Auto merge of #103659 - clubby789:improve-partialord-derive, r=nagisa

Special-case deriving `PartialOrd` for enums with dataless variants

I was able to get slightly better codegen by flipping the derived `PartialOrd` logic for two-variant enums.  I also tried to document the implementation of the derive macro to make the special-case logic a little clearer.
```rs
#[derive(PartialEq, PartialOrd)]
pub enum A<T> {
    A,
    B(T)
}
```
```diff
impl<T: ::core::cmp::PartialOrd> ::core::cmp::PartialOrd for A<T> {
   #[inline]
   fn partial_cmp(
       &self,
       other: &A<T>,
   ) -> ::core::option::Option<::core::cmp::Ordering> {
       let __self_tag = ::core::intrinsics::discriminant_value(self);
       let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-      match ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag) {
-          ::core::option::Option::Some(::core::cmp::Ordering::Equal) => {
-              match (self, other) {
-                  (A::B(__self_0), A::B(__arg1_0)) => {
-                      ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0)
-                  }
-                  _ => ::core::option::Option::Some(::core::cmp::Ordering::Equal),
-              }
+      match (self, other) {
+          (A::B(__self_0), A::B(__arg1_0)) => {
+              ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0)
           }
-          cmp => cmp,
+          _ => ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag),
       }
   }
}
```
Godbolt: [Current](https://godbolt.org/z/GYjEzG1T8), [New](https://godbolt.org/z/GoK78qx15)
I'm not sure how common a case comparing two enums like this (such as `Option`) is, and if it's worth the slowdown of adding a special case to the derive. If it causes overall regressions it might be worth just manually implementing this for `Option`.

1 files changed, 73 insertions, 9 deletions

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 c9dc8921262..2fc30d8e05f 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
@@ -1,7 +1,7 @@
 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 use crate::deriving::{path_std, pathvec_std};
-use rustc_ast::MetaItem;
+use rustc_ast::{ExprKind, ItemKind, MetaItem, PatKind};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
@@ -21,6 +21,27 @@ pub fn expand_deriving_partial_ord(
 
     let attrs = thin_vec![cx.attr_word(sym::inline, span)];
 
+    // Order in which to perform matching
+    let tag_then_data = if let Annotatable::Item(item) = item
+        && let ItemKind::Enum(def, _) = &item.kind {
+            let dataful: Vec<bool> = def.variants.iter().map(|v| !v.data.fields().is_empty()).collect();
+            match dataful.iter().filter(|&&b| b).count() {
+                // No data, placing the tag check first makes codegen simpler
+                0 => true,
+                1..=2 => false,
+                _ => {
+                    (0..dataful.len()-1).any(|i| {
+                        if dataful[i] && let Some(idx) = dataful[i+1..].iter().position(|v| *v) {
+                            idx >= 2
+                        } else {
+                            false
+                        }
+                    })
+                }
+            }
+        } else {
+            true
+        };
     let partial_cmp_def = MethodDef {
         name: sym::partial_cmp,
         generics: Bounds::empty(),
@@ -30,7 +51,7 @@ pub fn expand_deriving_partial_ord(
         attributes: attrs,
         fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
         combine_substructure: combine_substructure(Box::new(|cx, span, substr| {
-            cs_partial_cmp(cx, span, substr)
+            cs_partial_cmp(cx, span, substr, tag_then_data)
         })),
     };
 
@@ -47,7 +68,12 @@ pub fn expand_deriving_partial_ord(
     trait_def.expand(cx, mitem, item, push)
 }
 
-pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
+fn cs_partial_cmp(
+    cx: &mut ExtCtxt<'_>,
+    span: Span,
+    substr: &Substructure<'_>,
+    tag_then_data: bool,
+) -> BlockOrExpr {
     let test_id = Ident::new(sym::cmp, span);
     let equal_path = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));
     let partial_cmp_path = cx.std_path(&[sym::cmp, sym::PartialOrd, sym::partial_cmp]);
@@ -74,12 +100,50 @@ pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_
                 let args = vec![field.self_expr.clone(), other_expr.clone()];
                 cx.expr_call_global(field.span, partial_cmp_path.clone(), args)
             }
-            CsFold::Combine(span, expr1, expr2) => {
-                let eq_arm =
-                    cx.arm(span, cx.pat_some(span, cx.pat_path(span, equal_path.clone())), expr1);
-                let neq_arm =
-                    cx.arm(span, cx.pat_ident(span, test_id), cx.expr_ident(span, test_id));
-                cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
+            CsFold::Combine(span, mut expr1, expr2) => {
+                // When the item is an enum, this expands to
+                // ```
+                // match (expr2) {
+                //     Some(Ordering::Equal) => expr1,
+                //     cmp => cmp
+                // }
+                // ```
+                // where `expr2` is `partial_cmp(self_tag, other_tag)`, and `expr1` is a `match`
+                //  against the enum variants. This means that we begin by comparing the enum tags,
+                // before either inspecting their contents (if they match), or returning
+                // the `cmp::Ordering` of comparing the enum tags.
+                // ```
+                // match partial_cmp(self_tag, other_tag) {
+                //     Some(Ordering::Equal) => match (self, other)  {
+                //         (Self::A(self_0), Self::A(other_0)) => partial_cmp(self_0, other_0),
+                //         (Self::B(self_0), Self::B(other_0)) => partial_cmp(self_0, other_0),
+                //         _ => Some(Ordering::Equal)
+                //     }
+                //     cmp => cmp
+                // }
+                // ```
+                // If we have any certain enum layouts, flipping this results in better codegen
+                // ```
+                // match (self, other) {
+                //     (Self::A(self_0), Self::A(other_0)) => partial_cmp(self_0, other_0),
+                //     _ => partial_cmp(self_tag, other_tag)
+                // }
+                // ```
+                // Reference: https://github.com/rust-lang/rust/pull/103659#issuecomment-1328126354
+
+                if !tag_then_data
+                    && let ExprKind::Match(_, arms) = &mut expr1.kind
+                    && let Some(last) = arms.last_mut()
+                    && let PatKind::Wild = last.pat.kind {
+                        last.body = expr2;
+                        expr1
+                } else {
+                    let eq_arm =
+                        cx.arm(span, cx.pat_some(span, cx.pat_path(span, equal_path.clone())), expr1);
+                    let neq_arm =
+                        cx.arm(span, cx.pat_ident(span, test_id), cx.expr_ident(span, test_id));
+                    cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
+                }
             }
             CsFold::Fieldless => cx.expr_some(span, cx.expr_path(equal_path.clone())),
         },