Rollup merge of rust-lang#41939 - eddyb:trait-assoc-const-default, r=petrochenkov

rustc_resolve: don't deny outer type parameters in embedded constants.

This solves a problem noted at rust-lang#29646 (comment), where an associated const default in a trait couldn't refer to `Self` or type parameters, due to inaccuracies in lexical scoping.

I've also allowed "embedded expressions" (`[T; expr]`,  `[x; expr]`, `typeof expr`) to refer to type parameters in scope. *However*, the typesystem still doesn't handle rust-lang#34344.
Fully resolving that issue requires breaking cycles more aggressively (e.g. lazy evaluation), *even* in when the expression doesn't depend on type parameters, to type-check it at all, and then also type-level "constant projections" (in the vein of `{expr}` from const generics).

Author: Mark-Simulacrum

src/librustc_resolve/diagnostics.rs

@@ -1222,27 +1222,26 @@ fn foo() {
 "##,
 
 E0435: r##"
-A non-constant value was used to initialise a constant.
+A non-constant value was used in a constant expression.
 
 Erroneous code example:
 
 ```compile_fail,E0435
-let foo = 42u32;
-const FOO : u32 = foo; // error: attempt to use a non-constant value in a
-                       //        constant
+let foo = 42;
+let a: [u8; foo]; // error: attempt to use a non-constant value in a constant
 ```
 
 To fix this error, please replace the value with a constant. Example:
 
 ```
-const FOO : u32 = 42u32; // ok!
+let a: [u8; 42]; // ok!
 ```
 
 Or:
 
 ```
-const OTHER_FOO : u32 = 42u32;
-const FOO : u32 = OTHER_FOO; // ok!
+const FOO: usize = 42;
+let a: [u8; FOO]; // ok!
 ```
 "##,
 
@@ -1560,7 +1559,7 @@ register_diagnostics! {
 //  E0157, unused error code
 //  E0257,
 //  E0258,
-    E0402, // cannot use an outer type parameter in this context
+//  E0402, // cannot use an outer type parameter in this context
 //  E0406, merged into 420
 //  E0410, merged into 408
 //  E0413, merged into 530

src/librustc_resolve/lib.rs

@@ -127,8 +127,6 @@ impl Ord for BindingError {
 enum ResolutionError<'a> {
     /// error E0401: can't use type parameters from outer function
     TypeParametersFromOuterFunction,
-    /// error E0402: cannot use an outer type parameter in this context
-    OuterTypeParameterContext,
     /// error E0403: the name is already used for a type parameter in this type parameter list
     NameAlreadyUsedInTypeParameterList(Name, &'a Span),
     /// error E0407: method is not a member of trait
@@ -187,12 +185,6 @@ fn resolve_struct_error<'sess, 'a>(resolver: &'sess Resolver,
             err.span_label(span, "use of type variable from outer function");
             err
         }
-        ResolutionError::OuterTypeParameterContext => {
-            struct_span_err!(resolver.session,
-                             span,
-                             E0402,
-                             "cannot use an outer type parameter in this context")
-        }
         ResolutionError::NameAlreadyUsedInTypeParameterList(name, first_use_span) => {
              let mut err = struct_span_err!(resolver.session,
                                             span,
@@ -1671,16 +1663,16 @@ impl<'a> Resolver<'a> {
                             this.check_proc_macro_attrs(&trait_item.attrs);
 
                             match trait_item.node {
-                                TraitItemKind::Const(_, ref default) => {
+                                TraitItemKind::Const(ref ty, ref default) => {
+                                    this.visit_ty(ty);
+
                                     // Only impose the restrictions of
-                                    // ConstRibKind if there's an actual constant
+                                    // ConstRibKind for an actual constant
                                     // expression in a provided default.
-                                    if default.is_some() {
+                                    if let Some(ref expr) = *default{
                                         this.with_constant_rib(|this| {
-                                            visit::walk_trait_item(this, trait_item)
+                                            this.visit_expr(expr);
                                         });
-                                    } else {
-                                        visit::walk_trait_item(this, trait_item)
                                     }
                                 }
                                 TraitItemKind::Method(ref sig, _) => {
@@ -1709,9 +1701,13 @@ impl<'a> Resolver<'a> {
                 });
             }
 
-            ItemKind::Const(..) | ItemKind::Static(..) => {
-                self.with_constant_rib(|this| {
-                    visit::walk_item(this, item);
+            ItemKind::Static(ref ty, _, ref expr) |
+            ItemKind::Const(ref ty, ref expr) => {
+                self.with_item_rib(|this| {
+                    this.visit_ty(ty);
+                    this.with_constant_rib(|this| {
+                        this.visit_expr(expr);
+                    });
                 });
             }
 
@@ -1782,13 +1778,21 @@ impl<'a> Resolver<'a> {
         self.label_ribs.pop();
     }
 
+    fn with_item_rib<F>(&mut self, f: F)
+        where F: FnOnce(&mut Resolver)
+    {
+        self.ribs[ValueNS].push(Rib::new(ItemRibKind));
+        self.ribs[TypeNS].push(Rib::new(ItemRibKind));
+        f(self);
+        self.ribs[TypeNS].pop();
+        self.ribs[ValueNS].pop();
+    }
+
     fn with_constant_rib<F>(&mut self, f: F)
         where F: FnOnce(&mut Resolver)
     {
         self.ribs[ValueNS].push(Rib::new(ConstantItemRibKind));
-        self.ribs[TypeNS].push(Rib::new(ConstantItemRibKind));
         f(self);
-        self.ribs[TypeNS].pop();
         self.ribs[ValueNS].pop();
     }
 
@@ -2755,7 +2759,8 @@ impl<'a> Resolver<'a> {
                 for rib in ribs {
                     match rib.kind {
                         NormalRibKind | MethodRibKind(_) | ClosureRibKind(..) |
-                        ModuleRibKind(..) | MacroDefinition(..) | ForwardTyParamBanRibKind => {
+                        ModuleRibKind(..) | MacroDefinition(..) | ForwardTyParamBanRibKind |
+                        ConstantItemRibKind => {
                             // Nothing to do. Continue.
                         }
                         ItemRibKind => {
@@ -2767,14 +2772,6 @@ impl<'a> Resolver<'a> {
                             }
                             return Def::Err;
                         }
-                        ConstantItemRibKind => {
-                            // see #9186
-                            if record_used {
-                                resolve_error(self, span,
-                                              ResolutionError::OuterTypeParameterContext);
-                            }
-                            return Def::Err;
-                        }
                     }
                 }
             }

src/test/compile-fail/E0435.rs

@@ -10,6 +10,6 @@
 
 fn main () {
     let foo = 42u32;
-    const FOO : u32 = foo; //~ ERROR E0435
+    let _: [u8; foo]; //~ ERROR E0435
     //~| NOTE non-constant used with constant
 }

src/test/compile-fail/associated-const-type-parameter-arrays-2.rs

@@ -26,7 +26,7 @@ impl Foo for Def {
 
 pub fn test<A: Foo, B: Foo>() {
     let _array = [4; <A as Foo>::Y];
-    //~^ ERROR cannot use an outer type parameter in this context [E0402]
+    //~^ ERROR the trait bound `A: Foo` is not satisfied [E0277]
 }
 
 fn main() {

src/test/compile-fail/associated-const-type-parameter-arrays.rs

@@ -26,7 +26,7 @@ impl Foo for Def {
 
 pub fn test<A: Foo, B: Foo>() {
     let _array: [u32; <A as Foo>::Y];
-    //~^ ERROR cannot use an outer type parameter in this context [E0402]
+    //~^ ERROR the trait bound `A: Foo` is not satisfied [E0277]
 }
 
 fn main() {

src/test/compile-fail/inner-static-type-parameter.rs

@@ -14,7 +14,7 @@ enum Bar<T> { What } //~ ERROR parameter `T` is never used
 
 fn foo<T>() {
     static a: Bar<T> = Bar::What;
-    //~^ ERROR cannot use an outer type parameter in this context
+//~^ ERROR can't use type parameters from outer function; try using a local type parameter instead
 }
 
 fn main() {

src/test/compile-fail/issue-27433.rs

@@ -11,5 +11,5 @@
 fn main() {
     let foo = 42u32;
     const FOO : u32 = foo;
-                   //~^ ERROR attempt to use a non-constant value in a constant
+                   //~^ ERROR can't capture dynamic environment
 }

src/test/compile-fail/issue-3521-2.rs

@@ -12,7 +12,7 @@ fn main() {
     let foo = 100;
 
     static y: isize = foo + 1;
-    //~^ ERROR attempt to use a non-constant value in a constant
+    //~^ ERROR can't capture dynamic environment
 
     println!("{}", y);
 }

src/test/compile-fail/issue-3668-2.rs

@@ -10,7 +10,7 @@
 
 fn f(x:isize) {
     static child: isize = x + 1;
-    //~^ ERROR attempt to use a non-constant value in a constant
+    //~^ ERROR can't capture dynamic environment
 }
 
 fn main() {}

src/test/compile-fail/issue-3668.rs

@@ -16,7 +16,7 @@ trait PTrait {
 impl PTrait for P {
    fn getChildOption(&self) -> Option<Box<P>> {
        static childVal: Box<P> = self.child.get();
-       //~^ ERROR attempt to use a non-constant value in a constant
+       //~^ ERROR can't capture dynamic environment
        panic!();
    }
 }

src/test/compile-fail/issue-39559-2.rs

@@ -0,0 +1,28 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+trait Dim {
+    fn dim() -> usize;
+}
+
+enum Dim3 {}
+
+impl Dim for Dim3 {
+    fn dim() -> usize {
+        3
+    }
+}
+
+fn main() {
+    let array: [usize; Dim3::dim()]
+    //~^ ERROR calls in constants are limited to constant functions
+        = [0; Dim3::dim()];
+        //~^ ERROR calls in constants are limited to constant functions
+}

src/test/compile-fail/issue-39559.rs

@@ -22,12 +22,7 @@ impl Dim for Dim3 {
 
 pub struct Vector<T, D: Dim> {
     entries: [T; D::dim()]
-    //~^ ERROR cannot use an outer type parameter in this context
+    //~^ ERROR no associated item named `dim` found for type `D` in the current scope
 }
 
-fn main() {
-    let array: [usize; Dim3::dim()]
-    //~^ ERROR calls in constants are limited to constant functions
-        = [0; Dim3::dim()];
-        //~^ ERROR calls in constants are limited to constant functions
-}
+fn main() {}

src/test/run-pass/associated-const-type-parameters.rs

@@ -37,6 +37,10 @@ fn sub<A: Foo, B: Foo>() -> i32 {
     A::X - B::X
 }
 
+trait Bar: Foo {
+    const Y: i32 = Self::X;
+}
+
 fn main() {
     assert_eq!(11, Abc::X);
     assert_eq!(97, Def::X);