Auto merge of rust-lang#80547 - lqd:const_generics_defaults, r=varkor

In which we start to parse const generics defaults

As discussed in this [zulip topic](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/const.20generic.20defaults), this PR extracts the parsing parts from `@JulianKnodt's` PR rust-lang#75384 for a better user-experience using the newly stabilized `min_const_generics` (albeit temporary) as shown in rust-lang#80507: trying to use default values on const generics currently results in parse errors, as if the user didn't use the correct syntax (which is somewhat true but also misleading).

This PR extracts (and slightly modifies in a couple places) `@JulianKnodt's` parsing code (with attribution if I've done everything correctly), AST and HIR changes, and feature gate setup.

This feature is now marked as "incomplete" and thus will also print out the expected "const generics default values are unstable" error instead of a syntax error. Note that, as I've only extracted the parsing part, the actual feature will not work at all if enabled. There will be ICEs, and inference errors on the const generics default values themselves.

Fixes rust-lang#80507.

Once this merges, I'll:
- modify the const generics tracking issue to refer to the `const_generics_defaults` gate rather than the older temporary name it uses there.
- create the GH `F-const_generics_defaults` label

r? `@varkor`

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -368,6 +368,8 @@ pub enum GenericParamKind {
         ty: P<Ty>,
         /// Span of the `const` keyword.
         kw_span: Span,
+        /// Optional default value for the const generic param
+        default: Option<AnonConst>,
     },
 }
 

compiler/rustc_ast/src/mut_visit.rs

@@ -790,8 +790,9 @@ pub fn noop_flat_map_generic_param<T: MutVisitor>(
         GenericParamKind::Type { default } => {
             visit_opt(default, |default| vis.visit_ty(default));
         }
-        GenericParamKind::Const { ty, kw_span: _ } => {
+        GenericParamKind::Const { ty, kw_span: _, default } => {
             vis.visit_ty(ty);
+            visit_opt(default, |default| vis.visit_anon_const(default));
         }
     }
     smallvec![param]

compiler/rustc_ast/src/visit.rs

@@ -578,7 +578,12 @@ pub fn walk_generic_param<'a, V: Visitor<'a>>(visitor: &mut V, param: &'a Generi
     match param.kind {
         GenericParamKind::Lifetime => (),
         GenericParamKind::Type { ref default } => walk_list!(visitor, visit_ty, default),
-        GenericParamKind::Const { ref ty, .. } => visitor.visit_ty(ty),
+        GenericParamKind::Const { ref ty, ref default, .. } => {
+            visitor.visit_ty(ty);
+            if let Some(default) = default {
+                visitor.visit_anon_const(default);
+            }
+        }
     }
 }
 

compiler/rustc_ast_lowering/src/lib.rs

@@ -2242,13 +2242,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                 (hir::ParamName::Plain(param.ident), kind)
             }
-            GenericParamKind::Const { ref ty, kw_span: _ } => {
+            GenericParamKind::Const { ref ty, kw_span: _, ref default } => {
                 let ty = self
                     .with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
                         this.lower_ty(&ty, ImplTraitContext::disallowed())
                     });
+                let default = default.as_ref().map(|def| self.lower_anon_const(def));
 
-                (hir::ParamName::Plain(param.ident), hir::GenericParamKind::Const { ty })
+                (hir::ParamName::Plain(param.ident), hir::GenericParamKind::Const { ty, default })
             }
         };
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -733,7 +733,7 @@ fn validate_generic_param_order(
         let (ord_kind, ident) = match &param.kind {
             GenericParamKind::Lifetime => (ParamKindOrd::Lifetime, ident),
             GenericParamKind::Type { default: _ } => (ParamKindOrd::Type, ident),
-            GenericParamKind::Const { ref ty, kw_span: _ } => {
+            GenericParamKind::Const { ref ty, kw_span: _, default: _ } => {
                 let ty = pprust::ty_to_string(ty);
                 let unordered = sess.features_untracked().const_generics;
                 (ParamKindOrd::Const { unordered }, Some(format!("const {}: {}", param.ident, ty)))
@@ -774,8 +774,8 @@ fn validate_generic_param_order(
                 }
                 GenericParamKind::Type { default: None } => (),
                 GenericParamKind::Lifetime => (),
-                // FIXME(const_generics:defaults)
-                GenericParamKind::Const { ty: _, kw_span: _ } => (),
+                // FIXME(const_generics_defaults)
+                GenericParamKind::Const { ty: _, kw_span: _, default: _ } => (),
             }
             first = false;
         }

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_ast_pretty/src/pprust/state.rs

@@ -2668,13 +2668,17 @@ impl<'a> State<'a> {
                         s.print_type(default)
                     }
                 }
-                ast::GenericParamKind::Const { ref ty, kw_span: _ } => {
+                ast::GenericParamKind::Const { ref ty, kw_span: _, ref default } => {
                     s.word_space("const");
                     s.print_ident(param.ident);
                     s.s.space();
                     s.word_space(":");
                     s.print_type(ty);
-                    s.print_type_bounds(":", &param.bounds)
+                    s.print_type_bounds(":", &param.bounds);
+                    if let Some(ref _default) = default {
+                        // FIXME(const_generics_defaults): print the `default` value here
+                        todo!();
+                    }
                 }
             }
         });

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -145,7 +145,8 @@ fn inject_impl_of_structural_trait(
                 *default = None;
                 ast::GenericArg::Type(cx.ty_ident(span, param.ident))
             }
-            ast::GenericParamKind::Const { ty: _, kw_span: _ } => {
+            ast::GenericParamKind::Const { ty: _, kw_span: _, default } => {
+                *default = None;
                 ast::GenericArg::Const(cx.const_ident(span, param.ident))
             }
         })

compiler/rustc_feature/src/active.rs

@@ -581,7 +581,7 @@ declare_features! (
     /// Allows `if let` guard in match arms.
     (active, if_let_guard, "1.47.0", Some(51114), None),
 
-    /// Allows non-trivial generic constants which have to be manually propageted upwards.
+    /// Allows non-trivial generic constants which have to be manually propagated upwards.
     (active, const_evaluatable_checked, "1.48.0", Some(76560), None),
 
     /// Allows basic arithmetic on floating point types in a `const fn`.
@@ -623,6 +623,9 @@ declare_features! (
     /// `:pat2018` and `:pat2021` macro matchers.
     (active, edition_macro_pats, "1.51.0", Some(54883), None),
 
+    /// Allows const generics to have default values (e.g. `struct Foo<const N: usize = 3>(...);`).
+    (active, const_generics_defaults, "1.51.0", Some(44580), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -647,6 +650,7 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::repr128,
     sym::unsized_locals,
     sym::capture_disjoint_fields,
+    sym::const_generics_defaults,
 ];
 
 /// Some features are not allowed to be used together at the same time, if

compiler/rustc_hir/src/hir.rs

@@ -418,6 +418,8 @@ pub enum GenericParamKind<'hir> {
     },
     Const {
         ty: &'hir Ty<'hir>,
+        /// Optional default value for the const generic param
+        default: Option<AnonConst>,
     },
 }