Rollup merge of #79016 - fanzier:underscore-expressions, r=petrochenkov

Make `_` an expression, to discard values in destructuring assignments

This is the third and final step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: #71126). This PR is the third and final part of #71156, which was split up to allow for easier review.

With this PR, an underscore `_` is parsed as an expression but is allowed *only* on the left-hand side of a destructuring assignment. There it simply discards a value, similarly to the wildcard `_` in patterns. For instance,
```rust
(a, _) = (1, 2)
```
will simply assign 1 to `a` and discard the 2. Note that for consistency,
```
_ = foo
```
is also allowed and equivalent to just `foo`.

Thanks to ````@varkor```` who helped with the implementation, particularly around pre-expansion gating.

r? ````@petrochenkov````

Author: jonas-schievink

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,
@@ -1324,6 +1325,8 @@ pub enum ExprKind {
     Index(P<Expr>, P<Expr>),
     /// A range (e.g., `1..2`, `1..`, `..2`, `1..=2`, `..=2`; and `..` in destructuring assingment).
     Range(Option<P<Expr>>, Option<P<Expr>>, RangeLimits),
+    /// An underscore, used in destructuring assignment to ignore a value.
+    Underscore,
 
     /// Variable reference, possibly containing `::` and/or type
     /// parameters (e.g., `foo::bar::<baz>`).

compiler/rustc_ast/src/mut_visit.rs

@@ -1232,6 +1232,7 @@ 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

@@ -806,6 +806,7 @@ 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_lowering/src/expr.rs

@@ -164,6 +164,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::Range(ref e1, ref e2, lims) => {
                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
                 }
+                ExprKind::Underscore => {
+                    self.sess
+                        .struct_span_err(
+                            e.span,
+                            "in expressions, `_` can only be used on the left-hand side of an assignment",
+                        )
+                        .span_label(e.span, "`_` not allowed here")
+                        .emit();
+                    hir::ExprKind::Err
+                }
                 ExprKind::Path(ref qself, ref path) => {
                     let qpath = self.lower_qpath(
                         e.id,
@@ -863,7 +873,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // Return early in case of an ordinary assignment.
         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
             match &lhs.kind {
-                ExprKind::Array(..) | ExprKind::Struct(..) | ExprKind::Tup(..) => false,
+                ExprKind::Array(..)
+                | ExprKind::Struct(..)
+                | ExprKind::Tup(..)
+                | ExprKind::Underscore => false,
                 // Check for tuple struct constructor.
                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
                 ExprKind::Paren(e) => {
@@ -943,6 +956,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         assignments: &mut Vec<hir::Stmt<'hir>>,
     ) -> &'hir hir::Pat<'hir> {
         match &lhs.kind {
+            // Underscore pattern.
+            ExprKind::Underscore => {
+                return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
+            }
             // Slice patterns.
             ExprKind::Array(elements) => {
                 let (pats, rest) =

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -630,7 +630,11 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(const_trait_impl, "const trait impls are experimental");
     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
     gate_all!(inline_const, "inline-const is experimental");
-    gate_all!(destructuring_assignment, "destructuring assignments are unstable");
+    if sess.parse_sess.span_diagnostic.err_count() == 0 {
+        // Errors for `destructuring_assignment` can get quite noisy, especially where `_` is
+        // involved, so we only emit errors where there are no other parsing errors.
+        gate_all!(destructuring_assignment, "destructuring assignments are unstable");
+    }
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -2067,6 +2067,7 @@ 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

@@ -1089,6 +1089,9 @@ impl<'a> Parser<'a> {
             self.parse_yield_expr(attrs)
         } else if self.eat_keyword(kw::Let) {
             self.parse_let_expr(attrs)
+        } else if self.eat_keyword(kw::Underscore) {
+            self.sess.gated_spans.gate(sym::destructuring_assignment, self.prev_token.span);
+            Ok(self.mk_expr(self.prev_token.span, ExprKind::Underscore, attrs))
         } else if !self.unclosed_delims.is_empty() && self.check(&token::Semi) {
             // Don't complain about bare semicolons after unclosed braces
             // recovery in order to keep the error count down. Fixing the

src/test/ui/cross/cross-file-errors/main.rs

@@ -3,5 +3,6 @@ mod underscore;
 
 fn main() {
     underscore!();
-    //~^ ERROR expected expression, found reserved identifier `_`
+    //~^ ERROR `_` can only be used on the left-hand side of an assignment
+    //~| ERROR destructuring assignments are unstable
 }

src/test/ui/cross/cross-file-errors/main.stderr

@@ -1,15 +1,31 @@
-error: expected expression, found reserved identifier `_`
+error[E0658]: destructuring assignments are unstable
   --> $DIR/underscore.rs:8:9
    |
 LL |         _
-   |         ^ expected expression
+   |         ^
    | 
   ::: $DIR/main.rs:5:5
    |
 LL |     underscore!();
    |     -------------- in this macro invocation
    |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to previous error
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/underscore.rs:8:9
+   |
+LL |         _
+   |         ^ `_` not allowed here
+   | 
+  ::: $DIR/main.rs:5:5
+   |
+LL |     underscore!();
+   |     -------------- in this macro invocation
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/destructuring-assignment/nested_destructure.rs

@@ -14,4 +14,7 @@ fn main() {
     Struct { a: TupleStruct((a, b), c), b: [d] } =
         Struct { a: TupleStruct((0, 1), 2), b: [3] };
     assert_eq!((a, b, c, d), (0, 1, 2, 3));
+
+    // unnested underscore: just discard
+    _ = 1;
 }