Auto merge of #31977 - bluss:partial-eq-save, r=brson

derive: Avoid emitting provided PartialEq, PartialOrd methods for c-like enums

derive: Avoid emitting provided PartialEq, PartialOrd method for c-like enums

`ne` is completely symmetrical with the method `eq`, and we can save
rust code size and compilation time here if we only emit one of them
when possible.

One case where it's easy to recognize is when it's a C-like enum. Most
other cases can not omit ne, because any value field may have a custom
PartialEq implementation.

src/libsyntax_ext/deriving/cmp/partial_eq.rs

@@ -80,6 +80,14 @@ fn cs_ne(cx: &mut ExtCtxt, span: Span, substr: &Substructure) -> P<Expr> {
         } }
     }
 
+    // avoid defining `ne` if we can
+    // c-like enums, enums without any fields and structs without fields
+    // can safely define only `eq`.
+    let mut methods = vec![md!("eq", cs_eq)];
+    if !is_type_without_fields(item) {
+        methods.push(md!("ne", cs_ne));
+    }
+
     let trait_def = TraitDef {
         span: span,
         attributes: Vec::new(),
@@ -87,10 +95,7 @@ fn cs_ne(cx: &mut ExtCtxt, span: Span, substr: &Substructure) -> P<Expr> {
         additional_bounds: Vec::new(),
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
-        methods: vec!(
-            md!("eq", cs_eq),
-            md!("ne", cs_ne)
-        ),
+        methods: methods,
         associated_types: Vec::new(),
     };
     trait_def.expand(cx, mitem, item, push)

src/libsyntax_ext/deriving/cmp/partial_ord.rs

@@ -67,6 +67,21 @@ pub fn expand_deriving_partial_ord(cx: &mut ExtCtxt,
         }))
     };
 
+    // avoid defining extra methods if we can
+    // c-like enums, enums without any fields and structs without fields
+    // can safely define only `partial_cmp`.
+    let methods = if is_type_without_fields(item) {
+        vec![partial_cmp_def]
+    } else {
+        vec![
+            partial_cmp_def,
+            md!("lt", true, false),
+            md!("le", true, true),
+            md!("gt", false, false),
+            md!("ge", false, true)
+        ]
+    };
+
     let trait_def = TraitDef {
         span: span,
         attributes: vec![],
@@ -74,13 +89,7 @@ pub fn expand_deriving_partial_ord(cx: &mut ExtCtxt,
         additional_bounds: vec![],
         generics: LifetimeBounds::empty(),
         is_unsafe: false,
-        methods: vec![
-            partial_cmp_def,
-            md!("lt", true, false),
-            md!("le", true, true),
-            md!("gt", false, false),
-            md!("ge", false, true)
-        ],
+        methods: methods,
         associated_types: Vec::new(),
     };
     trait_def.expand(cx, mitem, item, push)

src/libsyntax_ext/deriving/generic/mod.rs

@@ -1647,3 +1647,21 @@ pub fn cs_same_method<F>(f: F,
         }
     }
 }
+
+/// Return true if the type has no value fields
+/// (for an enum, no variant has any fields)
+pub fn is_type_without_fields(item: &Annotatable) -> bool {
+    if let Annotatable::Item(ref item) = *item {
+        match item.node {
+            ast::ItemKind::Enum(ref enum_def, _) => {
+                enum_def.variants.iter().all(|v| v.node.data.fields().is_empty())
+            }
+            ast::ItemKind::Struct(ref variant_data, _) => {
+                variant_data.fields().is_empty()
+            }
+            _ => false
+        }
+    } else {
+        false
+    }
+}