Auto merge of #45175 - petrochenkov:dyn, r=nikomatsakis

Implement `dyn Trait` syntax (RFC 2113)

cc https://github.com/rust-lang/rust/issues/44662
r? @nikomatsakis

7 files changed, 67 insertions, 32 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 021f96aff8c..f445def9e03 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1419,7 +1419,7 @@ pub enum TyKind {
     Path(Option<QSelf>, Path),
     /// A trait object type `Bound1 + Bound2 + Bound3`
     /// where `Bound` is a trait or a lifetime.
-    TraitObject(TyParamBounds),
+    TraitObject(TyParamBounds, TraitObjectSyntax),
     /// An `impl Bound1 + Bound2 + Bound3` type
     /// where `Bound` is a trait or a lifetime.
     ImplTrait(TyParamBounds),
@@ -1438,6 +1438,13 @@ pub enum TyKind {
     Err,
 }
 
+/// Syntax used to declare a trait object.
+#[derive(Clone, Copy, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
+pub enum TraitObjectSyntax {
+    Dyn,
+    None,
+}
+
 /// Inline assembly dialect.
 ///
 /// E.g. `"intel"` as in `asm!("mov eax, 2" : "={eax}"(result) : : : "intel")``
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index a2f29181a20..9060a613bc1 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -398,6 +398,9 @@ declare_features! (
 
     // Default match binding modes (RFC 2005)
     (active, match_default_bindings, "1.22.0", Some(42640)),
+
+    // Trait object syntax with `dyn` prefix
+    (active, dyn_trait, "1.22.0", Some(44662)),
 );
 
 declare_features! (
@@ -1417,6 +1420,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 gate_feature_post!(&self, never_type, ty.span,
                                    "The `!` type is experimental");
             },
+            ast::TyKind::TraitObject(_, ast::TraitObjectSyntax::Dyn) => {
+                gate_feature_post!(&self, dyn_trait, ty.span,
+                                   "`dyn Trait` syntax is unstable");
+            }
             _ => {}
         }
         visit::walk_ty(self, ty)
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 03c47b71d02..d7d491db71f 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -400,8 +400,8 @@ pub fn noop_fold_ty<T: Folder>(t: P<Ty>, fld: &mut T) -> P<Ty> {
             TyKind::Typeof(expr) => {
                 TyKind::Typeof(fld.fold_expr(expr))
             }
-            TyKind::TraitObject(bounds) => {
-                TyKind::TraitObject(bounds.move_map(|b| fld.fold_ty_param_bound(b)))
+            TyKind::TraitObject(bounds, syntax) => {
+                TyKind::TraitObject(bounds.move_map(|b| fld.fold_ty_param_bound(b)), syntax)
             }
             TyKind::ImplTrait(bounds) => {
                 TyKind::ImplTrait(bounds.move_map(|b| fld.fold_ty_param_bound(b)))
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 04e6cdcfe58..bd400ef6dd6 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -33,7 +33,7 @@ use ast::{Stmt, StmtKind};
 use ast::{VariantData, StructField};
 use ast::StrStyle;
 use ast::SelfKind;
-use ast::{TraitItem, TraitRef};
+use ast::{TraitItem, TraitRef, TraitObjectSyntax};
 use ast::{Ty, TyKind, TypeBinding, TyParam, TyParamBounds};
 use ast::{ViewPath, ViewPathGlob, ViewPathList, ViewPathSimple};
 use ast::{Visibility, WhereClause};
@@ -364,6 +364,13 @@ fn is_ident_or_underscore(t: &token::Token) -> bool {
     t.is_ident() || *t == token::Underscore
 }
 
+// Returns true if `IDENT t` can start a type - `IDENT::a::b`, `IDENT<u8, u8>`,
+// `IDENT<<u8 as Trait>::AssocTy>`, `IDENT(u8, u8) -> u8`.
+fn can_continue_type_after_ident(t: &token::Token) -> bool {
+    t == &token::ModSep || t == &token::Lt ||
+    t == &token::BinOp(token::Shl) || t == &token::OpenDelim(token::Paren)
+}
+
 /// Information about the path to a module.
 pub struct ModulePath {
     pub name: String,
@@ -1428,7 +1435,7 @@ impl<'a> Parser<'a> {
                     TyKind::Path(None, ref path) if maybe_bounds => {
                         self.parse_remaining_bounds(Vec::new(), path.clone(), lo, true)?
                     }
-                    TyKind::TraitObject(ref bounds)
+                    TyKind::TraitObject(ref bounds, TraitObjectSyntax::None)
                             if maybe_bounds && bounds.len() == 1 && !trailing_plus => {
                         let path = match bounds[0] {
                             TraitTyParamBound(ref pt, ..) => pt.trait_ref.path.clone(),
@@ -1472,27 +1479,6 @@ impl<'a> Parser<'a> {
         } else if self.eat(&token::Underscore) {
             // A type to be inferred `_`
             TyKind::Infer
-        } else if self.eat_lt() {
-            // Qualified path
-            let (qself, path) = self.parse_qpath(PathStyle::Type)?;
-            TyKind::Path(Some(qself), path)
-        } else if self.token.is_path_start() {
-            // Simple path
-            let path = self.parse_path(PathStyle::Type)?;
-            if self.eat(&token::Not) {
-                // Macro invocation in type position
-                let (_, tts) = self.expect_delimited_token_tree()?;
-                TyKind::Mac(respan(lo.to(self.span), Mac_ { path: path, tts: tts }))
-            } else {
-                // Just a type path or bound list (trait object type) starting with a trait.
-                //   `Type`
-                //   `Trait1 + Trait2 + 'a`
-                if allow_plus && self.check(&token::BinOp(token::Plus)) {
-                    self.parse_remaining_bounds(Vec::new(), path, lo, true)?
-                } else {
-                    TyKind::Path(None, path)
-                }
-            }
         } else if self.token_is_bare_fn_keyword() {
             // Function pointer type
             self.parse_ty_bare_fn(Vec::new())?
@@ -1512,10 +1498,37 @@ impl<'a> Parser<'a> {
         } else if self.eat_keyword(keywords::Impl) {
             // FIXME: figure out priority of `+` in `impl Trait1 + Trait2` (#34511).
             TyKind::ImplTrait(self.parse_ty_param_bounds()?)
+        } else if self.check_keyword(keywords::Dyn) &&
+                  self.look_ahead(1, |t| t.can_begin_bound() && !can_continue_type_after_ident(t)) {
+            // FIXME: figure out priority of `+` in `dyn Trait1 + Trait2` (#34511).
+            self.bump(); // `dyn`
+            TyKind::TraitObject(self.parse_ty_param_bounds()?, TraitObjectSyntax::Dyn)
         } else if self.check(&token::Question) ||
-                  self.check_lifetime() && self.look_ahead(1, |t| t == &token::BinOp(token::Plus)){
+                  self.check_lifetime() && self.look_ahead(1, |t| t == &token::BinOp(token::Plus)) {
             // Bound list (trait object type)
-            TyKind::TraitObject(self.parse_ty_param_bounds_common(allow_plus)?)
+            TyKind::TraitObject(self.parse_ty_param_bounds_common(allow_plus)?,
+                                TraitObjectSyntax::None)
+        } else if self.eat_lt() {
+            // Qualified path
+            let (qself, path) = self.parse_qpath(PathStyle::Type)?;
+            TyKind::Path(Some(qself), path)
+        } else if self.token.is_path_start() {
+            // Simple path
+            let path = self.parse_path(PathStyle::Type)?;
+            if self.eat(&token::Not) {
+                // Macro invocation in type position
+                let (_, tts) = self.expect_delimited_token_tree()?;
+                TyKind::Mac(respan(lo.to(self.span), Mac_ { path: path, tts: tts }))
+            } else {
+                // Just a type path or bound list (trait object type) starting with a trait.
+                //   `Type`
+                //   `Trait1 + Trait2 + 'a`
+                if allow_plus && self.check(&token::BinOp(token::Plus)) {
+                    self.parse_remaining_bounds(Vec::new(), path, lo, true)?
+                } else {
+                    TyKind::Path(None, path)
+                }
+            }
         } else {
             let msg = format!("expected type, found {}", self.this_token_descr());
             return Err(self.fatal(&msg));
@@ -1538,7 +1551,7 @@ impl<'a> Parser<'a> {
             self.bump(); // `+`
             bounds.append(&mut self.parse_ty_param_bounds()?);
         }
-        Ok(TyKind::TraitObject(bounds))
+        Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
     }
 
     fn maybe_recover_from_bad_type_plus(&mut self, allow_plus: bool, ty: &Ty) -> PResult<'a, ()> {
@@ -4256,6 +4269,7 @@ impl<'a> Parser<'a> {
     fn parse_ty_param_bounds_common(&mut self, allow_plus: bool) -> PResult<'a, TyParamBounds> {
         let mut bounds = Vec::new();
         loop {
+            // This needs to be syncronized with `Token::can_begin_bound`.
             let is_bound_start = self.check_path() || self.check_lifetime() ||
                                  self.check(&token::Question) ||
                                  self.check_keyword(keywords::For) ||
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 4888654fac9..20db87cfc82 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -258,6 +258,12 @@ impl Token {
         }
     }
 
+    /// Returns `true` if the token can appear at the start of a generic bound.
+    pub fn can_begin_bound(&self) -> bool {
+        self.is_path_start() || self.is_lifetime() || self.is_keyword(keywords::For) ||
+        self == &Question || self == &OpenDelim(Paren)
+    }
+
     /// Returns `true` if the token is any literal
     pub fn is_lit(&self) -> bool {
         match *self {
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 959dd4ef30f..02f871c58c7 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1049,8 +1049,9 @@ impl<'a> State<'a> {
             ast::TyKind::Path(Some(ref qself), ref path) => {
                 self.print_qpath(path, qself, false)?
             }
-            ast::TyKind::TraitObject(ref bounds) => {
-                self.print_bounds("", &bounds[..])?;
+            ast::TyKind::TraitObject(ref bounds, syntax) => {
+                let prefix = if syntax == ast::TraitObjectSyntax::Dyn { "dyn " } else { "" };
+                self.print_bounds(prefix, &bounds[..])?;
             }
             ast::TyKind::ImplTrait(ref bounds) => {
                 self.print_bounds("impl ", &bounds[..])?;
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 05077d42a0b..521c6030eba 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -348,7 +348,7 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
             visitor.visit_ty(ty);
             visitor.visit_expr(expression)
         }
-        TyKind::TraitObject(ref bounds) |
+        TyKind::TraitObject(ref bounds, ..) |
         TyKind::ImplTrait(ref bounds) => {
             walk_list!(visitor, visit_ty_param_bound, bounds);
         }