Add pre-expansion gating

Author: varkor

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -630,6 +630,7 @@ 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");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).

compiler/rustc_parse/src/parser/expr.rs

@@ -1117,6 +1117,7 @@ impl<'a> Parser<'a> {
                 self.parse_lit_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 {
             self.parse_lit_expr(attrs)
@@ -2106,6 +2107,7 @@ impl<'a> Parser<'a> {
                 let exp_span = self.prev_token.span;
                 // We permit `.. }` on the left-hand side of a destructuring assignment.
                 if self.token == token::CloseDelim(token::Brace) {
+                    self.sess.gated_spans.gate(sym::destructuring_assignment, self.prev_token.span);
                     base = StructRest::Rest(self.prev_token.span.shrink_to_hi());
                     break;
                 }

src/test/ui/destructuring-assignment/underscore-range-expr-gating.rs

@@ -0,0 +1,10 @@
+fn main() {}
+
+struct S { x : u32 }
+
+#[cfg(FALSE)]
+fn foo() {
+    _; //~ ERROR destructuring assignments are unstable
+
+    S { x: 5, .. }; //~ ERROR destructuring assignments are unstable
+}

src/test/ui/destructuring-assignment/underscore-range-expr-gating.stderr

@@ -0,0 +1,21 @@
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/underscore-range-expr-gating.rs:7:5
+   |
+LL |     _;
+   |     ^
+   |
+   = 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
+
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/underscore-range-expr-gating.rs:9:15
+   |
+LL |     S { x: 5, .. };
+   |               ^^
+   |
+   = 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
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.