Implement Anonymous{Struct, Union} in the AST

Add unnamed_fields feature gate and gate unnamed fields on parsing

src/test/ui/feature-gates/feature-gate-unnamed_fields.rs

@@ -0,0 +1,27 @@
+struct Foo {
+    foo: u8,
+    _: union { //~ ERROR unnamed fields are not yet fully implemented [E0658]
+    //~^ ERROR unnamed fields are not yet fully implemented [E0658]
+    //~| ERROR anonymous unions are unimplemented
+        bar: u8,
+        baz: u16
+    }
+}
+
+union Bar {
+    foobar: u8,
+    _: struct { //~ ERROR unnamed fields are not yet fully implemented [E0658]
+    //~^ ERROR unnamed fields are not yet fully implemented [E0658]
+    //~| ERROR anonymous structs are unimplemented
+    //~| ERROR unions may not contain fields that need dropping [E0740]
+        foobaz: u8,
+        barbaz: u16
+    }
+}
+
+struct S;
+struct Baz {
+    _: S //~ ERROR unnamed fields are not yet fully implemented [E0658]
+}
+
+fn main(){}

src/test/ui/feature-gates/feature-gate-unnamed_fields.stderr

@@ -0,0 +1,111 @@
+error[E0658]: unnamed fields are not yet fully implemented
+  --> $DIR/feature-gate-unnamed_fields.rs:3:5
+   |
+LL |     _: union {
+   |     ^
+   |
+   = note: see issue #49804 <https://github.com/rust-lang/rust/issues/49804> for more information
+   = help: add `#![feature(unnamed_fields)]` to the crate attributes to enable
+
+error[E0658]: unnamed fields are not yet fully implemented
+  --> $DIR/feature-gate-unnamed_fields.rs:3:8
+   |
+LL |       _: union {
+   |  ________^
+LL | |
+LL | |
+LL | |         bar: u8,
+LL | |         baz: u16
+LL | |     }
+   | |_____^
+   |
+   = note: see issue #49804 <https://github.com/rust-lang/rust/issues/49804> for more information
+   = help: add `#![feature(unnamed_fields)]` to the crate attributes to enable
+
+error[E0658]: unnamed fields are not yet fully implemented
+  --> $DIR/feature-gate-unnamed_fields.rs:13:5
+   |
+LL |     _: struct {
+   |     ^
+   |
+   = note: see issue #49804 <https://github.com/rust-lang/rust/issues/49804> for more information
+   = help: add `#![feature(unnamed_fields)]` to the crate attributes to enable
+
+error[E0658]: unnamed fields are not yet fully implemented
+  --> $DIR/feature-gate-unnamed_fields.rs:13:8
+   |
+LL |       _: struct {
+   |  ________^
+LL | |
+LL | |
+LL | |
+LL | |         foobaz: u8,
+LL | |         barbaz: u16
+LL | |     }
+   | |_____^
+   |
+   = note: see issue #49804 <https://github.com/rust-lang/rust/issues/49804> for more information
+   = help: add `#![feature(unnamed_fields)]` to the crate attributes to enable
+
+error[E0658]: unnamed fields are not yet fully implemented
+  --> $DIR/feature-gate-unnamed_fields.rs:24:5
+   |
+LL |     _: S
+   |     ^
+   |
+   = note: see issue #49804 <https://github.com/rust-lang/rust/issues/49804> for more information
+   = help: add `#![feature(unnamed_fields)]` to the crate attributes to enable
+
+error: anonymous unions are unimplemented
+  --> $DIR/feature-gate-unnamed_fields.rs:3:8
+   |
+LL |       _: union {
+   |  ________^
+LL | |
+LL | |
+LL | |         bar: u8,
+LL | |         baz: u16
+LL | |     }
+   | |_____^
+
+error: anonymous structs are unimplemented
+  --> $DIR/feature-gate-unnamed_fields.rs:13:8
+   |
+LL |       _: struct {
+   |  ________^
+LL | |
+LL | |
+LL | |
+LL | |         foobaz: u8,
+LL | |         barbaz: u16
+LL | |     }
+   | |_____^
+
+error[E0740]: unions may not contain fields that need dropping
+  --> $DIR/feature-gate-unnamed_fields.rs:13:5
+   |
+LL | /     _: struct {
+LL | |
+LL | |
+LL | |
+LL | |         foobaz: u8,
+LL | |         barbaz: u16
+LL | |     }
+   | |_____^
+   |
+note: `std::mem::ManuallyDrop` can be used to wrap the type
+  --> $DIR/feature-gate-unnamed_fields.rs:13:5
+   |
+LL | /     _: struct {
+LL | |
+LL | |
+LL | |
+LL | |         foobaz: u8,
+LL | |         barbaz: u16
+LL | |     }
+   | |_____^
+
+error: aborting due to 8 previous errors
+
+Some errors have detailed explanations: E0658, E0740.
+For more information about an error, try `rustc --explain E0658`.