Reject defaultness on free consts

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1008,12 +1008,14 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     _ => {}
                 }
             }
-            ItemKind::Const(box ConstItem { defaultness, expr: None, .. }) => {
+            ItemKind::Const(box ConstItem { defaultness, expr, .. }) => {
                 self.check_defaultness(item.span, *defaultness);
-                self.session.emit_err(errors::ConstWithoutBody {
-                    span: item.span,
-                    replace_span: self.ending_semi_or_hi(item.span),
-                });
+                if expr.is_none() {
+                    self.session.emit_err(errors::ConstWithoutBody {
+                        span: item.span,
+                        replace_span: self.ending_semi_or_hi(item.span),
+                    });
+                }
             }
             ItemKind::Static(box StaticItem { expr: None, .. }) => {
                 self.session.emit_err(errors::StaticWithoutBody {

tests/ui/parser/defaultness-invalid-places-fail-semantic.rs

@@ -0,0 +1,12 @@
+#![feature(specialization)] //~ WARN the feature `specialization` is incomplete
+
+fn main() {}
+
+trait X {
+    default const A: u8; //~ ERROR `default` is only allowed on items in trait impls
+    default const B: u8 = 0;  //~ ERROR `default` is only allowed on items in trait impls
+    default type D; //~ ERROR `default` is only allowed on items in trait impls
+    default type C: Ord; //~ ERROR `default` is only allowed on items in trait impls
+    default fn f1(); //~ ERROR `default` is only allowed on items in trait impls
+    default fn f2() {} //~ ERROR `default` is only allowed on items in trait impls
+}

tests/ui/parser/defaultness-invalid-places-fail-semantic.stderr

@@ -0,0 +1,60 @@
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:6:5
+   |
+LL |     default const A: u8;
+   |     -------^^^^^^^^^^^^^
+   |     |
+   |     `default` because of this
+
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:7:5
+   |
+LL |     default const B: u8 = 0;
+   |     -------^^^^^^^^^^^^^^^^^
+   |     |
+   |     `default` because of this
+
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:8:5
+   |
+LL |     default type D;
+   |     -------^^^^^^^^
+   |     |
+   |     `default` because of this
+
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:9:5
+   |
+LL |     default type C: Ord;
+   |     -------^^^^^^^^^^^^^
+   |     |
+   |     `default` because of this
+
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:10:5
+   |
+LL |     default fn f1();
+   |     -------^^^^^^^^^
+   |     |
+   |     `default` because of this
+
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:11:5
+   |
+LL |     default fn f2() {}
+   |     -------^^^^^^^^
+   |     |
+   |     `default` because of this
+
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:1:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+   = help: consider using `min_specialization` instead, which is more stable and complete
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: aborting due to 6 previous errors; 1 warning emitted
+

tests/ui/parser/trait-item-with-defaultness-fail-semantic.rs

@@ -7,6 +7,10 @@ trait X {
     default const B: u8 = 0;  //~ ERROR `default` is only allowed on items in trait impls
     default type D; //~ ERROR `default` is only allowed on items in trait impls
     default type C: Ord; //~ ERROR `default` is only allowed on items in trait impls
-    default fn f1(); //~ ERROR `default` is only allowed on items in trait impls
-    default fn f2() {} //~ ERROR `default` is only allowed on items in trait impls
+    default fn f(); //~ ERROR `default` is only allowed on items in trait impls
+    default fn g() {} //~ ERROR `default` is only allowed on items in trait impls
 }
+
+default const E: u8 = 0; //~ ERROR `default` is only allowed on items in trait impls
+default type F = (); //~ ERROR `default` is only allowed on items in trait impls
+default fn h() {} //~ ERROR `default` is only allowed on items in trait impls

tests/ui/parser/trait-item-with-defaultness-fail-semantic.stderr

@@ -33,19 +33,43 @@ LL |     default type C: Ord;
 error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:10:5
    |
-LL |     default fn f1();
-   |     -------^^^^^^^^^
+LL |     default fn f();
+   |     -------^^^^^^^^
    |     |
    |     `default` because of this
 
 error: `default` is only allowed on items in trait impls
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:11:5
    |
-LL |     default fn f2() {}
-   |     -------^^^^^^^^
+LL |     default fn g() {}
+   |     -------^^^^^^^
    |     |
    |     `default` because of this
 
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:14:1
+   |
+LL | default const E: u8 = 0;
+   | -------^^^^^^^^^^^^^^^^^
+   | |
+   | `default` because of this
+
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:15:1
+   |
+LL | default type F = ();
+   | -------^^^^^^^^^^^^^
+   | |
+   | `default` because of this
+
+error: `default` is only allowed on items in trait impls
+  --> $DIR/trait-item-with-defaultness-fail-semantic.rs:16:1
+   |
+LL | default fn h() {}
+   | -------^^^^^^^
+   | |
+   | `default` because of this
+
 warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/trait-item-with-defaultness-fail-semantic.rs:1:12
    |
@@ -56,5 +80,5 @@ LL | #![feature(specialization)]
    = help: consider using `min_specialization` instead, which is more stable and complete
    = note: `#[warn(incomplete_features)]` on by default
 
-error: aborting due to 6 previous errors; 1 warning emitted
+error: aborting due to 9 previous errors; 1 warning emitted