view-types: store view types in the AST

compiler/rustc_ast/src/ast.rs

@@ -2565,6 +2565,8 @@ pub enum TyKind {
     /// Usually not written directly in user code but indirectly via the macro
     /// `core::field::field_of!(...)`.
     FieldOf(Box<Ty>, Option<Ident>, Ident),
+    /// A view of a type. `T.{ field_1, field_2 }`.
+    View(Box<Ty>, #[visitable(ignore)] ThinVec<Ident>),
     /// Sometimes we need a dummy value when no error has occurred.
     Dummy,
     /// Placeholder for a kind that has failed to be defined.
@@ -2929,6 +2931,12 @@ pub struct Param {
 /// Alternative representation for `Arg`s describing `self` parameter of methods.
 ///
 /// E.g., `&mut self` as in `fn foo(&mut self)`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct SelfParam {
+    pub kind: SelfKind,
+    pub view: ViewKind,
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum SelfKind {
     /// `self`, `mut self`
@@ -2955,28 +2963,48 @@ impl SelfKind {
     }
 }
 
-pub type ExplicitSelf = Spanned<SelfKind>;
+/// Represents how a type is viewed.
+#[derive(Clone, Encodable, Decodable, Debug, Walkable)]
+pub enum ViewKind {
+    /// `T`. All the fields can be observed.
+    Full,
+    /// `T.{ foo, bar }`. Only `foo` and `bar` can be observed.
+    // FIXME(scrabsha): in the future, we may want to express more complex situations, such as
+    // mutably observing some fields while immutably observing others. Something like
+    // `T.{ foo, mut bar }`.
+    Partial {
+        #[visitable(ignore)]
+        fields: ThinVec<Ident>,
+    },
+}
+
+pub type ExplicitSelf = Spanned<SelfParam>;
 
 impl Param {
     /// Attempts to cast parameter to `ExplicitSelf`.
     pub fn to_self(&self) -> Option<ExplicitSelf> {
         if let PatKind::Ident(BindingMode(ByRef::No, mutbl), ident, _) = self.pat.kind {
             if ident.name == kw::SelfLower {
-                return match self.ty.kind {
-                    TyKind::ImplicitSelf => Some(respan(self.pat.span, SelfKind::Value(mutbl))),
+                let (ty, view) = match &self.ty.kind {
+                    TyKind::View(ty, fields) => {
+                        (ty.as_ref(), ViewKind::Partial { fields: fields.clone() })
+                    }
+                    _ => (&*self.ty, ViewKind::Full),
+                };
+                let (kind, span) = match ty.kind {
+                    TyKind::ImplicitSelf => (SelfKind::Value(mutbl), self.pat.span),
                     TyKind::Ref(lt, MutTy { ref ty, mutbl }) if ty.kind.is_implicit_self() => {
-                        Some(respan(self.pat.span, SelfKind::Region(lt, mutbl)))
+                        (SelfKind::Region(lt, mutbl), self.pat.span)
                     }
                     TyKind::PinnedRef(lt, MutTy { ref ty, mutbl })
                         if ty.kind.is_implicit_self() =>
                     {
-                        Some(respan(self.pat.span, SelfKind::Pinned(lt, mutbl)))
+                        (SelfKind::Pinned(lt, mutbl), self.pat.span)
                     }
-                    _ => Some(respan(
-                        self.pat.span.to(self.ty.span),
-                        SelfKind::Explicit(self.ty.clone(), mutbl),
-                    )),
+                    _ => (SelfKind::Explicit(self.ty.clone(), mutbl), self.ty.span),
                 };
+                let param = respan(span, SelfParam { view, kind });
+                return Some(param);
             }
         }
         None
@@ -3000,7 +3028,7 @@ impl Param {
             span: eself_ident.span,
             tokens: None,
         });
-        let (mutbl, ty) = match eself.node {
+        let (mutbl, mut ty) = match eself.node.kind {
             SelfKind::Explicit(ty, mutbl) => (mutbl, ty),
             SelfKind::Value(mutbl) => (mutbl, infer_ty),
             SelfKind::Region(lt, mutbl) => (
@@ -3022,6 +3050,14 @@ impl Param {
                 }),
             ),
         };
+        if let ViewKind::Partial { fields } = eself.node.view {
+            ty = Box::new(Ty {
+                id: DUMMY_NODE_ID,
+                kind: TyKind::View(ty, fields),
+                span,
+                tokens: None,
+            });
+        }
         Param {
             attrs,
             pat: Box::new(Pat {

compiler/rustc_ast/src/util/classify.rs

@@ -167,6 +167,8 @@ pub enum TrailingBrace<'a> {
     /// Trailing brace in any other expression, such as `a + B {}`. We will
     /// suggest wrapping the innermost expression in parentheses: `a + (B {})`.
     Expr(&'a ast::Expr),
+    /// Trailing brace in a type, like the one in `&Foo.{ bar }`
+    Type(&'a ast::Ty),
 }
 
 /// If an expression ends with `}`, returns the innermost expression ending in the `}`
@@ -205,7 +207,7 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
             | ConstBlock(_) => break Some(TrailingBrace::Expr(expr)),
 
             Cast(_, ty) => {
-                break type_trailing_braced_mac_call(ty).map(TrailingBrace::MacCall);
+                break type_trailing_brace(ty);
             }
 
             MacCall(mac) => {
@@ -246,13 +248,17 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
     }
 }
 
-/// If the type's last token is `}`, it must be due to a braced macro call, such
-/// as in `*const brace! { ... }`. Returns that trailing macro call.
-fn type_trailing_braced_mac_call(mut ty: &ast::Ty) -> Option<&ast::MacCall> {
+/// If a type ends with `}`, return the innermost node ending with the `}`.
+///
+/// This may be caused by a macro call (`*const brace! { ... }`) or a view type
+/// (`&Foo.{ bar }`).
+fn type_trailing_brace(mut ty: &ast::Ty) -> Option<TrailingBrace<'_>> {
     loop {
         match &ty.kind {
             ast::TyKind::MacCall(mac) => {
-                break (mac.args.delim == Delimiter::Brace).then_some(mac);
+                break (mac.args.delim == Delimiter::Brace)
+                    .then_some(mac.as_ref())
+                    .map(TrailingBrace::MacCall);
             }
 
             ast::TyKind::Ptr(mut_ty)
@@ -261,6 +267,8 @@ fn type_trailing_braced_mac_call(mut ty: &ast::Ty) -> Option<&ast::MacCall> {
                 ty = &mut_ty.ty;
             }
 
+            ast::TyKind::View(..) => break Some(TrailingBrace::Type(ty)),
+
             ast::TyKind::UnsafeBinder(binder) => {
                 ty = &binder.inner_ty;
             }

compiler/rustc_ast/src/visit.rs

@@ -486,6 +486,7 @@ macro_rules! common_visitor_and_walkers {
             UnsafeBinderTy,
             UnsafeSource,
             UseTreeKind,
+            ViewKind,
             VisibilityKind,
             WhereBoundPredicate,
             WhereClause,

compiler/rustc_ast_lowering/src/lib.rs

@@ -1594,6 +1594,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 );
                 hir::TyKind::Err(guar)
             }
+            TyKind::View(ty, _) => {
+                // FIXME(scrabsha): lower view types to HIR.
+                return self.lower_ty(ty, itctx);
+            }
             TyKind::Dummy => panic!("`TyKind::Dummy` should never be lowered"),
         };
 

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -17,7 +17,7 @@ use rustc_ast::util::comments::{Comment, CommentStyle};
 use rustc_ast::{
     self as ast, AttrArgs, BindingMode, BlockCheckMode, ByRef, DelimArgs, GenericArg, GenericBound,
     InlineAsmOperand, InlineAsmOptions, InlineAsmRegOrRegClass, InlineAsmTemplatePiece, PatKind,
-    RangeEnd, RangeSyntax, Safety, SelfKind, Term, attr,
+    RangeEnd, RangeSyntax, Safety, SelfKind, Term, ViewKind, attr,
 };
 use rustc_span::edition::Edition;
 use rustc_span::source_map::SourceMap;
@@ -1255,6 +1255,20 @@ impl<'a> State<'a> {
         }
     }
 
+    fn print_view(&mut self, fields: &[Ident]) {
+        self.word(".{");
+
+        if !fields.is_empty() {
+            self.space();
+            self.commasep(Consistent, fields, |s, field| {
+                s.print_ident(*field);
+            });
+            self.space();
+        }
+
+        self.word("}");
+    }
+
     pub fn print_assoc_item_constraint(&mut self, constraint: &ast::AssocItemConstraint) {
         self.print_ident(constraint.ident);
         if let Some(args) = constraint.gen_args.as_ref() {
@@ -1437,6 +1451,10 @@ impl<'a> State<'a> {
                 self.end(ib);
                 self.pclose();
             }
+            ast::TyKind::View(ty, fields) => {
+                self.print_type(ty);
+                self.print_view(fields);
+            }
         }
         self.end(ib);
     }
@@ -2053,7 +2071,7 @@ impl<'a> State<'a> {
     }
 
     fn print_explicit_self(&mut self, explicit_self: &ast::ExplicitSelf) {
-        match &explicit_self.node {
+        match &explicit_self.node.kind {
             SelfKind::Value(m) => {
                 self.print_mutability(*m, false);
                 self.word("self")
@@ -2078,6 +2096,10 @@ impl<'a> State<'a> {
                 self.print_type(typ)
             }
         }
+
+        if let ViewKind::Partial { fields } = &explicit_self.node.view {
+            self.print_view(fields);
+        }
     }
 
     fn print_coroutine_kind(&mut self, coroutine_kind: ast::CoroutineKind) {

compiler/rustc_ast_pretty/src/pprust/tests.rs

@@ -1,6 +1,6 @@
 use rustc_ast as ast;
 use rustc_span::{DUMMY_SP, Ident, create_default_session_globals_then};
-use thin_vec::ThinVec;
+use thin_vec::{ThinVec, thin_vec};
 
 use super::*;
 
@@ -22,6 +22,12 @@ fn variant_to_string(var: &ast::Variant) -> String {
     to_string(|s| s.print_variant(var))
 }
 
+fn ty_to_string(ty: &ast::Ty) -> String {
+    to_string(|s| {
+        s.print_type(ty);
+    })
+}
+
 #[test]
 fn test_fun_to_string() {
     create_default_session_globals_then(|| {
@@ -60,3 +66,26 @@ fn test_variant_to_string() {
         assert_eq!(varstr, "principal_skinner");
     })
 }
+
+#[test]
+fn test_field_view() {
+    create_default_session_globals_then(|| {
+        let ty = ast::Ty {
+            id: ast::DUMMY_NODE_ID,
+            kind: ast::TyKind::View(
+                Box::new(ast::Ty {
+                    id: ast::DUMMY_NODE_ID,
+                    kind: ast::TyKind::Dummy,
+                    span: DUMMY_SP,
+                    tokens: None,
+                }),
+                thin_vec![Ident::from_str("milhouse"), Ident::from_str("apu")],
+            ),
+            span: DUMMY_SP,
+            tokens: None,
+        };
+
+        let ty_str = ty_to_string(&ty);
+        assert_eq!(ty_str, "(/*DUMMY*/).{ milhouse, apu }");
+    });
+}

compiler/rustc_builtin_macros/src/deriving/generic/ty.rs

@@ -2,7 +2,9 @@
 //! when specifying impls to be derived.
 
 pub(crate) use Ty::*;
-use rustc_ast::{self as ast, Expr, GenericArg, GenericParamKind, Generics, SelfKind, TyKind};
+use rustc_ast::{
+    self as ast, Expr, GenericArg, GenericParamKind, Generics, SelfKind, TyKind, ViewKind,
+};
 use rustc_expand::base::ExtCtxt;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, respan};
 use thin_vec::ThinVec;
@@ -200,6 +202,8 @@ impl Bounds {
 pub(crate) fn get_explicit_self(cx: &ExtCtxt<'_>, span: Span) -> (Box<Expr>, ast::ExplicitSelf) {
     // This constructs a fresh `self` path.
     let self_path = cx.expr_self(span);
-    let self_ty = respan(span, SelfKind::Region(None, ast::Mutability::Not));
+    let kind = SelfKind::Region(None, ast::Mutability::Not);
+    let view = ViewKind::Full;
+    let self_ty = respan(span, ast::SelfParam { kind, view });
     (self_path, self_ty)
 }

compiler/rustc_parse/src/errors.rs

@@ -1058,11 +1058,9 @@ pub(crate) struct LabeledLoopInBreak {
 
 #[derive(Subdiagnostic)]
 pub(crate) enum WrapInParentheses {
-    #[multipart_suggestion(
-        "wrap the expression in parentheses",
-        applicability = "machine-applicable"
-    )]
-    Expression {
+    #[multipart_suggestion("wrap the {$kind} in parentheses", applicability = "machine-applicable")]
+    NonMacro {
+        kind: WrapInParenthesesNodeKind,
         #[suggestion_part(code = "(")]
         left: Span,
         #[suggestion_part(code = ")")]
@@ -1080,6 +1078,22 @@ pub(crate) enum WrapInParentheses {
     },
 }
 
+pub(crate) enum WrapInParenthesesNodeKind {
+    Expression,
+    Type,
+}
+
+impl IntoDiagArg for WrapInParenthesesNodeKind {
+    fn into_diag_arg(self, _path: &mut Option<PathBuf>) -> DiagArgValue {
+        let str = match self {
+            WrapInParenthesesNodeKind::Expression => "expression",
+            WrapInParenthesesNodeKind::Type => "type",
+        };
+
+        DiagArgValue::Str(Cow::Borrowed(str))
+    }
+}
+
 #[derive(Diagnostic)]
 #[diag("this is a block expression, not an array")]
 pub(crate) struct ArrayBracketsInsteadOfBraces {

compiler/rustc_parse/src/parser/expr.rs

@@ -1900,7 +1900,8 @@ impl<'a> Parser<'a> {
             let lexpr = self.parse_expr_labeled(label, true)?;
             self.dcx().emit_err(errors::LabeledLoopInBreak {
                 span: lexpr.span,
-                sub: errors::WrapInParentheses::Expression {
+                sub: errors::WrapInParentheses::NonMacro {
+                    kind: errors::WrapInParenthesesNodeKind::Expression,
                     left: lexpr.span.shrink_to_lo(),
                     right: lexpr.span.shrink_to_hi(),
                 },