Allow _ as an expression

Author: fanzier

compiler/rustc_ast/src/ast.rs

@@ -1192,6 +1192,7 @@ impl Expr {
             ExprKind::Field(..) => ExprPrecedence::Field,
             ExprKind::Index(..) => ExprPrecedence::Index,
             ExprKind::Range(..) => ExprPrecedence::Range,
+            ExprKind::Underscore => ExprPrecedence::Path,
             ExprKind::Path(..) => ExprPrecedence::Path,
             ExprKind::AddrOf(..) => ExprPrecedence::AddrOf,
             ExprKind::Break(..) => ExprPrecedence::Break,
@@ -1314,6 +1315,8 @@ pub enum ExprKind {
     Index(P<Expr>, P<Expr>),
     /// A range (e.g., `1..2`, `1..`, `..2`, `1..=2`, `..=2`).
     Range(Option<P<Expr>>, Option<P<Expr>>, RangeLimits),
+    /// An underscore.
+    Underscore,
 
     /// Variable reference, possibly containing `::` and/or type
     /// parameters (e.g., `foo::bar::<baz>`).

compiler/rustc_ast/src/mut_visit.rs

@@ -1218,6 +1218,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
             visit_opt(e1, |e1| vis.visit_expr(e1));
             visit_opt(e2, |e2| vis.visit_expr(e2));
         }
+        ExprKind::Underscore => {}
         ExprKind::Path(qself, path) => {
             vis.visit_qself(qself);
             vis.visit_path(path);

compiler/rustc_ast/src/visit.rs

@@ -807,6 +807,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             walk_list!(visitor, visit_expr, start);
             walk_list!(visitor, visit_expr, end);
         }
+        ExprKind::Underscore => {}
         ExprKind::Path(ref maybe_qself, ref path) => {
             if let Some(ref qself) = *maybe_qself {
                 visitor.visit_ty(&qself.ty);

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -2069,6 +2069,7 @@ impl<'a> State<'a> {
                     self.print_expr_maybe_paren(e, fake_prec);
                 }
             }
+            ast::ExprKind::Underscore => self.s.word("_"),
             ast::ExprKind::Path(None, ref path) => self.print_path(path, true, 0),
             ast::ExprKind::Path(Some(ref qself), ref path) => self.print_qpath(path, qself, true),
             ast::ExprKind::Break(opt_label, ref opt_expr) => {

compiler/rustc_parse/src/parser/expr.rs

@@ -1116,6 +1116,8 @@ impl<'a> Parser<'a> {
             } else {
                 self.parse_lit_expr(attrs)
             }
+        } else if self.eat_keyword(kw::Underscore) {
+            Ok(self.mk_expr(self.prev_token.span, ExprKind::Underscore, attrs))
         } else {
             self.parse_lit_expr(attrs)
         }

src/test/ui/issues/issue-34334.rs

@@ -1,6 +1,12 @@
 fn main () {
     let sr: Vec<(u32, _, _) = vec![];
     //~^ ERROR expected one of `,` or `>`, found `=`
+    //~| ERROR expected value, found struct `Vec`
+    //~| ERROR expected value, found builtin type `u32`
+    //~| ERROR mismatched types
+    //~| ERROR invalid left-hand side of assignment
+    //~| ERROR expected expression, found reserved identifier `_`
+    //~| ERROR expected expression, found reserved identifier `_`
     let sr2: Vec<(u32, _, _)> = sr.iter().map(|(faction, th_sender, th_receiver)| {}).collect();
     //~^ ERROR a value of type `Vec<(u32, _, _)>` cannot be built
 }

src/test/ui/issues/issue-34334.stderr

@@ -6,14 +6,56 @@ LL |     let sr: Vec<(u32, _, _) = vec![];
    |         |
    |         while parsing the type for `sr`
 
-error[E0277]: a value of type `Vec<(u32, _, _)>` cannot be built from an iterator over elements of type `()`
-  --> $DIR/issue-34334.rs:4:87
+error[E0423]: expected value, found struct `Vec`
+  --> $DIR/issue-34334.rs:2:13
+   |
+LL |     let sr: Vec<(u32, _, _) = vec![];
+   |             ^^^ did you mean `Vec { /* fields */ }`?
+
+error[E0423]: expected value, found builtin type `u32`
+  --> $DIR/issue-34334.rs:2:18
+   |
+LL |     let sr: Vec<(u32, _, _) = vec![];
+   |                  ^^^ not a value
+
+error: expected expression, found reserved identifier `_`
+  --> $DIR/issue-34334.rs:2:23
+   |
+LL |     let sr: Vec<(u32, _, _) = vec![];
+   |                       ^ expected expression
+
+error: expected expression, found reserved identifier `_`
+  --> $DIR/issue-34334.rs:2:26
+   |
+LL |     let sr: Vec<(u32, _, _) = vec![];
+   |                          ^ expected expression
+
+error[E0308]: mismatched types
+  --> $DIR/issue-34334.rs:2:31
+   |
+LL |     let sr: Vec<(u32, _, _) = vec![];
+   |                               ^^^^^^ expected `bool`, found struct `std::vec::Vec`
+   |
+   = note: expected type `bool`
+            found struct `std::vec::Vec<_>`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0070]: invalid left-hand side of assignment
+  --> $DIR/issue-34334.rs:2:29
+   |
+LL |     let sr: Vec<(u32, _, _) = vec![];
+   |             --------------- ^
+   |             |
+   |             cannot assign to this expression
+
+error[E0599]: no method named `iter` found for unit type `()` in the current scope
+  --> $DIR/issue-34334.rs:10:36
    |
 LL |     let sr2: Vec<(u32, _, _)> = sr.iter().map(|(faction, th_sender, th_receiver)| {}).collect();
    |                                                                                       ^^^^^^^ value of type `Vec<(u32, _, _)>` cannot be built from `std::iter::Iterator<Item=()>`
    |
    = help: the trait `FromIterator<()>` is not implemented for `Vec<(u32, _, _)>`
 
-error: aborting due to 2 previous errors
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0277`.