resolve: Minimize hacks in name resolution of primitive types

src/librustc_resolve/diagnostics.rs

@@ -205,51 +205,6 @@ about what constitutes an Item declaration and what does not:
 https://doc.rust-lang.org/reference.html#statements
 "##,
 
-E0317: r##"
-User-defined types or type parameters cannot shadow the primitive types.
-This error indicates you tried to define a type, struct or enum with the same
-name as an existing primitive type:
-
-```compile_fail
-struct u8 {
-    // ...
-}
-```
-
-To fix this, simply name it something else.
-
-Such an error may also occur if you define a type parameter which shadows a
-primitive type. An example would be something like:
-
-```compile_fail
-impl<u8> MyTrait for Option<u8> {
-    // ...
-}
-```
-
-In such a case, if you meant for `u8` to be a generic type parameter (i.e. any
-type can be used in its place), use something like `T` instead:
-
-```ignore
-impl<T> MyTrait for Option<T> {
-    // ...
-}
-```
-
-On the other hand, if you wished to refer to the specific type `u8`, remove it
-from the type parameter list:
-
-```ignore
-impl MyTrait for Option<u8> {
-    // ...
-}
-
-See the Types section of the reference for more information about the primitive
-types:
-
-https://doc.rust-lang.org/reference.html#types
-"##,
-
 E0364: r##"
 Private items cannot be publicly re-exported.  This error indicates that you
 attempted to `pub use` a type or value that was not itself public.

src/librustc_resolve/lib.rs

@@ -1615,15 +1615,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         intravisit::walk_crate(self, krate);
     }
 
-    fn check_if_primitive_type_name(&self, name: Name, span: Span) {
-        if let Some(_) = self.primitive_type_table.primitive_types.get(&name) {
-            span_err!(self.session,
-                      span,
-                      E0317,
-                      "user-defined types or type parameters cannot shadow the primitive types");
-        }
-    }
-
     fn resolve_item(&mut self, item: &Item) {
         let name = item.name;
 
@@ -1633,8 +1624,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             ItemEnum(_, ref generics) |
             ItemTy(_, ref generics) |
             ItemStruct(_, ref generics) => {
-                self.check_if_primitive_type_name(name, item.span);
-
                 self.with_type_parameter_rib(HasTypeParameters(generics, TypeSpace, ItemRibKind),
                                              |this| intravisit::walk_item(this, item));
             }
@@ -1655,8 +1644,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             }
 
             ItemTrait(_, ref generics, ref bounds, ref trait_items) => {
-                self.check_if_primitive_type_name(name, item.span);
-
                 // Create a new rib for the trait-wide type parameters.
                 self.with_type_parameter_rib(HasTypeParameters(generics,
                                                                TypeSpace,
@@ -1691,8 +1678,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                                     });
                                 }
                                 hir::TypeTraitItem(..) => {
-                                    this.check_if_primitive_type_name(trait_item.name,
-                                                                      trait_item.span);
                                     this.with_type_parameter_rib(NoTypeParameters, |this| {
                                         intravisit::walk_trait_item(this, trait_item)
                                     });
@@ -1716,28 +1701,8 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             }
 
             ItemUse(ref view_path) => {
-                // check for imports shadowing primitive types
-                let check_rename = |this: &Self, id, name| {
-                    match this.def_map.borrow().get(&id).map(|d| d.full_def()) {
-                        Some(Def::Enum(..)) | Some(Def::TyAlias(..)) | Some(Def::Struct(..)) |
-                        Some(Def::Trait(..)) | None => {
-                            this.check_if_primitive_type_name(name, item.span);
-                        }
-                        _ => {}
-                    }
-                };
-
                 match view_path.node {
-                    hir::ViewPathSimple(name, _) => {
-                        check_rename(self, item.id, name);
-                    }
                     hir::ViewPathList(ref prefix, ref items) => {
-                        for item in items {
-                            if let Some(name) = item.node.rename() {
-                                check_rename(self, item.node.id(), name);
-                            }
-                        }
-
                         // Resolve prefix of an import with empty braces (issue #28388)
                         if items.is_empty() && !prefix.segments.is_empty() {
                             match self.resolve_crate_relative_path(prefix.span,
@@ -1918,9 +1883,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
     }
 
     fn resolve_generics(&mut self, generics: &Generics) {
-        for type_parameter in generics.ty_params.iter() {
-            self.check_if_primitive_type_name(type_parameter.name, type_parameter.span);
-        }
         for predicate in &generics.where_clause.predicates {
             match predicate {
                 &hir::WherePredicate::BoundPredicate(_) |
@@ -2654,15 +2616,37 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
 
         // Try to find a path to an item in a module.
         let last_ident = segments.last().unwrap().identifier;
-        if segments.len() <= 1 {
-            let unqualified_def = self.resolve_identifier(last_ident, namespace, true);
-            return unqualified_def.and_then(|def| self.adjust_local_def(def, span))
-                                  .map(|def| {
-                                      PathResolution::new(def, path_depth)
-                                  });
-        }
+        // Resolve a single identifier with fallback to primitive types
+        let resolve_identifier_with_fallback = |this: &mut Self, record_used| {
+            let def = this.resolve_identifier(last_ident, namespace, record_used);
+            match def {
+                None | Some(LocalDef{def: Def::Mod(..), ..}) if namespace == TypeNS =>
+                    this.primitive_type_table
+                        .primitive_types
+                        .get(&last_ident.unhygienic_name)
+                        .map_or(def, |prim_ty| Some(LocalDef::from_def(Def::PrimTy(*prim_ty)))),
+                _ => def
+            }
+        };
 
-        let unqualified_def = self.resolve_identifier(last_ident, namespace, false);
+        if segments.len() == 1 {
+            // In `a(::assoc_item)*` `a` cannot be a module. If `a` does resolve to a module we
+            // don't report an error right away, but try to fallback to a primitive type.
+            // So, we are still able to successfully resolve something like
+            //
+            // use std::u8; // bring module u8 in scope
+            // fn f() -> u8 { // OK, resolves to primitive u8, not to std::u8
+            //     u8::max_value() // OK, resolves to associated function <u8>::max_value,
+            //                     // not to non-existent std::u8::max_value
+            // }
+            //
+            // Such behavior is required for backward compatibility.
+            // The same fallback is used when `a` resolves to nothing.
+            let unqualified_def = resolve_identifier_with_fallback(self, true);
+            return unqualified_def.and_then(|def| self.adjust_local_def(def, span)).map(mk_res);
+        }
+
+        let unqualified_def = resolve_identifier_with_fallback(self, false);
         let def = self.resolve_module_relative_path(span, segments, namespace);
         match (def, unqualified_def) {
             (Some(d), Some(ref ud)) if d == ud.def => {
@@ -2688,15 +2672,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             return Some(LocalDef::from_def(Def::Err));
         }
 
-        // First, check to see whether the name is a primitive type.
-        if namespace == TypeNS {
-            if let Some(&prim_ty) = self.primitive_type_table
-                                        .primitive_types
-                                        .get(&identifier.unhygienic_name) {
-                return Some(LocalDef::from_def(Def::PrimTy(prim_ty)));
-            }
-        }
-
         self.resolve_identifier_in_local_ribs(identifier, namespace, record_used)
     }
 

src/test/compile-fail/issue-20427.rs → src/test/run-pass/issue-20427.rs

@@ -1,4 +1,4 @@
-// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -9,17 +9,17 @@
 // except according to those terms.
 
 // aux-build:i8.rs
+// ignore-pretty (#23623)
+
 extern crate i8;
 use std::string as i16;
 static i32: i32 = 0;
 const i64: i64 = 0;
 fn u8(f32: f32) {}
 fn f<f64>(f64: f64) {}
-//~^ ERROR user-defined types or type parameters cannot shadow the primitive types
-type u16 = u16; //~ ERROR user-defined types or type parameters cannot shadow the primitive types
-enum u32 {} //~ ERROR user-defined types or type parameters cannot shadow the primitive types
-struct u64; //~ ERROR user-defined types or type parameters cannot shadow the primitive types
-trait bool {} //~ ERROR user-defined types or type parameters cannot shadow the primitive types
+enum u32 {}
+struct u64;
+trait bool {}
 
 mod char {
     extern crate i8;
@@ -40,29 +40,52 @@ mod char {
         use super::u8_ as u8;
         use super::f_ as f64;
         use super::u16_ as u16;
-        //~^ ERROR user-defined types or type parameters cannot shadow the primitive types
         use super::u32_ as u32;
-        //~^ ERROR user-defined types or type parameters cannot shadow the primitive types
         use super::u64_ as u64;
-        //~^ ERROR user-defined types or type parameters cannot shadow the primitive types
         use super::bool_ as bool;
-        //~^ ERROR user-defined types or type parameters cannot shadow the primitive types
         use super::{bool_ as str};
-        //~^ ERROR user-defined types or type parameters cannot shadow the primitive types
         use super::char_ as char;
     }
 }
 
 trait isize_ {
-    type isize; //~ ERROR user-defined types or type parameters cannot shadow the primitive types
+    type isize;
 }
 
 fn usize<'usize>(usize: &'usize usize) -> &'usize usize { usize }
 
+mod reuse {
+    use std::mem::size_of;
+
+    type u8 = u64;
+    use std::string::String as i16;
+
+    pub fn check<u16>() {
+        assert_eq!(size_of::<u8>(), 8);
+        assert_eq!(size_of::<::u64>(), 0);
+        assert_eq!(size_of::<i16>(), 3 * size_of::<*const ()>());
+        assert_eq!(size_of::<u16>(), 0);
+    }
+}
+
+mod guard {
+    pub fn check() {
+        use std::u8; // bring module u8 in scope
+        fn f() -> u8 { // OK, resolves to primitive u8, not to std::u8
+            u8::max_value() // OK, resolves to associated function <u8>::max_value,
+                            // not to non-existent std::u8::max_value
+        }
+        assert_eq!(f(), u8::MAX); // OK, resolves to std::u8::MAX
+    }
+}
+
 fn main() {
     let bool = true;
-    match bool {
+    let _ = match bool {
         str @ true => if str { i32 as i64 } else { i64 },
         false => i64,
     };
+
+    reuse::check::<u64>();
+    guard::check();
 }