Rollup merge of #41052 - topecongiro:overlapping_inherent_impls, r=estebank

Make 'overlapping_inherent_impls' lint a hard error

This is ought to be implemented in PR #40728. Unfortunately, when I rebased the PR to resolve merge conflict, the "hard error" code disappeared. This PR complements the initial PR.

Now the following rust code gives the following error:
```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}

fn main() {}
```
```
error[E0592]: duplicate definitions with name `id`
 --> /home/topecongiro/test.rs:4:5
  |
4 |     fn id() {}
  |     ^^^^^^^^^^ duplicate definitions for `id`
...
8 |     fn id() {}
  |     ---------- other definition for `id`

error: aborting due to previous error
```

src/test/ui/codemap_tests/coherence-overlapping-inherent-impl-trait.rs

@@ -0,0 +1,16 @@
+// 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.
+
+#![allow(dead_code)]
+
+trait C {}
+impl C { fn f() {} }
+impl C { fn f() {} }
+fn main() { }

src/test/ui/codemap_tests/coherence-overlapping-inherent-impl-trait.stderr

@@ -0,0 +1,10 @@
+error[E0592]: duplicate definitions with name `f`
+  --> $DIR/coherence-overlapping-inherent-impl-trait.rs:14:10
+   |
+14 | impl C { fn f() {} }
+   |          ^^^^^^^^^ duplicate definitions for `f`
+15 | impl C { fn f() {} }
+   |          --------- other definition for `f`
+
+error: aborting due to previous error
+

src/test/ui/codemap_tests/overlapping_inherent_impls.rs

@@ -0,0 +1,46 @@
+// 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.
+
+#![allow(unused)]
+
+struct Foo;
+
+impl Foo {
+    fn id() {}
+}
+
+impl Foo {
+    fn id() {}
+}
+
+struct Bar<T>(T);
+
+impl<T> Bar<T> {
+    fn bar(&self) {}
+}
+
+impl Bar<u32> {
+    fn bar(&self) {}
+}
+
+struct Baz<T>(T);
+
+impl<T: Copy> Baz<T> {
+    fn baz(&self) {}
+}
+
+impl<T> Baz<Vec<T>> {
+    fn baz(&self) {}
+}
+
+fn main() {}

src/test/ui/codemap_tests/overlapping_inherent_impls.stderr

@@ -0,0 +1,29 @@
+error[E0592]: duplicate definitions with name `id`
+  --> $DIR/overlapping_inherent_impls.rs:19:5
+   |
+19 |     fn id() {}
+   |     ^^^^^^^^^^ duplicate definitions for `id`
+...
+23 |     fn id() {}
+   |     ---------- other definition for `id`
+
+error[E0592]: duplicate definitions with name `bar`
+  --> $DIR/overlapping_inherent_impls.rs:29:5
+   |
+29 |     fn bar(&self) {}
+   |     ^^^^^^^^^^^^^^^^ duplicate definitions for `bar`
+...
+33 |     fn bar(&self) {}
+   |     ---------------- other definition for `bar`
+
+error[E0592]: duplicate definitions with name `baz`
+  --> $DIR/overlapping_inherent_impls.rs:39:5
+   |
+39 |     fn baz(&self) {}
+   |     ^^^^^^^^^^^^^^^^ duplicate definitions for `baz`
+...
+43 |     fn baz(&self) {}
+   |     ---------------- other definition for `baz`
+
+error: aborting due to 3 previous errors
+