From 95a824c02c3f9a50bb4fa49e7af7d3a5a2faf51e Mon Sep 17 00:00:00 2001
From: clubby789 <jamie@hill-daniel.co.uk>
Date: Thu, 19 Jan 2023 15:52:29 +0000
Subject: [PATCH] Special case `derive(Debug)` for fieldless enums

---
 .../src/deriving/debug.rs                     | 51 ++++++++++++++++++-
 tests/ui/deriving/deriving-all-codegen.stdout | 11 ++--
 2 files changed, 55 insertions(+), 7 deletions(-)

diff --git a/compiler/rustc_builtin_macros/src/deriving/debug.rs b/compiler/rustc_builtin_macros/src/deriving/debug.rs
index a586964d0c3..e0f487e8648 100644
--- a/compiler/rustc_builtin_macros/src/deriving/debug.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/debug.rs
@@ -2,6 +2,7 @@ use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 use crate::deriving::path_std;
 
+use ast::EnumDef;
 use rustc_ast::{self as ast, MetaItem};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident, Symbol};
@@ -31,7 +32,8 @@ pub fn expand_deriving_debug(
             nonself_args: vec![(fmtr, sym::f)],
             ret_ty: Path(path_std!(fmt::Result)),
             attributes: ast::AttrVec::new(),
-            fieldless_variants_strategy: FieldlessVariantsStrategy::Default,
+            fieldless_variants_strategy:
+                FieldlessVariantsStrategy::SpecializeIfAllVariantsFieldless,
             combine_substructure: combine_substructure(Box::new(|a, b, c| {
                 show_substructure(a, b, c)
             })),
@@ -49,7 +51,8 @@ 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),
-        AllFieldlessEnum(..) | EnumTag(..) | StaticStruct(..) | StaticEnum(..) => {
+        AllFieldlessEnum(enum_def) => return show_fieldless_enum(cx, span, enum_def, substr),
+        EnumTag(..) | StaticStruct(..) | StaticEnum(..) => {
             cx.span_bug(span, "nonsensical .fields in `#[derive(Debug)]`")
         }
     };
@@ -174,3 +177,47 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
         BlockOrExpr::new_mixed(stmts, Some(expr))
     }
 }
+
+/// Special case for enums with no fields. Builds:
+/// ```text
+/// impl ::core::fmt::Debug for A {
+///     fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+///          ::core::fmt::Formatter::write_str(f,
+///             match self {
+///                 A::A => "A",
+///                 A::B() => "B",
+///                 A::C {} => "C",
+///             })
+///     }
+/// }
+/// ```
+fn show_fieldless_enum(
+    cx: &mut ExtCtxt<'_>,
+    span: Span,
+    def: &EnumDef,
+    substr: &Substructure<'_>,
+) -> BlockOrExpr {
+    let fmt = substr.nonselflike_args[0].clone();
+    let arms = def
+        .variants
+        .iter()
+        .map(|v| {
+            let variant_path = cx.path(span, vec![substr.type_ident, v.ident]);
+            let pat = match &v.data {
+                ast::VariantData::Tuple(fields, _) => {
+                    debug_assert!(fields.is_empty());
+                    cx.pat_tuple_struct(span, variant_path, vec![])
+                }
+                ast::VariantData::Struct(fields, _) => {
+                    debug_assert!(fields.is_empty());
+                    cx.pat_struct(span, variant_path, vec![])
+                }
+                ast::VariantData::Unit(_) => cx.pat_path(span, variant_path),
+            };
+            cx.arm(span, pat, cx.expr_str(span, v.ident.name))
+        })
+        .collect::<Vec<_>>();
+    let name = cx.expr_match(span, cx.expr_self(span), arms);
+    let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
+    BlockOrExpr::new_expr(cx.expr_call_global(span, fn_path_write_str, vec![fmt, name]))
+}
diff --git a/tests/ui/deriving/deriving-all-codegen.stdout b/tests/ui/deriving/deriving-all-codegen.stdout
index a63cbd4ca7e..e6ee11a783b 100644
--- a/tests/ui/deriving/deriving-all-codegen.stdout
+++ b/tests/ui/deriving/deriving-all-codegen.stdout
@@ -731,11 +731,12 @@ impl ::core::marker::Copy for Fieldless { }
 #[automatically_derived]
 impl ::core::fmt::Debug for Fieldless {
     fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
-        match self {
-            Fieldless::A => ::core::fmt::Formatter::write_str(f, "A"),
-            Fieldless::B => ::core::fmt::Formatter::write_str(f, "B"),
-            Fieldless::C => ::core::fmt::Formatter::write_str(f, "C"),
-        }
+        ::core::fmt::Formatter::write_str(f,
+            match self {
+                Fieldless::A => "A",
+                Fieldless::B => "B",
+                Fieldless::C => "C",
+            })
     }
 }
 #[automatically_derived]