Parse view type syntax, mark it as unstable

Author: scrabsha

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_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 }),

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

@@ -1,16 +1,17 @@
-//@ known-bug: #155938
-
 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

@@ -1,49 +1,33 @@
-error: expected parameter name, found `{`
-  --> $DIR/feature-gate-view-types.rs:8:20
+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 }) {
-   |                    ^ 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 `,`
+   = 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: expected parameter name, found `{`
-  --> $DIR/feature-gate-view-types.rs:8:39
+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 }) {
-   |                                       ^ 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 `,`
+   = 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[E0061]: this function takes 4 arguments but 2 arguments were supplied
-  --> $DIR/feature-gate-view-types.rs:15:5
+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);
-   |     ^^^-------------------- 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 */, /* _ */);
-   |                           +++++++++++++++++++++++++
+   |     --- --------  ^^^^^^^^ second mutable borrow occurs here
+   |     |   |
+   |     |   first mutable borrow occurs here
+   |     first borrow later used by call
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0061`.
+Some errors have detailed explanations: E0499, E0658.
+For more information about an error, try `rustc --explain E0499`.