Add feature gate for view_types experiment

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -510,6 +510,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(try_blocks_heterogeneous, "`try bikeshed` expression is experimental");
     gate_all!(unsafe_binders, "unsafe binder types are experimental");
     gate_all!(unsafe_fields, "`unsafe` fields are experimental");
+    gate_all!(view_types, "view types are experimental");
     gate_all!(where_clause_attrs, "attributes in `where` clause are unstable");
     gate_all!(yeet_expr, "`do yeet` expression is experimental");
     // tidy-alphabetical-end

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_parse/src/parser/ty.rs

@@ -19,7 +19,7 @@ use crate::errors::{
     NeedPlusAfterTraitObjectLifetime, NestedCVariadicType, ReturnTypesUseThinArrow,
 };
 use crate::parser::item::FrontMatterParsingMode;
-use crate::parser::{FnContext, FnParseMode};
+use crate::parser::{ExpTokenPair, FnContext, FnParseMode};
 use crate::{exp, maybe_recover_from_interpolated_ty_qpath};
 
 /// Signals whether parsing a type should allow `+`.
@@ -768,6 +768,25 @@ impl<'a> Parser<'a> {
             self.bump_with((dyn_tok, dyn_tok_sp));
         }
         let ty = self.parse_ty_no_plus()?;
+        if self.token == TokenKind::Dot && self.look_ahead(1, |t| t.kind == TokenKind::OpenBrace) {
+            // & [mut] <type> . { <fields> }
+            //                ^
+            //                we are here
+            let view_start_span = self.token.span;
+            self.bump();
+            let fields = self
+                .parse_delim_comma_seq(
+                    ExpTokenPair { tok: TokenKind::OpenBrace, token_type: TokenType::OpenBrace },
+                    ExpTokenPair { tok: TokenKind::CloseBrace, token_type: TokenType::CloseBrace },
+                    |p| p.parse_ident(),
+                )?
+                .0;
+            // FIXME(scrabsha): actually propagate field view in the AST.
+            let _ = fields;
+            let view_end_span = self.prev_token.span;
+            let span = view_start_span.to(view_end_span);
+            self.psess.gated_spans.gate(sym::view_types, span);
+        }
         Ok(match pinned {
             Pinnedness::Not => TyKind::Ref(opt_lifetime, MutTy { ty, mutbl }),
             Pinnedness::Pinned => TyKind::PinnedRef(opt_lifetime, MutTy { ty, mutbl }),

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 +1,17 @@
+struct Foo {
+    a: usize,
+    b: usize,
+}
+
+fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+    //~^ ERROR view types are experimental
+    //~| ERROR view types are experimental
+    a.a += 1;
+    b.b += 1;
+}
+
+fn main() {
+    let mut foo = Foo { a: 0, b: 0 };
+    bar(&mut foo, &mut foo);
+    //~^ ERROR cannot borrow `foo` as mutable more than once at a time
+}

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

@@ -0,0 +1,33 @@
+error[E0658]: view types are experimental
+  --> $DIR/feature-gate-view-types.rs:6:19
+   |
+LL | fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+   |                   ^^^^^^
+   |
+   = note: see issue #155938 <https://github.com/rust-lang/rust/issues/155938> for more information
+   = help: add `#![feature(view_types)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: view types are experimental
+  --> $DIR/feature-gate-view-types.rs:6:38
+   |
+LL | fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
+   |                                      ^^^^^^
+   |
+   = note: see issue #155938 <https://github.com/rust-lang/rust/issues/155938> for more information
+   = help: add `#![feature(view_types)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0499]: cannot borrow `foo` as mutable more than once at a time
+  --> $DIR/feature-gate-view-types.rs:15:19
+   |
+LL |     bar(&mut foo, &mut foo);
+   |     --- --------  ^^^^^^^^ second mutable borrow occurs here
+   |     |   |
+   |     |   first mutable borrow occurs here
+   |     first borrow later used by call
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0499, E0658.
+For more information about an error, try `rustc --explain E0499`.