Add feature gate for view_types experiment

compiler/rustc_feature/src/unstable.rs

@@ -723,6 +723,8 @@ declare_features! (
     (internal, unsized_fn_params, "1.49.0", Some(48055)),
     /// Allows using the `#[used(linker)]` (or `#[used(compiler)]`) attribute.
     (unstable, used_with_arg, "1.60.0", Some(93798)),
+    /// Allows view types.
+    (unstable, view_types, "CURRENT_RUSTC_VERSION", Some(155938)),
     /// Target features on wasm.
     (unstable, wasm_target_feature, "1.30.0", Some(150260)),
     /// Allows use of attributes in `where` clauses.

compiler/rustc_span/src/symbol.rs

@@ -2232,6 +2232,7 @@ symbols! {
         verbatim,
         version,
         vfp2,
+        view_types,
         vis,
         visible_private_types,
         volatile,

tests/ui/feature-gates/feature-gate-view-types.rs

@@ -0,0 +1,16 @@
+//@ known-bug: #155938
+
+struct Foo {
+    a: usize,
+    b: usize,
+}
+
+fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+    a.a += 1;
+    b.b += 1;
+}
+
+fn main() {
+    let mut foo = Foo { a: 0, b: 0 };
+    bar(&mut foo, &mut foo);
+}

tests/ui/feature-gates/feature-gate-view-types.stderr

@@ -0,0 +1,49 @@
+error: expected parameter name, found `{`
+  --> $DIR/feature-gate-view-types.rs:8:20
+   |
+LL | fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+   |                    ^ expected parameter name
+
+error: expected one of `!`, `(`, `)`, `,`, `::`, or `<`, found `.`
+  --> $DIR/feature-gate-view-types.rs:8:19
+   |
+LL | fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+   |                   ^
+   |                   |
+   |                   expected one of `!`, `(`, `)`, `,`, `::`, or `<`
+   |                   help: missing `,`
+
+error: expected parameter name, found `{`
+  --> $DIR/feature-gate-view-types.rs:8:39
+   |
+LL | fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+   |                                       ^ expected parameter name
+
+error: expected one of `!`, `(`, `)`, `,`, `::`, or `<`, found `.`
+  --> $DIR/feature-gate-view-types.rs:8:38
+   |
+LL | fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+   |                                      ^
+   |                                      |
+   |                                      expected one of `!`, `(`, `)`, `,`, `::`, or `<`
+   |                                      help: missing `,`
+
+error[E0061]: this function takes 4 arguments but 2 arguments were supplied
+  --> $DIR/feature-gate-view-types.rs:15:5
+   |
+LL |     bar(&mut foo, &mut foo);
+   |     ^^^-------------------- two arguments are missing
+   |
+note: function defined here
+  --> $DIR/feature-gate-view-types.rs:8:4
+   |
+LL | fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+   |    ^^^                    -----------------
+help: provide the arguments
+   |
+LL |     bar(&mut foo, &mut foo, /* &mut Foo */, /* _ */);
+   |                           +++++++++++++++++++++++++
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0061`.