Auto merge of rust-lang#66104 - yodaldevoid:generic-arg-disambiguation, r=petrochenkov

Generic arg disambiguation

Using the tactic suggested by @petrochenkov in rust-lang#60804 (comment) and on [zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/generic.20argument.20disambiguation), this change checks type arguments to see if they are really incorrectly-parsed const arguments.

it should be noted that `segments.len() == 1 && segments[0].arg.is_none()` was reduced to `segments.len() == 1` as suggested by @petrochenkov in [zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/generic.20argument.20disambiguation/near/177848002). This change allowed a few more existing tests to have their braces removed.

There are a couple of "problems" with these changes that I should note. First, there was a regression in the error messages found in "src/test/ui/privacy-ns1.rs" and "src/test/ui/privacy-ns1.rs". Second, some braces were unable to be removed from "src/test/ui/const-generics/fn-const-param-infer.rs". Those on line 24 caused the statement to stop equating when removed, and those on line 20 cause a statement that should not equate to produce no error when removed.

I have not looked further into any of these issues yet, though I would be willing to look into them before landing this. I simply wanted to get some other eyes on this before going further.

Fixes rust-lang#60804

cc @varkor @jplatte

Author: bors

src/librustc/hir/def.rs

@@ -127,6 +127,34 @@ impl DefKind {
             _ => "a",
         }
     }
+
+    pub fn matches_ns(&self, ns: Namespace) -> bool {
+        match self {
+            DefKind::Mod
+            | DefKind::Struct
+            | DefKind::Union
+            | DefKind::Enum
+            | DefKind::Variant
+            | DefKind::Trait
+            | DefKind::OpaqueTy
+            | DefKind::TyAlias
+            | DefKind::ForeignTy
+            | DefKind::TraitAlias
+            | DefKind::AssocTy
+            | DefKind::AssocOpaqueTy
+            | DefKind::TyParam => ns == Namespace::TypeNS,
+
+            DefKind::Fn
+            | DefKind::Const
+            | DefKind::ConstParam
+            | DefKind::Static
+            | DefKind::Ctor(..)
+            | DefKind::Method
+            | DefKind::AssocConst => ns == Namespace::ValueNS,
+
+            DefKind::Macro(..) => ns == Namespace::MacroNS,
+        }
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, HashStable)]
@@ -427,4 +455,14 @@ impl<Id> Res<Id> {
             _ => None,
         }
     }
+
+    pub fn matches_ns(&self, ns: Namespace) -> bool {
+        match self {
+            Res::Def(kind, ..) => kind.matches_ns(ns),
+            Res::PrimTy(..) | Res::SelfTy(..) | Res::ToolMod => ns == Namespace::TypeNS,
+            Res::SelfCtor(..) | Res::Local(..) => ns == Namespace::ValueNS,
+            Res::NonMacroAttr(..) => ns == Namespace::MacroNS,
+            Res::Err => true,
+        }
+    }
 }

src/librustc/hir/lowering.rs

@@ -1155,13 +1155,64 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
-    fn lower_generic_arg(&mut self,
-                         arg: &ast::GenericArg,
-                         itctx: ImplTraitContext<'_>)
-                         -> hir::GenericArg {
+    fn lower_generic_arg(
+        &mut self,
+        arg: &ast::GenericArg,
+        itctx: ImplTraitContext<'_>
+    ) -> hir::GenericArg {
         match arg {
             ast::GenericArg::Lifetime(lt) => GenericArg::Lifetime(self.lower_lifetime(&lt)),
-            ast::GenericArg::Type(ty) => GenericArg::Type(self.lower_ty_direct(&ty, itctx)),
+            ast::GenericArg::Type(ty) => {
+                // We parse const arguments as path types as we cannot distiguish them durring
+                // parsing. We try to resolve that ambiguity by attempting resolution in both the
+                // type and value namespaces. If we resolved the path in the value namespace, we
+                // transform it into a generic const argument.
+                if let TyKind::Path(ref qself, ref path) = ty.kind {
+                    if let Some(partial_res) = self.resolver.get_partial_res(ty.id) {
+                        let res = partial_res.base_res();
+                        if !res.matches_ns(Namespace::TypeNS) {
+                            debug!(
+                                "lower_generic_arg: Lowering type argument as const argument: {:?}",
+                                ty,
+                            );
+
+                            // Construct a AnonConst where the expr is the "ty"'s path.
+
+                            let parent_def_index =
+                                self.current_hir_id_owner.last().unwrap().0;
+                            let node_id = self.resolver.next_node_id();
+
+                            // Add a definition for the in-band const def.
+                            self.resolver.definitions().create_def_with_parent(
+                                parent_def_index,
+                                node_id,
+                                DefPathData::AnonConst,
+                                ExpnId::root(),
+                                ty.span,
+                            );
+
+                            let path_expr = Expr {
+                                id: ty.id,
+                                kind: ExprKind::Path(qself.clone(), path.clone()),
+                                span: ty.span,
+                                attrs: ThinVec::new(),
+                            };
+
+                            let ct = self.with_new_scopes(|this| {
+                                hir::AnonConst {
+                                    hir_id: this.lower_node_id(node_id),
+                                    body: this.lower_const_body(&path_expr),
+                                }
+                            });
+                            return GenericArg::Const(ConstArg {
+                                value: ct,
+                                span: ty.span,
+                            });
+                        }
+                    }
+                }
+                GenericArg::Type(self.lower_ty_direct(&ty, itctx))
+            }
             ast::GenericArg::Const(ct) => {
                 GenericArg::Const(ConstArg {
                     value: self.lower_anon_const(&ct),

src/librustc_resolve/late.rs

@@ -546,6 +546,52 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
             self.visit_where_predicate(p);
         }
     }
+
+    fn visit_generic_arg(&mut self, arg: &'tcx GenericArg) {
+        debug!("visit_generic_arg({:?})", arg);
+        match arg {
+            GenericArg::Type(ref ty) => {
+                // We parse const arguments as path types as we cannot distiguish them durring
+                // parsing. We try to resolve that ambiguity by attempting resolution the type
+                // namespace first, and if that fails we try again in the value namespace. If
+                // resolution in the value namespace succeeds, we have an generic const argument on
+                // our hands.
+                if let TyKind::Path(ref qself, ref path) = ty.kind {
+                    // We cannot disambiguate multi-segment paths right now as that requires type
+                    // checking.
+                    if path.segments.len() == 1 && path.segments[0].args.is_none() {
+                        let mut check_ns = |ns| self.resolve_ident_in_lexical_scope(
+                            path.segments[0].ident, ns, None, path.span
+                        ).is_some();
+
+                        if !check_ns(TypeNS) && check_ns(ValueNS) {
+                            // This must be equivalent to `visit_anon_const`, but we cannot call it
+                            // directly due to visitor lifetimes so we have to copy-paste some code.
+                            self.with_constant_rib(|this| {
+                                this.smart_resolve_path(
+                                    ty.id,
+                                    qself.as_ref(),
+                                    path,
+                                    PathSource::Expr(None)
+                                );
+
+                                if let Some(ref qself) = *qself {
+                                    this.visit_ty(&qself.ty);
+                                }
+                                this.visit_path(path, ty.id);
+                            });
+
+                            return;
+                        }
+                    }
+                }
+
+                self.visit_ty(ty);
+            }
+            GenericArg::Lifetime(lt) => self.visit_lifetime(lt),
+            GenericArg::Const(ct) => self.visit_anon_const(ct),
+        }
+    }
 }
 
 impl<'a, 'b> LateResolutionVisitor<'a, '_> {

src/librustc_resolve/lib.rs

@@ -1726,6 +1726,14 @@ impl<'a> Resolver<'a> {
             }
         }
 
+        if ns == TypeNS {
+            if let Some(prim_ty) = self.primitive_type_table.primitive_types.get(&ident.name) {
+                let binding = (Res::PrimTy(*prim_ty), ty::Visibility::Public,
+                               DUMMY_SP, ExpnId::root()).to_name_binding(self.arenas);
+                return Some(LexicalScopeBinding::Item(binding));
+            }
+        }
+
         None
     }
 

src/test/ui/const-generics/const-generic-array-wrapper.rs

@@ -3,11 +3,11 @@
 #![feature(const_generics)]
 //~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
 
-struct Foo<T, const N: usize>([T; {N}]);
+struct Foo<T, const N: usize>([T; N]);
 
-impl<T, const N: usize> Foo<T, {N}> {
+impl<T, const N: usize> Foo<T, N> {
     fn foo(&self) -> usize {
-        {N}
+        N
     }
 }
 

src/test/ui/const-generics/fn-const-param-call.rs

@@ -9,12 +9,12 @@ fn function() -> u32 {
 
 struct Wrapper<const F: fn() -> u32>;
 
-impl<const F: fn() -> u32> Wrapper<{F}> {
+impl<const F: fn() -> u32> Wrapper<F> {
     fn call() -> u32 {
         F()
     }
 }
 
 fn main() {
-    assert_eq!(Wrapper::<{function}>::call(), 17);
+    assert_eq!(Wrapper::<function>::call(), 17);
 }

src/test/ui/const-generics/fn-const-param-infer.rs

@@ -11,15 +11,15 @@ fn generic_arg<T>(val: T) -> bool { true }
 fn generic<T>(val: usize) -> bool { val != 1 }
 
 fn main() {
-    let _: Option<Checked<{not_one}>> = None;
-    let _: Checked<{not_one}> = Checked::<{not_one}>;
-    let _: Checked<{not_one}> = Checked::<{not_two}>; //~ mismatched types
+    let _: Option<Checked<not_one>> = None;
+    let _: Checked<not_one> = Checked::<not_one>;
+    let _: Checked<not_one> = Checked::<not_two>; //~ mismatched types
 
-    let _ = Checked::<{generic_arg}>;
+    let _ = Checked::<generic_arg>;
     let _ = Checked::<{generic_arg::<usize>}>;
     let _ = Checked::<{generic_arg::<u32>}>;  //~ mismatched types
 
-    let _ = Checked::<{generic}>; //~ type annotations needed
+    let _ = Checked::<generic>; //~ type annotations needed
     let _ = Checked::<{generic::<u16>}>;
     let _: Checked<{generic::<u16>}> = Checked::<{generic::<u16>}>;
     let _: Checked<{generic::<u32>}> = Checked::<{generic::<u16>}>; //~ mismatched types

src/test/ui/const-generics/fn-const-param-infer.stderr

@@ -7,10 +7,10 @@ LL | #![feature(const_generics, const_compare_raw_pointers)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0308]: mismatched types
-  --> $DIR/fn-const-param-infer.rs:16:33
+  --> $DIR/fn-const-param-infer.rs:16:31
    |
-LL |     let _: Checked<{not_one}> = Checked::<{not_two}>;
-   |                                 ^^^^^^^^^^^^^^^^^^^^ expected `not_one`, found `not_two`
+LL |     let _: Checked<not_one> = Checked::<not_two>;
+   |                               ^^^^^^^^^^^^^^^^^^ expected `not_one`, found `not_two`
    |
    = note: expected type `Checked<not_one>`
               found type `Checked<not_two>`
@@ -25,10 +25,10 @@ LL |     let _ = Checked::<{generic_arg::<u32>}>;
               found type `fn(u32) -> bool {generic_arg::<u32>}`
 
 error[E0282]: type annotations needed
-  --> $DIR/fn-const-param-infer.rs:22:24
+  --> $DIR/fn-const-param-infer.rs:22:23
    |
-LL |     let _ = Checked::<{generic}>;
-   |                        ^^^^^^^ cannot infer type for `T`
+LL |     let _ = Checked::<generic>;
+   |                       ^^^^^^^ cannot infer type for `T`
 
 error[E0308]: mismatched types
   --> $DIR/fn-const-param-infer.rs:25:40

src/test/ui/const-generics/impl-const-generic-struct.rs

@@ -5,7 +5,7 @@
 
 struct S<const X: u32>;
 
-impl<const X: u32> S<{X}> {
+impl<const X: u32> S<X> {
     fn x() -> u32 {
         X
     }

src/test/ui/const-generics/raw-ptr-const-param-deref.rs

@@ -6,7 +6,7 @@ const A: u32 = 3;
 
 struct Const<const P: *const u32>;
 
-impl<const P: *const u32> Const<{P}> {
+impl<const P: *const u32> Const<P> {
     fn get() -> u32 {
         unsafe {
             *P