progress, stuff compiles now

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1150,20 +1150,23 @@ fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
     }
 
     fn visit_generics(&mut self, generics: &'a Generics) {
-        let mut prev_ty_default = None;
+        let cg_defaults = self.session.features_untracked().const_generics_defaults;
+
+        let mut prev_param_default = None;
         for param in &generics.params {
             match param.kind {
                 GenericParamKind::Lifetime => (),
-                GenericParamKind::Type { default: Some(_), .. } => {
-                    prev_ty_default = Some(param.ident.span);
+                GenericParamKind::Type { default: Some(_), .. }
+                | GenericParamKind::Const { default: Some(_), .. } => {
+                    prev_param_default = Some(param.ident.span);
                 }
                 GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
-                    if let Some(span) = prev_ty_default {
+                    if let Some(span) = prev_param_default {
                         let mut err = self.err_handler().struct_span_err(
                             span,
-                            "type parameters with a default must be trailing",
+                            "generic parameters with a default must be trailing",
                         );
-                        if matches!(param.kind, GenericParamKind::Const { .. }) {
+                        if matches!(param.kind, GenericParamKind::Const { .. }) && !cg_defaults {
                             err.note(
                                 "using type defaults and const parameters \
                                  in the same parameter list is currently not permitted",
@@ -1174,17 +1177,6 @@ fn visit_generics(&mut self, generics: &'a Generics) {
                     }
                 }
             }
-            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_generic_defaults)]` to the crate attributes to enable");
-                    err.emit();
-                    break;
-                }
-            }
         }
 
         validate_generic_param_order(

compiler/rustc_error_codes/src/error_codes/E0128.md

@@ -7,7 +7,7 @@ struct Foo<T = U, U = ()> {
     field1: T,
     field2: U,
 }
-// error: type parameters with a default cannot use forward declared
+// error: generic parameters with a default cannot use forward declared
 //        identifiers
 ```
 

compiler/rustc_hir/src/intravisit.rs

@@ -366,6 +366,9 @@ fn visit_ty(&mut self, t: &'v Ty<'v>) {
     fn visit_generic_param(&mut self, p: &'v GenericParam<'v>) {
         walk_generic_param(self, p)
     }
+    fn visit_const_param_default(&mut self, _param: HirId, ct: &'v AnonConst) {
+        walk_const_param_default(self, ct)
+    }
     fn visit_generics(&mut self, g: &'v Generics<'v>) {
         walk_generics(self, g)
     }
@@ -869,13 +872,17 @@ pub fn walk_generic_param<'v, V: Visitor<'v>>(visitor: &mut V, param: &'v Generi
         GenericParamKind::Const { ref ty, ref default } => {
             visitor.visit_ty(ty);
             if let Some(ref default) = default {
-                visitor.visit_anon_const(default);
+                visitor.visit_const_param_default(param.hir_id, default);
             }
         }
     }
     walk_list!(visitor, visit_param_bound, param.bounds);
 }
 
+pub fn walk_const_param_default<'v, V: Visitor<'v>>(visitor: &mut V, ct: &'v AnonConst) {
+    visitor.visit_anon_const(ct)
+}
+
 pub fn walk_generics<'v, V: Visitor<'v>>(visitor: &mut V, generics: &'v Generics<'v>) {
     walk_list!(visitor, visit_generic_param, generics.params);
     walk_list!(visitor, visit_where_predicate, generics.where_clause.predicates);

compiler/rustc_middle/src/hir/map/collector.rs

@@ -395,6 +395,10 @@ fn visit_generic_param(&mut self, param: &'hir GenericParam<'hir>) {
         }
     }
 
+    fn visit_const_param_default(&mut self, param: HirId, ct: &'hir AnonConst) {
+        self.with_parent(param, |this| intravisit::walk_const_param_default(this, ct))
+    }
+
     fn visit_trait_item(&mut self, ti: &'hir TraitItem<'hir>) {
         self.with_dep_node_owner(ti.def_id, ti, |this, hash| {
             this.insert_with_hash(ti.span, ti.hir_id(), Node::TraitItem(ti), hash);

compiler/rustc_middle/src/ty/consts.rs

@@ -44,7 +44,11 @@ pub fn from_opt_const_arg_anon_const(
         let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
 
         let body_id = match tcx.hir().get(hir_id) {
-            hir::Node::AnonConst(ac) => ac.body,
+            hir::Node::AnonConst(ac)
+            | hir::Node::GenericParam(hir::GenericParam {
+                kind: hir::GenericParamKind::Const { ty: _, default: Some(ac) },
+                ..
+            }) => ac.body,
             _ => span_bug!(
                 tcx.def_span(def.did.to_def_id()),
                 "from_anon_const can only process anonymous constants"

compiler/rustc_resolve/src/lib.rs

@@ -228,7 +228,7 @@ enum ResolutionError<'a> {
     ),
     /// Error E0530: `X` bindings cannot shadow `Y`s.
     BindingShadowsSomethingUnacceptable(&'static str, Symbol, &'a NameBinding<'a>),
-    /// Error E0128: type parameters with a default cannot use forward-declared identifiers.
+    /// Error E0128: generic parameters with a default cannot use forward-declared identifiers.
     ForwardDeclaredTyParam, // FIXME(const_generics_defaults)
     /// ERROR E0770: the type of const parameters must not depend on other generic parameters.
     ParamInTyOfConstParam(Symbol),
@@ -238,7 +238,7 @@ enum ResolutionError<'a> {
     ///
     /// This error is only emitted when using `min_const_generics`.
     ParamInNonTrivialAnonConst { name: Symbol, is_type: bool },
-    /// Error E0735: type parameters with a default cannot use `Self`
+    /// Error E0735: generic parameters with a default cannot use `Self`
     SelfInTyParamDefault,
     /// Error E0767: use of unreachable label
     UnreachableLabel { name: Symbol, definition_span: Span, suggestion: Option<LabelSuggestion> },

compiler/rustc_typeck/src/astconv/generics.rs

@@ -499,7 +499,9 @@ pub(crate) fn check_generic_arg_count(
             let expected_min = if infer_args {
                 0
             } else {
-                param_counts.consts + named_type_param_count - default_counts.types
+                param_counts.consts + named_type_param_count
+                    - default_counts.types
+                    - default_counts.consts
             };
 
             check_generics(

compiler/rustc_typeck/src/astconv/mod.rs

@@ -505,34 +505,17 @@ fn inferred_kind(
                         }
                     }
                     GenericParamDefKind::Const { has_default } => {
-                        let ty = tcx.at(self.span).type_of(param.def_id);
                         if !infer_args && has_default {
-                            let c = substs.unwrap()[param.index as usize].expect_const();
-                            ty::subst::GenericArg::from(c)
-                        } else if infer_args {
-                            self.astconv.ct_infer(ty, Some(param), self.span).into()
+                            ty::Const::from_anon_const(tcx, param.def_id.expect_local()).into()
                         } else {
-                            // We've already errored above about the mismatch.
-                            tcx.const_error(ty).into()
-                        }
-                        // FIXME(const_generic_defaults)
-                        /*
-                        if !infer_args && has_default {
-                            /*
-                            if default_needs_object_self(param) {
-                                missing_type_params.push(param.name.to_string());
-                                tcx.const_error(ty).into()
+                            let ty = tcx.at(self.span).type_of(param.def_id);
+                            if infer_args {
+                                self.astconv.ct_infer(ty, Some(param), self.span).into()
                             } else {
+                                // We've already errored above about the mismatch.
+                                tcx.const_error(ty).into()
                             }
-                            */
-                        } else if infer_args {
-                            // No const parameters were provided, we can infer all.
-                            self.astconv.ct_infer(ty, Some(param), self.span).into()
-                        } else {
-                            // We've already errored above about the mismatch.
-                            tcx.const_error(ty).into()
                         }
-                        */
                     }
                 }
             }

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -1444,12 +1444,11 @@ fn inferred_kind(
                         }
                     }
                     GenericParamDefKind::Const { has_default, .. } => {
-                        if infer_args || !has_default {
-                            return self.fcx.var_for_def(self.span, param);
+                        if !infer_args && has_default {
+                            ty::Const::from_anon_const(tcx, param.def_id.expect_local()).into()
+                        } else {
+                            self.fcx.var_for_def(self.span, param)
                         }
-                        // FIXME(const_generic_defaults)
-                        // No const parameters were provided, we have to infer them.
-                        todo!()
                     }
                 }
             }

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -759,7 +759,7 @@ fn check_where_clauses<'tcx, 'fcx>(
                 fcx.tcx.mk_param_from_def(param)
             }
 
-            GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
+            GenericParamDefKind::Type { .. } => {
                 // If the param has a default, ...
                 if is_our_default(param) {
                     let default_ty = fcx.tcx.type_of(param.def_id);
@@ -772,6 +772,16 @@ fn check_where_clauses<'tcx, 'fcx>(
 
                 fcx.tcx.mk_param_from_def(param)
             }
+            GenericParamDefKind::Const { .. } => {
+                if is_our_default(param) {
+                    let default_ct = ty::Const::from_anon_const(tcx, param.def_id.expect_local());
+                    // Const params have to currently be concrete.
+                    assert!(!default_ct.needs_subst());
+                    default_ct.into()
+                } else {
+                    fcx.tcx.mk_param_from_def(param)
+                }
+            }
         }
     });
 

compiler/rustc_typeck/src/collect.rs

@@ -1527,7 +1527,7 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::Generics {
                         |lint| {
                             lint.build(
                                 "defaults for type parameters are only allowed in \
-                                 `struct`, `enum`, `type`, or `trait` definitions.",
+                                 `struct`, `enum`, `type`, or `trait` definitions",
                             )
                             .emit();
                         },
@@ -1554,6 +1554,14 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::Generics {
             Some(param_def)
         }
         GenericParamKind::Const { default, .. } => {
+            if !allow_defaults && default.is_some() {
+                tcx.sess.span_err(
+                    param.span,
+                    "defaults for const parameters are only allowed in \
+                    `struct`, `enum`, `type`, or `trait` definitions",
+                );
+            }
+
             let param_def = ty::GenericParamDef {
                 index: type_start + i as u32,
                 name: param.name.ident().name,

compiler/rustc_typeck/src/collect/type_of.rs

@@ -436,6 +436,12 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                     .discr_type()
                     .to_ty(tcx),
 
+                Node::GenericParam(&GenericParam {
+                    hir_id: param_hir_id,
+                    kind: GenericParamKind::Const { default: Some(ct), .. },
+                    ..
+                }) if ct.hir_id == hir_id => tcx.type_of(tcx.hir().local_def_id(param_hir_id)),
+
                 x => tcx.ty_error_with_message(
                     DUMMY_SP,
                     &format!("unexpected const parent in type_of_def_id(): {:?}", x),

src/test/ui/const-generics/defaults/const-default.rs

-// check-pass
+// run-pass
 
 #![feature(const_generics)]
-#![feature(const_generic_defaults)]
+#![feature(const_generics_defaults)]
 #![allow(incomplete_features)]
 
 
-pub struct ConstDefault<const N: usize = 3> {}
+pub struct ConstDefault<const N: usize = 3>;
+
+impl<const N: usize> ConstDefault<N> {
+    fn foo(self) -> usize {
+        N
+    }
+}
+
+impl ConstDefault {
+    fn new() -> Self {
+        ConstDefault
+    }
+
+    fn bar(self) {}
+}
 
 pub fn main() {
-  let s = ConstDefault::default();
+    let s = ConstDefault::new();
+    assert_eq!(s.foo(), 3);
+
+    let w = ConstDefault::<3>;
+    w.bar();
 }

src/test/ui/const-generics/defaults/wrong-order.full.stderr

-error: type parameters with a default must be trailing
+error: generic parameters with a default must be trailing
   --> $DIR/wrong-order.rs:4:10
    |
 LL | struct A<T = u32, const N: usize> {

src/test/ui/const-generics/defaults/wrong-order.min.stderr

-error: type parameters with a default must be trailing
+error: generic parameters with a default must be trailing
   --> $DIR/wrong-order.rs:4:10
    |
 LL | struct A<T = u32, const N: usize> {

src/test/ui/const-generics/defaults/wrong-order.rs

@@ -2,7 +2,7 @@
 #![cfg_attr(full, feature(const_generics))] //[full]~WARN the feature `const_generics` is incomplete
 
 struct A<T = u32, const N: usize> {
-    //~^ ERROR type parameters with a default must be trailing
+    //~^ ERROR generic parameters with a default must be trailing
     arg: T,
 }
 

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

 #![crate_type = "lib"]
 #![feature(const_generics_defaults)]
-#![feature(min_const_generics)]
 #![allow(incomplete_features)]
 
 fn foo<const SIZE: usize = 5usize>() {}
+//~^ ERROR defaults for const parameters are

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

-error[E0282]: type annotations needed
-  --> $DIR/default_function_param.rs:6:28
+error: defaults for const parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/default_function_param.rs:5:14
    |
-LL | fn foo<const SIZE: usize = 5>() {}
-   |                            ^ cannot infer type for type `{integer}`
+LL | fn foo<const SIZE: usize = 5usize>() {}
+   |              ^^^^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0282`.

src/test/ui/const-generics/params-in-ct-in-ty-param-lazy-norm.full.stderr

-error: type parameters with a default must be trailing
+error: generic parameters with a default must be trailing
   --> $DIR/params-in-ct-in-ty-param-lazy-norm.rs:11:12
    |
 LL | struct Bar<T = [u8; N], const N: usize>(T);

src/test/ui/const-generics/params-in-ct-in-ty-param-lazy-norm.min.stderr

-error: type parameters with a default must be trailing
+error: generic parameters with a default must be trailing
   --> $DIR/params-in-ct-in-ty-param-lazy-norm.rs:11:12
    |
 LL | struct Bar<T = [u8; N], const N: usize>(T);

src/test/ui/const-generics/params-in-ct-in-ty-param-lazy-norm.rs

@@ -10,6 +10,6 @@
 // FIXME(const_generics_defaults): We still don't know how to deal with type defaults.
 struct Bar<T = [u8; N], const N: usize>(T);
 //~^ ERROR constant values inside of type parameter defaults
-//~| ERROR type parameters with a default
+//~| ERROR generic parameters with a default
 
 fn main() {}

src/test/ui/generics/generic-non-trailing-defaults.rs

 struct Heap;
 
 struct Vec<A = Heap, T>(A, T);
-//~^ ERROR type parameters with a default must be trailing
+//~^ ERROR generic parameters with a default must be trailing
 
 struct Foo<A, B = Vec<C>, C>(A, B, C);
-//~^ ERROR type parameters with a default must be trailing
-//~| ERROR type parameters with a default cannot use forward declared identifiers
+//~^ ERROR generic parameters with a default must be trailing
+//~| ERROR generic parameters with a default cannot use forward declared identifiers
 
 fn main() {}

src/test/ui/generics/generic-non-trailing-defaults.stderr

-error: type parameters with a default must be trailing
+error: generic parameters with a default must be trailing
   --> $DIR/generic-non-trailing-defaults.rs:3:12
    |
 LL | struct Vec<A = Heap, T>(A, T);
    |            ^
 
-error: type parameters with a default must be trailing
+error: generic parameters with a default must be trailing
   --> $DIR/generic-non-trailing-defaults.rs:6:15
    |
 LL | struct Foo<A, B = Vec<C>, C>(A, B, C);

src/test/ui/generics/generic-type-params-forward-mention.rs

 // Ensure that we get an error and not an ICE for this problematic case.
 struct Foo<T = Option<U>, U = bool>(T, U);
-//~^ ERROR type parameters with a default cannot use forward declared identifiers
+//~^ ERROR generic parameters with a default cannot use forward declared identifiers
 fn main() {
     let x: Foo;
 }

src/test/ui/issues/issue-26812.rs

 #![feature(default_type_parameter_fallback)]
 
 fn avg<T=T::Item>(_: T) {}
-//~^ ERROR type parameters with a default cannot use forward declared identifiers
+//~^ ERROR generic parameters with a default cannot use forward declared identifiers
 
 fn main() {}