Auto merge of #31925 - aturon:inherent-overlap, r=nikomatsakis

Forbid items with the same name from appearing in overlapping inherent impl blocks

For example, the following is now correctly illegal:

```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}
```

"Overlapping" here is determined the same way it is for traits (and in fact shares the same code path): roughly, there must be some way of substituting any generic types to unify the impls, such that none of the `where` clauses are provably unsatisfiable under such a unification.

Along the way, this PR also introduces an `ImplHeader` abstraction (the first commit) that makes it easier to work with impls abstractly (without caring whether they are trait or inherent impl blocks); see the first commit.

Closes #22889
r? @nikomatsakis

src/test/compile-fail/associated-item-duplicate-names-2.rs

@@ -14,7 +14,7 @@ struct Foo;
 
 impl Foo {
     const bar: bool = true;
-    fn bar() {} //~ ERROR duplicate associated function
+    fn bar() {} //~ ERROR duplicate definitions
 }
 
 fn main() {}

src/test/compile-fail/associated-item-duplicate-names-3.rs

@@ -20,7 +20,7 @@ struct Baz;
 
 impl Foo for Baz {
     type Bar = i16;
-    type Bar = u16; //~ ERROR duplicate associated type
+    type Bar = u16; //~ ERROR duplicate definitions
 }
 
 fn main() {

src/test/compile-fail/associated-item-duplicate-names.rs

@@ -19,9 +19,9 @@ trait Foo {
 
 impl Foo for () {
     type Ty = ();
-    type Ty = usize; //~ ERROR duplicate associated type
+    type Ty = usize; //~ ERROR duplicate definitions
     const BAR: u32 = 7;
-    const BAR: u32 = 8; //~ ERROR duplicate associated constant
+    const BAR: u32 = 8; //~ ERROR duplicate definitions
 }
 
 fn main() {

src/test/compile-fail/impl-duplicate-methods.rs

@@ -11,7 +11,7 @@
 struct Foo;
 impl Foo {
     fn orange(&self){}
-    fn orange(&self){}   //~ ERROR duplicate method
+    fn orange(&self){}   //~ ERROR duplicate definitions
 }
 
 fn main() {}

src/test/compile-fail/inherent-overlap.rs

@@ -0,0 +1,44 @@
+// 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.
+//
+// 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.
+
+// Test that you cannot define items with the same name in overlapping inherent
+// impl blocks.
+
+struct Foo;
+
+impl Foo {
+    fn id() {} //~ ERROR E0201
+}
+
+impl Foo {
+    fn id() {}
+}
+
+struct Bar<T>(T);
+
+impl<T> Bar<T> {
+    fn bar(&self) {} //~ ERROR E0201
+}
+
+impl Bar<u32> {
+    fn bar(&self) {}
+}
+
+struct Baz<T>(T);
+
+impl<T: Copy> Baz<T> {
+    fn baz(&self) {} //~ ERROR E0201
+}
+
+impl<T> Baz<Vec<T>> {
+    fn baz(&self) {}
+}
+
+fn main() {}

src/test/compile-fail/issue-4265.rs

@@ -17,7 +17,7 @@ impl Foo {
         Foo { baz: 0 }.bar();
     }
 
-    fn bar() { //~ ERROR duplicate associated function
+    fn bar() { //~ ERROR duplicate definitions
     }
 }
 

src/test/compile-fail/issue-8153.rs

@@ -18,7 +18,7 @@ trait Bar {
 
 impl Bar for Foo {
     fn bar(&self) -> isize {1}
-    fn bar(&self) -> isize {2} //~ ERROR duplicate method
+    fn bar(&self) -> isize {2} //~ ERROR duplicate definitions
 }
 
 fn main() {

src/test/compile-fail/method-macro-backtrace.rs

@@ -29,7 +29,7 @@ impl S {
 
     // Cause an error. It shouldn't have any macro backtrace frames.
     fn bar(&self) { }
-    fn bar(&self) { } //~ ERROR duplicate method
+    fn bar(&self) { } //~ ERROR duplicate definitions
 }
 
 fn main() { }

src/test/rustdoc/issue-25001.rs

@@ -17,15 +17,15 @@ pub trait Bar {
     fn quux(self);
 }
 
-impl<T> Foo<T> {
+impl Foo<u8> {
     // @has - '//*[@id="method.pass"]//code' 'fn pass()'
     pub fn pass() {}
 }
-impl<T> Foo<T> {
+impl Foo<u16> {
     // @has - '//*[@id="method.pass-1"]//code' 'fn pass() -> usize'
     pub fn pass() -> usize { 42 }
 }
-impl<T> Foo<T> {
+impl Foo<u32> {
     // @has - '//*[@id="method.pass-2"]//code' 'fn pass() -> isize'
     pub fn pass() -> isize { 42 }
 }