless = variants.iter().all(|v| v.data.fields().is_empty());
+        if all_fieldless {
+            if uses_tags && variants.len() > 1 {
+                // If the type is fieldless and the trait uses the tag and
+                // there are multiple variants, we need just an operation on
+                // the tag(s).
+                let (tag_field, mut tag_let_stmts) = get_tag_pieces(cx);
+                let mut tag_check = self.call_substructure_method(
+                    cx,
+                    trait_,
+                    type_ident,
+                    nonselflike_args,
+                    &EnumTag(tag_field, None),
+                );
+                tag_let_stmts.append(&mut tag_check.0);
+                return BlockOrExpr(tag_let_stmts, tag_check.1);
+            }
+
+            if variants.len() == 1 {
+                // If there is a single variant, we don't need an operation on
+                // the tag(s). Just use the most degenerate result.
+                return self.call_substructure_method(
+                    cx,
+                    trait_,
+                    type_ident,
+                    nonselflike_args,
+                    &EnumMatching(0, 1, &variants[0], Vec::new()),
+                );
+            };
+        }
 
         // These arms are of the form:
         // (Variant1, Variant1, ...) => Body1
         // (Variant2, Variant2, ...) => Body2
         // ...
         // where each tuple has length = selflike_args.len()
-
         let mut match_arms: Vec<ast::Arm> = variants
             .iter()
             .enumerate()
@@ -1233,7 +1269,7 @@ impl<'a> MethodDef<'a> {
                     use_ref_pat,
                 );
 
-                // Here is the pat = `(&VariantK, &VariantK, ...)`
+                // `(VariantK, VariantK, ...)` or just `VariantK`.
                 let single_pat = if subpats.len() == 1 {
                     subpats.pop().unwrap()
                 } else {
@@ -1263,27 +1299,28 @@ impl<'a> MethodDef<'a> {
             })
             .collect();
 
+        // Add a default arm to the match, if necessary.
+        let first_fieldless = variants.iter().find(|v| v.data.fields().is_empty());
         let default = match first_fieldless {
             Some(v) if self.unify_fieldless_variants => {
-                // We need a default case that handles the fieldless variants.
-                // The index and actual variant aren't meaningful in this case,
-                // so just use whatever
-                let substructure = EnumMatching(0, variants.len(), v, Vec::new());
+                // We need a default case that handles all the fieldless
+                // variants. The index and actual variant aren't meaningful in
+                // this case, so just use dummy values.
                 Some(
                     self.call_substructure_method(
                         cx,
                         trait_,
                         type_ident,
                         nonselflike_args,
-                        &substructure,
+                        &EnumMatching(0, variants.len(), v, Vec::new()),
                     )
                     .into_expr(cx, span),
                 )
             }
             _ if variants.len() > 1 && selflike_args.len() > 1 => {
-                // Since we know that all the arguments will match if we reach
+                // Because we know that all the arguments will match if we reach
                 // the match expression we add the unreachable intrinsics as the
-                // result of the catch all which should help llvm in optimizing it
+                // result of the default which should help llvm in optimizing it.
                 Some(deriving::call_unreachable(cx, span))
             }
             _ => None,
@@ -1292,92 +1329,41 @@ impl<'a> MethodDef<'a> {
             match_arms.push(cx.arm(span, cx.pat_wild(span), arm));
         }
 
-        // We will usually need the catch-all after matching the
-        // tuples `(VariantK, VariantK, ...)` for each VariantK of the
-        // enum.  But:
-        //
-        // * when there is only one Self arg, the arms above suffice
-        // (and the deriving we call back into may not be prepared to
-        // handle EnumNonMatchCollapsed), and,
-        //
-        // * when the enum has only one variant, the single arm that
-        // is already present always suffices.
-        //
-        // * In either of the two cases above, if we *did* add a
-        //   catch-all `_` match, it would trigger the
-        //   unreachable-pattern error.
-        //
-        if variants.len() > 1 && selflike_args.len() > 1 {
-            // Build a series of let statements mapping each selflike_arg
-            // to its discriminant value.
-            //
-            // i.e., for `enum E<T> { A, B(1), C(T, T) }` for `PartialEq::eq`,
-            // builds two statements:
-            // ```
-            // let __self_tag = ::core::intrinsics::discriminant_value(self);
-            // let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-            // ```
-            let mut index_let_stmts: Vec<ast::Stmt> = Vec::with_capacity(tag_idents.len() + 1);
-
-            // We also build an expression which checks whether all discriminants are equal, e.g.
-            // `__self_tag == __arg1_tag`.
-            let mut discriminant_test = cx.expr_bool(span, true);
-            for (i, (&ident, selflike_arg)) in iter::zip(&tag_idents, &selflike_args).enumerate() {
-                let variant_value = deriving::call_intrinsic(
-                    cx,
-                    span,
-                    sym::discriminant_value,
-                    vec![selflike_arg.clone()],
-                );
-                let let_stmt = cx.stmt_let(span, false, ident, variant_value);
-                index_let_stmts.push(let_stmt);
-
-                if i > 0 {
-                    let id0 = cx.expr_ident(span, tag_idents[0]);
-                    let id = cx.expr_ident(span, ident);
-                    let test = cx.expr_binary(span, BinOpKind::Eq, id0, id);
-                    discriminant_test = if i == 1 {
-                        test
-                    } else {
-                        cx.expr_binary(span, BinOpKind::And, discriminant_test, test)
-                    };
-                }
-            }
-
-            let arm_expr = self
-                .call_substructure_method(
-                    cx,
-                    trait_,
-                    type_ident,
-                    nonselflike_args,
-                    &catch_all_substructure,
-                )
-                .into_expr(cx, span);
-
-            let match_arg = cx.expr(span, ast::ExprKind::Tup(selflike_args));
-
-            // Lastly we create an expression which branches on all discriminants being equal, e.g.
-            //  if __self_tag == _arg1_tag {
-            //      match (self, other) {
-            //          (Variant1, Variant1, ...) => Body1
-            //          (Variant2, Variant2, ...) => Body2,
-            //          ...
-            //          _ => ::core::intrinsics::unreachable()
-            //      }
-            //  }
-            //  else {
-            //      <delegated expression referring to __self_tag, et al.>
-            //  }
-            let all_match = cx.expr_match(span, match_arg, match_arms);
-            let arm_expr = cx.expr_if(span, discriminant_test, all_match, Some(arm_expr));
-            BlockOrExpr(index_let_stmts, Some(arm_expr))
-        } else {
+        // Create a match expression with one arm per discriminant plus
+        // possibly a default arm, e.g.:
+        //      match (self, other) {
+        //          (Variant1, Variant1, ...) => Body1
+        //          (Variant2, Variant2, ...) => Body2,
+        //          ...
+        //          _ => ::core::intrinsics::unreachable()
+        //      }
+        let get_match_expr = |mut selflike_args: Vec<P<Expr>>| {
             let match_arg = if selflike_args.len() == 1 {
                 selflike_args.pop().unwrap()
             } else {
                 cx.expr(span, ast::ExprKind::Tup(selflike_args))
             };
-            BlockOrExpr(vec![], Some(cx.expr_match(span, match_arg, match_arms)))
+            cx.expr_match(span, match_arg, match_arms)
+        };
+
+        // If the trait uses the tag and there are multiple variants, we need
+        // to add a tag check operation before the match. Otherwise, the match
+        // is enough.
+        if uses_tags && variants.len() > 1 {
+            let (tag_field, mut tag_let_stmts) = get_tag_pieces(cx);
+
+            // Combine a tag check with the match.
+            let mut tag_check_plus_match = self.call_substructure_method(
+                cx,
+                trait_,
+                type_ident,
+                nonselflike_args,
+                &EnumTag(tag_field, Some(get_match_expr(selflike_args))),
+            );
+            tag_let_stmts.append(&mut tag_check_plus_match.0);
+            BlockOrExpr(tag_let_stmts, tag_check_plus_match.1)
+        } else {
+            BlockOrExpr(vec![], Some(get_match_expr(selflike_args)))
         }
     }
 
@@ -1591,11 +1577,6 @@ pub enum CsFold<'a> {
 
     // The fallback case for a struct or enum variant with no fields.
     Fieldless,
-
-    /// The fallback case for non-matching enum variants. The slice is the
-    /// identifiers holding the variant index value for each of the `Self`
-    /// arguments.
-    EnumNonMatching(Span, &'a [Ident]),
 }
 
 /// Folds over fields, combining the expressions for each field in a sequence.
@@ -1610,8 +1591,8 @@ pub fn cs_fold<F>(
 where
     F: FnMut(&mut ExtCtxt<'_>, CsFold<'_>) -> P<Expr>,
 {
-    match *substructure.fields {
-        EnumMatching(.., ref all_fields) | Struct(_, ref all_fields) => {
+    match substructure.fields {
+        EnumMatching(.., all_fields) | Struct(_, all_fields) => {
             if all_fields.is_empty() {
                 return f(cx, CsFold::Fieldless);
             }
@@ -1635,7 +1616,18 @@ where
                 rest.iter().rfold(base_expr, op)
             }
         }
-        EnumNonMatchingCollapsed(tuple) => f(cx, CsFold::EnumNonMatching(trait_span, tuple)),
+        EnumTag(tag_field, match_expr) => {
+            let tag_check_expr = f(cx, CsFold::Single(tag_field));
+            if let Some(match_expr) = match_expr {
+                if use_foldl {
+                    f(cx, CsFold::Combine(trait_span, tag_check_expr, match_expr.clone()))
+                } else {
+                    f(cx, CsFold::Combine(trait_span, match_expr.clone(), tag_check_expr))
+                }
+            } else {
+                tag_check_expr
+            }
+        }
         StaticEnum(..) | StaticStruct(..) => cx.span_bug(trait_span, "static function in `derive`"),
     }
 }
diff --git a/compiler/rustc_builtin_macros/src/deriving/hash.rs b/compiler/rustc_builtin_macros/src/deriving/hash.rs
index 52239eff5da..32ae3d34478 100644
--- a/compiler/rustc_builtin_macros/src/deriving/hash.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/hash.rs
@@ -1,6 +1,6 @@
 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
-use crate::deriving::{self, path_std, pathvec_std};
+use crate::deriving::{path_std, pathvec_std};
 
 use rustc_ast::{MetaItem, Mutability};
 use rustc_expand::base::{Annotatable, ExtCtxt};
@@ -61,32 +61,20 @@ fn hash_substructure(
         let expr = cx.expr_call(span, hash_path, vec![expr, state_expr.clone()]);
         cx.stmt_expr(expr)
     };
-    let mut stmts = Vec::new();
 
-    let fields = match substr.fields {
-        Struct(_, fs) | EnumMatching(_, 1, .., fs) => fs,
-        EnumMatching(.., fs) => {
-            let variant_value = cx.expr_addr_of(
-                trait_span,
-                deriving::call_intrinsic(
-                    cx,
-                    trait_span,
-                    sym::discriminant_value,
-                    vec![cx.expr_self(trait_span)],
-                ),
-            );
-
-            stmts.push(call_hash(trait_span, variant_value));
-
-            fs
+    let (stmts, match_expr) = match substr.fields {
+        Struct(_, fields) | EnumMatching(.., fields) => {
+            let stmts =
+                fields.iter().map(|field| call_hash(field.span, field.self_expr.clone())).collect();
+            (stmts, None)
+        }
+        EnumTag(tag_field, match_expr) => {
+            assert!(tag_field.other_selflike_exprs.is_empty());
+            let stmts = vec![call_hash(tag_field.span, tag_field.self_expr.clone())];
+            (stmts, match_expr.clone())
         }
         _ => cx.span_bug(trait_span, "impossible substructure in `derive(Hash)`"),
     };
 
-    stmts.extend(
-        fields
-            .iter()
-            .map(|FieldInfo { ref self_expr, span, .. }| call_hash(*span, self_expr.clone())),
-    );
-    BlockOrExpr::new_stmts(stmts)
+    BlockOrExpr::new_mixed(stmts, match_expr)
 }
diff --git a/src/test/ui/deriving/deriving-all-codegen.stdout b/src/test/ui/deriving/deriving-all-codegen.stdout
index 0e222d131d7..e129f25b0dd 100644
--- a/src/test/ui/deriving/deriving-all-codegen.stdout
+++ b/src/test/ui/deriving/deriving-all-codegen.stdout
@@ -766,17 +766,13 @@ enum Fieldless1 {
 #[allow(unused_qualifications)]
 impl ::core::clone::Clone for Fieldless1 {
     #[inline]
-    fn clone(&self) -> Fieldless1 {
-        match self { Fieldless1::A => Fieldless1::A, }
-    }
+    fn clone(&self) -> Fieldless1 { Fieldless1::A }
 }
 #[automatically_derived]
 #[allow(unused_qualifications)]
 impl ::core::fmt::Debug for Fieldless1 {
     fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
-        match self {
-            Fieldless1::A => ::core::fmt::Formatter::write_str(f, "A"),
-        }
+        ::core::fmt::Formatter::write_str(f, "A")
     }
 }
 #[automatically_derived]
@@ -788,18 +784,14 @@ impl ::core::default::Default for Fieldless1 {
 #[automatically_derived]
 #[allow(unused_qualifications)]
 impl ::core::hash::Hash for Fieldless1 {
-    fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
-        match self { _ => {} }
-    }
+    fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {}
 }
 impl ::core::marker::StructuralPartialEq for Fieldless1 {}
 #[automatically_derived]
 #[allow(unused_qualifications)]
 impl ::core::cmp::PartialEq for Fieldless1 {
     #[inline]
-    fn eq(&self, other: &Fieldless1) -> bool {
-        match (self, other) { _ => true, }
-    }
+    fn eq(&self, other: &Fieldless1) -> bool { true }
 }
 impl ::core::marker::StructuralEq for Fieldless1 {}
 #[automatically_derived]
@@ -816,9 +808,7 @@ impl ::core::cmp::PartialOrd for Fieldless1 {
     #[inline]
     fn partial_cmp(&self, other: &Fieldless1)
         -> ::core::option::Option<::core::cmp::Ordering> {
-        match (self, other) {
-            _ => ::core::option::Option::Some(::core::cmp::Ordering::Equal),
-        }
+        ::core::option::Option::Some(::core::cmp::Ordering::Equal)
     }
 }
 #[automatically_derived]
@@ -826,7 +816,7 @@ impl ::core::cmp::PartialOrd for Fieldless1 {
 impl ::core::cmp::Ord for Fieldless1 {
     #[inline]
     fn cmp(&self, other: &Fieldless1) -> ::core::cmp::Ordering {
-        match (self, other) { _ => ::core::cmp::Ordering::Equal, }
+        ::core::cmp::Ordering::Equal
     }
 }
 
@@ -868,11 +858,8 @@ impl ::core::default::Default for Fieldless {
 #[allow(unused_qualifications)]
 impl ::core::hash::Hash for Fieldless {
     fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
-        match self {
-            _ =>
-                ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state),
-        }
+        let __self_tag = ::core::intrinsics::discriminant_value(self);
+        ::core::hash::Hash::hash(&__self_tag, state)
     }
 }
 impl ::core::marker::StructuralPartialEq for Fieldless {}
@@ -883,9 +870,7 @@ impl ::core::cmp::PartialEq for Fieldless {
     fn eq(&self, other: &Fieldless) -> bool {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
-                match (self, other) { _ => true, }
-            } else { false }
+        __self_tag == __arg1_tag
     }
 }
 impl ::core::marker::StructuralEq for Fieldless {}
@@ -905,14 +890,7 @@ impl ::core::cmp::PartialOrd for Fieldless {
         -> ::core::option::Option<::core::cmp::Ordering> {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
-                match (self, other) {
-                    _ =>
-                        ::core::option::Option::Some(::core::cmp::Ordering::Equal),
-                }
-            } else {
-               ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag)
-           }
+        ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag)
     }
 }
 #[automatically_derived]
@@ -922,9 +900,7 @@ impl ::core::cmp::Ord for Fieldless {
     fn cmp(&self, other: &Fieldless) -> ::core::cmp::Ordering {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
-                match (self, other) { _ => ::core::cmp::Ordering::Equal, }
-            } else { ::core::cmp::Ord::cmp(&__self_tag, &__arg1_tag) }
+        ::core::cmp::Ord::cmp(&__self_tag, &__arg1_tag)
     }
 }
 
@@ -978,21 +954,15 @@ impl ::core::default::Default for Mixed {
 #[allow(unused_qualifications)]
 impl ::core::hash::Hash for Mixed {
     fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
+        let __self_tag = ::core::intrinsics::discriminant_value(self);
+        ::core::hash::Hash::hash(&__self_tag, state);
         match self {
-            Mixed::R(__self_0) => {
-                ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state);
-                ::core::hash::Hash::hash(__self_0, state)
-            }
+            Mixed::R(__self_0) => ::core::hash::Hash::hash(__self_0, state),
             Mixed::S { d1: __self_0, d2: __self_1 } => {
-                ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state);
                 ::core::hash::Hash::hash(__self_0, state);
                 ::core::hash::Hash::hash(__self_1, state)
             }
-            _ =>
-                ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state),
+            _ => {}
         }
     }
 }
@@ -1004,31 +974,29 @@ impl ::core::cmp::PartialEq for Mixed {
     fn eq(&self, other: &Mixed) -> bool {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
-                match (self, other) {
-                    (Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
-                        *__self_0 == *__arg1_0,
-                    (Mixed::S { d1: __self_0, d2: __self_1 }, Mixed::S {
-                        d1: __arg1_0, d2: __arg1_1 }) =>
-                        *__self_0 == *__arg1_0 && *__self_1 == *__arg1_1,
-                    _ => true,
-                }
-            } else { false }
+        __self_tag == __arg1_tag &&
+            match (self, other) {
+                (Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
+                    *__self_0 == *__arg1_0,
+                (Mixed::S { d1: __self_0, d2: __self_1 }, Mixed::S {
+                    d1: __arg1_0, d2: __arg1_1 }) =>
+                    *__self_0 == *__arg1_0 && *__self_1 == *__arg1_1,
+                _ => true,
+            }
     }
     #[inline]
     fn ne(&self, other: &Mixed) -> bool {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
-                match (self, other) {
-                    (Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
-                        *__self_0 != *__arg1_0,
-                    (Mixed::S { d1: __self_0, d2: __self_1 }, Mixed::S {
-                        d1: __arg1_0, d2: __arg1_1 }) =>
-                        *__self_0 != *__arg1_0 || *__self_1 != *__arg1_1,
-                    _ => false,
-                }
-            } else { true }
+        __self_tag != __arg1_tag ||
+            match (self, other) {
+                (Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
+                    *__self_0 != *__arg1_0,
+                (Mixed::S { d1: __self_0, d2: __self_1 }, Mixed::S {
+                    d1: __arg1_0, d2: __arg1_1 }) =>
+                    *__self_0 != *__arg1_0 || *__self_1 != *__arg1_1,
+                _ => false,
+            }
     }
 }
 impl ::core::marker::StructuralEq for Mixed {}
@@ -1050,7 +1018,8 @@ impl ::core::cmp::PartialOrd for Mixed {
         -> ::core::option::Option<::core::cmp::Ordering> {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
+        match ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag) {
+            ::core::option::Option::Some(::core::cmp::Ordering::Equal) =>
                 match (self, other) {
                     (Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
                         ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
@@ -1064,10 +1033,9 @@ impl ::core::cmp::PartialOrd for Mixed {
                         },
                     _ =>
                         ::core::option::Option::Some(::core::cmp::Ordering::Equal),
-                }
-            } else {
-               ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag)
-           }
+                },
+            cmp => cmp,
+        }
     }
 }
 #[automatically_derived]
@@ -1077,7 +1045,8 @@ impl ::core::cmp::Ord for Mixed {
     fn cmp(&self, other: &Mixed) -> ::core::cmp::Ordering {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
+        match ::core::cmp::Ord::cmp(&__self_tag, &__arg1_tag) {
+            ::core::cmp::Ordering::Equal =>
                 match (self, other) {
                     (Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
                         ::core::cmp::Ord::cmp(__self_0, __arg1_0),
@@ -1089,8 +1058,9 @@ impl ::core::cmp::Ord for Mixed {
                             cmp => cmp,
                         },
                     _ => ::core::cmp::Ordering::Equal,
-                }
-            } else { ::core::cmp::Ord::cmp(&__self_tag, &__arg1_tag) }
+                },
+            cmp => cmp,
+        }
     }
 }
 
@@ -1133,22 +1103,12 @@ impl ::core::fmt::Debug for Fielded {
 #[allow(unused_qualifications)]
 impl ::core::hash::Hash for Fielded {
     fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
+        let __self_tag = ::core::intrinsics::discriminant_value(self);
+        ::core::hash::Hash::hash(&__self_tag, state);
         match self {
-            Fielded::X(__self_0) => {
-                ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state);
-                ::core::hash::Hash::hash(__self_0, state)
-            }
-            Fielded::Y(__self_0) => {
-                ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state);
-                ::core::hash::Hash::hash(__self_0, state)
-            }
-            Fielded::Z(__self_0) => {
-                ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state);
-                ::core::hash::Hash::hash(__self_0, state)
-            }
+            Fielded::X(__self_0) => ::core::hash::Hash::hash(__self_0, state),
+            Fielded::Y(__self_0) => ::core::hash::Hash::hash(__self_0, state),
+            Fielded::Z(__self_0) => ::core::hash::Hash::hash(__self_0, state),
         }
     }
 }
@@ -1160,33 +1120,31 @@ impl ::core::cmp::PartialEq for Fielded {
     fn eq(&self, other: &Fielded) -> bool {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
-                match (self, other) {
-                    (Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
-                        *__self_0 == *__arg1_0,
-                    (Fielded::Y(__self_0), Fielded::Y(__arg1_0)) =>
-                        *__self_0 == *__arg1_0,
-                    (Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
-                        *__self_0 == *__arg1_0,
-                    _ => unsafe { ::core::intrinsics::unreachable() }
-                }
-            } else { false }
+        __self_tag == __arg1_tag &&
+            match (self, other) {
+                (Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
+                    *__self_0 == *__arg1_0,
+                (Fielded::Y(__self_0), Fielded::Y(__arg1_0)) =>
+                    *__self_0 == *__arg1_0,
+                (Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
+                    *__self_0 == *__arg1_0,
+                _ => unsafe { ::core::intrinsics::unreachable() }
+            }
     }
     #[inline]
     fn ne(&self, other: &Fielded) -> bool {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
-                match (self, other) {
-                    (Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
-                        *__self_0 != *__arg1_0,
-                    (Fielded::Y(__self_0), Fielded::Y(__arg1_0)) =>
-                        *__self_0 != *__arg1_0,
-                    (Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
-                        *__self_0 != *__arg1_0,
-                    _ => unsafe { ::core::intrinsics::unreachable() }
-                }
-            } else { true }
+        __self_tag != __arg1_tag ||
+            match (self, other) {
+                (Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
+                    *__self_0 != *__arg1_0,
+                (Fielded::Y(__self_0), Fielded::Y(__arg1_0)) =>
+                    *__self_0 != *__arg1_0,
+                (Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
+                    *__self_0 != *__arg1_0,
+                _ => unsafe { ::core::intrinsics::unreachable() }
+            }
     }
 }
 impl ::core::marker::StructuralEq for Fielded {}
@@ -1210,7 +1168,8 @@ impl ::core::cmp::PartialOrd for Fielded {
         -> ::core::option::Option<::core::cmp::Ordering> {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
+        match ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag) {
+            ::core::option::Option::Some(::core::cmp::Ordering::Equal) =>
                 match (self, other) {
                     (Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
                         ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
@@ -1219,10 +1178,9 @@ impl ::core::cmp::PartialOrd for Fielded {
                     (Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
                         ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
                     _ => unsafe { ::core::intrinsics::unreachable() }
-                }
-            } else {
-               ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag)
-           }
+                },
+            cmp => cmp,
+        }
     }
 }
 #[automatically_derived]
@@ -1232,7 +1190,8 @@ impl ::core::cmp::Ord for Fielded {
     fn cmp(&self, other: &Fielded) -> ::core::cmp::Ordering {
         let __self_tag = ::core::intrinsics::discriminant_value(self);
         let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-        if __self_tag == __arg1_tag {
+        match ::core::cmp::Ord::cmp(&__self_tag, &__arg1_tag) {
+            ::core::cmp::Ordering::Equal =>
                 match (self, other) {
                     (Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
                         ::core::cmp::Ord::cmp(__self_0, __arg1_0),
@@ -1241,8 +1200,9 @@ impl ::core::cmp::Ord for Fielded {
                     (Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
                         ::core::cmp::Ord::cmp(__self_0, __arg1_0),
                     _ => unsafe { ::core::intrinsics::unreachable() }
-                }
-            } else { ::core::cmp::Ord::cmp(&__self_tag, &__arg1_tag) }
+                },
+            cmp => cmp,
+        }
     }
 }