Clearer use of the error message and span labels to communicate
duplicaiton defitions/imports.
New error format:
```
error[E0428]: the name `Foo` is defined twice
--> example.rs:2:1
|
1 | trait Foo { }
| ------------- previous definition of the trait `Foo` here
2 | struct Foo { }
| ^^^^^^^^^^^^^^ `Foo` redefined here
= note: `Foo` must be defined only once in the type namespace of this module
error: aborting due to previous error
```
declared with the same name in the same scope.
This breaks several common patterns. First are unused imports:
use foo::bar;
use baz::bar;
Change this code to the following:
use baz::bar;
Second, this patch breaks globs that import names that are shadowed by
subsequent imports. For example:
use foo::*; // including `bar`
use baz::bar;
Change this code to remove the glob:
use foo::{boo, quux};
use baz::bar;
Or qualify all uses of `bar`:
use foo::{boo, quux};
use baz;
... baz::bar ...
Finally, this patch breaks code that, at top level, explicitly imports
`std` and doesn't disable the prelude.
extern crate std;
Because the prelude imports `std` implicitly, there is no need to
explicitly import it; just remove such directives.
The old behavior can be opted into via the `import_shadowing` feature
gate. Use of this feature gate is discouraged.
This implements RFC #116.
Closes#16464.
[breaking-change]
