stabilize pattern_parentheses feature

src/libsyntax/feature_gate.rs

@@ -412,9 +412,6 @@ declare_features! (
     // Multiple patterns with `|` in `if let` and `while let`
     (active, if_while_or_patterns, "1.26.0", Some(48215), None),
 
-    // Parentheses in patterns
-    (active, pattern_parentheses, "1.26.0", Some(51087), None),
-
     // Allows `#[repr(packed)]` attribute on structs
     (active, repr_packed, "1.26.0", Some(33158), None),
 
@@ -680,6 +677,8 @@ declare_features! (
     (accepted, extern_absolute_paths, "1.30.0", Some(44660), None),
     // Access to crate names passed via `--extern` through prelude
     (accepted, extern_prelude, "1.30.0", Some(44660), None),
+    // Parentheses in patterns
+    (accepted, pattern_parentheses, "1.31.0", Some(51087), None),
 );
 
 // If you change this, please modify src/doc/unstable-book as well. You must
@@ -1773,10 +1772,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 gate_feature_post!(&self, exclusive_range_pattern, pattern.span,
                                    "exclusive range pattern syntax is experimental");
             }
-            PatKind::Paren(..) => {
-                gate_feature_post!(&self, pattern_parentheses, pattern.span,
-                                   "parentheses in patterns are unstable");
-            }
             _ => {}
         }
         visit::walk_pat(self, pattern)

src/test/run-pass-fulldeps/ast_stmt_expr_attr.rs

@@ -284,9 +284,7 @@ fn run() {
     reject_stmt_parse("#[attr] #![attr] foo!{}");
 
     // FIXME: Allow attributes in pattern constexprs?
-    // would require parens in patterns to allow disambiguation...
-    // —which is now available under the `pattern_parentheses` feature gate
-    // (tracking issue #51087)
+    // note: requires parens in patterns to allow disambiguation
 
     reject_expr_parse("match 0 {
         0..=#[attr] 10 => ()

src/test/ui/run-pass/binding/pat-tuple-7.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 // run-pass
-#![feature(pattern_parentheses)]
 
 fn main() {
     match 0 {

src/test/ui/run-pass/binding/range-inclusive-pattern-precedence.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 // run-pass
-#![feature(box_patterns, pattern_parentheses)]
+#![feature(box_patterns)]
 
 const VALUE: usize = 21;