make const_generics_defaults use the unstable syntax mechanism

This is important to not accidentally stabilize the parsing of the syntax while it still is experimental and not formally accepted

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1166,20 +1166,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     }
                 }
             }
-            if !self.session.features_untracked().const_generics_defaults {
-                if let GenericParamKind::Const { default: Some(ref default), .. } = param.kind {
-                    let mut err = self.err_handler().struct_span_err(
-                        default.value.span,
-                        "default values for const generic parameters are unstable",
-                    );
-                    err.help(
-                        "add `#![feature(const_generics_defaults)]` \
-                         to the crate attributes to enable",
-                    );
-                    err.emit();
-                    break;
-                }
-            }
         }
 
         validate_generic_param_order(

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -619,6 +619,10 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
         extended_key_value_attributes,
         "arbitrary expressions in key-value attributes are unstable"
     );
+    gate_all!(
+        const_generics_defaults,
+        "default values for const generic parameters are experimental"
+    );
     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.

compiler/rustc_parse/src/parser/generics.rs

@@ -5,7 +5,7 @@ use rustc_ast::{
     self as ast, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause,
 };
 use rustc_errors::PResult;
-use rustc_span::symbol::kw;
+use rustc_span::symbol::{kw, sym};
 
 impl<'a> Parser<'a> {
     /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
@@ -56,8 +56,19 @@ impl<'a> Parser<'a> {
         self.expect(&token::Colon)?;
         let ty = self.parse_ty()?;
 
-        // Parse optional const generics default value.
+        // Parse optional const generics default value, taking care of feature gating the spans
-        let default = if self.eat(&token::Eq) { Some(self.parse_const_arg()?) } else { None };
+        // with the unstable syntax mechanism.
+        let default = if self.eat(&token::Eq) {
+            // The gated span goes from the `=` to the end of the const argument that follows (and
+            // which could be a block expression).
+            let start = self.prev_token.span;
+            let const_arg = self.parse_const_arg()?;
+            let span = start.to(const_arg.value.span);
+            self.sess.gated_spans.gate(sym::const_generics_defaults, span);
+            Some(const_arg)
+        } else {
+            None
+        };
 
         Ok(GenericParam {
             ident,

src/test/ui/const-generics/min_const_generics/default_function_param.rs

@@ -1,4 +1,4 @@
 fn foo<const SIZE: usize = 5>() {}
-//~^ ERROR default values for const generic parameters are unstable
+//~^ ERROR default values for const generic parameters are experimental
 
 fn main() {}

src/test/ui/const-generics/min_const_generics/default_function_param.stderr

@@ -1,10 +1,12 @@
-error: default values for const generic parameters are unstable
+error[E0658]: default values for const generic parameters are experimental
-  --> $DIR/default_function_param.rs:1:28
+  --> $DIR/default_function_param.rs:1:26
    |
 LL | fn foo<const SIZE: usize = 5>() {}
-   |                            ^
+   |                          ^^^
    |
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
    = help: add `#![feature(const_generics_defaults)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/const-generics/min_const_generics/default_trait_param.rs

@@ -1,4 +1,4 @@
 trait Foo<const KIND: bool = true> {}
-//~^ ERROR default values for const generic parameters are unstable
+//~^ ERROR default values for const generic parameters are experimental
 
 fn main() {}

src/test/ui/const-generics/min_const_generics/default_trait_param.stderr

@@ -1,10 +1,12 @@
-error: default values for const generic parameters are unstable
+error[E0658]: default values for const generic parameters are experimental
-  --> $DIR/default_trait_param.rs:1:30
+  --> $DIR/default_trait_param.rs:1:28
    |
 LL | trait Foo<const KIND: bool = true> {}
-   |                              ^^^^
+   |                            ^^^^^^
    |
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
    = help: add `#![feature(const_generics_defaults)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/feature-gates/feature-gate-const_generics_defaults.rs

@@ -1,7 +1,9 @@
+#[cfg(FALSE)]
 struct A<const N: usize = 3>;
-//~^ ERROR default values for const generic parameters are unstable
+//~^ ERROR default values for const generic parameters are experimental
 
-fn foo<const N: u8 = 6>() {}
+#[cfg(FALSE)]
-//~^ ERROR default values for const generic parameters are unstable
+fn foo<const B: bool = false>() {}
+//~^ ERROR default values for const generic parameters are experimental
 
 fn main() {}

src/test/ui/feature-gates/feature-gate-const_generics_defaults.stderr

@@ -1,18 +1,21 @@
-error: default values for const generic parameters are unstable
+error[E0658]: default values for const generic parameters are experimental
-  --> $DIR/feature-gate-const_generics_defaults.rs:1:27
+  --> $DIR/feature-gate-const_generics_defaults.rs:2:25
    |
 LL | struct A<const N: usize = 3>;
-   |                           ^
+   |                         ^^^
    |
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
    = help: add `#![feature(const_generics_defaults)]` to the crate attributes to enable
 
-error: default values for const generic parameters are unstable
+error[E0658]: default values for const generic parameters are experimental
-  --> $DIR/feature-gate-const_generics_defaults.rs:4:22
+  --> $DIR/feature-gate-const_generics_defaults.rs:6:22
    |
-LL | fn foo<const N: u8 = 6>() {}
+LL | fn foo<const B: bool = false>() {}
-   |                      ^
+   |                      ^^^^^^^
    |
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
    = help: add `#![feature(const_generics_defaults)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0658`.