deriv(Hash) for single-variant enum should not hash discriminant

Fixes #39137

src/libsyntax_ext/deriving/clone.rs

@@ -167,7 +167,7 @@ fn cs_clone(name: &str,
             all_fields = af;
             vdata = vdata_;
         }
-        EnumMatching(_, variant, ref af) => {
+        EnumMatching(.., variant, ref af) => {
             ctor_path = cx.path(trait_span, vec![substr.type_ident, variant.node.name]);
             all_fields = af;
             vdata = &variant.node.data;

src/libsyntax_ext/deriving/debug.rs

@@ -60,7 +60,7 @@ fn show_substructure(cx: &mut ExtCtxt, span: Span, substr: &Substructure) -> P<E
     // based on the "shape".
     let (ident, is_struct) = match *substr.fields {
         Struct(vdata, _) => (substr.type_ident, vdata.is_struct()),
-        EnumMatching(_, v, _) => (v.node.name, v.node.data.is_struct()),
+        EnumMatching(_, _, v, _) => (v.node.name, v.node.data.is_struct()),
         EnumNonMatchingCollapsed(..) |
         StaticStruct(..) |
         StaticEnum(..) => cx.span_bug(span, "nonsensical .fields in `#[derive(Debug)]`"),

src/libsyntax_ext/deriving/encodable.rs

@@ -233,7 +233,7 @@ fn encodable_substructure(cx: &mut ExtCtxt,
                                      blk])
         }
 
-        EnumMatching(idx, variant, ref fields) => {
+        EnumMatching(idx, _, variant, ref fields) => {
             // We're not generating an AST that the borrow checker is expecting,
             // so we need to generate a unique local variable to take the
             // mutable loan out on, otherwise we get conflicts which don't

src/libsyntax_ext/deriving/generic/mod.rs

@@ -305,10 +305,10 @@ pub enum StaticFields {
 /// A summary of the possible sets of fields.
 pub enum SubstructureFields<'a> {
     Struct(&'a ast::VariantData, Vec<FieldInfo<'a>>),
-    /// Matching variants of the enum: variant index, ast::Variant,
+    /// 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, &'a ast::Variant, Vec<FieldInfo<'a>>),
+    EnumMatching(usize, usize, &'a ast::Variant, Vec<FieldInfo<'a>>),
 
     /// Non-matching variants of the enum, but with all state hidden from
     /// the consequent code.  The first component holds `Ident`s for all of
@@ -1250,7 +1250,7 @@ impl<'a> MethodDef<'a> {
                 // expressions for referencing every field of every
                 // Self arg, assuming all are instances of VariantK.
                 // Build up code associated with such a case.
-                let substructure = EnumMatching(index, variant, field_tuples);
+                let substructure = EnumMatching(index, variants.len(), variant, field_tuples);
                 let arm_expr = self.call_substructure_method(cx,
                                                              trait_,
                                                              type_ident,
@@ -1267,12 +1267,13 @@ impl<'a> MethodDef<'a> {
                 // 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());
                 Some(self.call_substructure_method(cx,
                                                    trait_,
                                                    type_ident,
                                                    &self_args[..],
                                                    nonself_args,
-                                                   &EnumMatching(0, v, Vec::new())))
+                                                   &substructure))
             }
             _ if variants.len() > 1 && self_args.len() > 1 => {
                 // Since we know that all the arguments will match if we reach

src/libsyntax_ext/deriving/hash.rs

@@ -81,7 +81,7 @@ fn hash_substructure(cx: &mut ExtCtxt, trait_span: Span, substr: &Substructure)
     let mut stmts = Vec::new();
 
     let fields = match *substr.fields {
-        Struct(_, ref fs) => fs,
+        Struct(_, ref fs) | EnumMatching(_, 1, .., ref fs) => fs,
         EnumMatching(.., ref fs) => {
             let variant_value = deriving::call_intrinsic(cx,
                                                          trait_span,

src/test/run-pass/deriving-hash.rs

@@ -45,8 +45,8 @@ impl<'a> Hasher for FakeHasher<'a> {
     }
 }
 
-fn fake_hash(v: &mut Vec<u8>, e: E) {
+fn fake_hash<A: Hash>(v: &mut Vec<u8>, a: A) {
-    e.hash(&mut FakeHasher(v));
+    a.hash(&mut FakeHasher(v));
 }
 
 fn main() {
@@ -69,4 +69,13 @@ fn main() {
     fake_hash(&mut va, E::A);
     fake_hash(&mut vb, E::B);
     assert!(va != vb);
+
+    // issue #39137: single variant enum hash should not hash discriminant
+    #[derive(Hash)]
+    enum SingleVariantEnum {
+        A(u8),
+    }
+    let mut v = vec![];
+    fake_hash(&mut v, SingleVariantEnum::A(17));
+    assert_eq!(vec![17], v);
 }