Fix handling of discriminantless univariant enums in Rust

diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index 82cd3f9..63c7992 100644
--- a/gdb/rust-lang.c
+++ b/gdb/rust-lang.c
@@ -194,7 +194,18 @@  rust_get_disr_info (struct type *type, const
gdb_byte *valaddr,
      has changed its debuginfo format.  */
       error (_("Could not find enum discriminant field"));
     }
-
+  else if (TYPE_NFIELDS (type) == 1) {
+    /* Sometimes univariant enums are encoded without a
+    discriminant. In that case, treating it as an encoded enum
+    with the first field being the actual type works.  */
+    const char* field_name = TYPE_NAME (TYPE_FIELD_TYPE (type, 0));
+    ret.name = concat (TYPE_NAME (type), "::",
+                       rust_last_path_segment (field_name),
+                       (char *) NULL);
+    ret.field_no = RUST_ENCODED_ENUM_REAL;
+    ret.is_encoded = 1;
+    return ret;
+  }
   if (strcmp (TYPE_FIELD_NAME (disr_type, 0), "RUST$ENUM$DISR") != 0)
     error (_("Rust debug format has changed"));

@@ -1725,7 +1736,9 @@  tuple structs, and tuple-like enum variants"));
         variant_type = TYPE_FIELD_TYPE (type, disr.field_no);

         if (variant_type == NULL
-        || rust_tuple_variant_type_p (variant_type))
+            || (disr.is_encoded
+                ? rust_tuple_struct_type_p (variant_type)
+                : rust_tuple_variant_type_p (variant_type)))
           error(_("Attempting to access named field %s of tuple variant %s, \
 which has only anonymous fields"),
             field_name, disr.name);
diff --git a/gdb/testsuite/gdb.rust/simple.exp
b/gdb/testsuite/gdb.rust/simple.exp
index 5e00b03..14eae2e 100644
--- a/gdb/testsuite/gdb.rust/simple.exp
+++ b/gdb/testsuite/gdb.rust/simple.exp
@@ -103,6 +103,11 @@  gdb_test_sequence "ptype z" "" {
 }
 gdb_test "print z.1" " = 8"

+gdb_test "print univariant" " = simple::Univariant::Foo{a: 1}"
+gdb_test "print univariant.a" " = 1"
+gdb_test "print univariant_anon" " = simple::UnivariantAnon::Foo\\(1\\)"
+gdb_test "print univariant_anon.0" " = 1"
+
 gdb_test_sequence "ptype simple::ByeBob" "" {
     " = struct simple::ByeBob \\("
     "  i32,"
@@ -220,3 +225,9 @@  gdb_test "print (1,)" "Tuple expressions not supported yet"
 gdb_test "print (1)" " = 1"

 gdb_test "print 23..97.0" "Range expression with different types"
+
+gdb_test "print (*parametrized.next.val)" \
+    " = simple::ParametrizedStruct<i32> {next:
simple::ParametrizedEnum<Box<simple::ParametrizedStruct<i32>>>::Empty,
value: 1}"
+gdb_test "print parametrized.next.val" " =
\\(simple::ParametrizedStruct<i32> \\*\\) $hex"
+gdb_test "print parametrized" \
+    " = simple::ParametrizedStruct<i32> \\{next:
simple::ParametrizedEnum<Box<simple::ParametrizedStruct<i32>>>::Val\\{val:
$hex\\}, value: 0\\}"
\ No newline at end of file
diff --git a/gdb/testsuite/gdb.rust/simple.rs b/gdb/testsuite/gdb.rust/simple.rs
index eeff3d7..670f54e 100644
--- a/gdb/testsuite/gdb.rust/simple.rs
+++ b/gdb/testsuite/gdb.rust/simple.rs
@@ -63,6 +63,23 @@  enum SpaceSaver {
     Nothing,
 }

+enum Univariant {
+    Foo {a: u8}
+}
+enum UnivariantAnon {
+    Foo(u8)
+}
+
+enum ParametrizedEnum<T> {
+    Val { val: T },
+    Empty,
+}
+
+struct ParametrizedStruct<T> {
+    next: ParametrizedEnum<Box<ParametrizedStruct<T>>>,
+    value: T
+}
+
 fn main () {
     let a = ();
     let b : [i32; 0] = [];
@@ -93,6 +110,9 @@  fn main () {
     let y = HiBob {field1: 7, field2: 8};
     let z = ByeBob(7, 8);

+    let univariant = Univariant::Foo {a : 1};
+    let univariant_anon = UnivariantAnon::Foo(1);
+
     let slice = &w[2..3];
     let fromslice = slice[0];
     let slice2 = &slice[0..1];
@@ -117,6 +137,16 @@  fn main () {
     let custom_some = NonZeroOptimized::Value("hi".into());
     let custom_none = NonZeroOptimized::Empty;

+    let parametrized = ParametrizedStruct {
+        next: ParametrizedEnum::Val {
+            val: Box::new(ParametrizedStruct {
+                next: ParametrizedEnum::Empty,
+                value: 1,
+            })
+        },
+        value: 0,
+    };
+
     println!("{}, {}", x.0, x.1);        // set breakpoint here
     println!("{}", diff2(92, 45));
     empty();