Warn or error on multiple-`mod` code duplication

[Kimundi]

Sometimes people use multiple mod statements that refer to the same file in order to access that files content as a module in different modules/files, thinking all those instances would be identical. But in reality that just duplicates all definitions in the file at different places in the module hierarchy, which leads to confusing errors like two presumably identical types not being identical etc.

I think two mod statements in a crate that refer to the same file should at least emit a code duplication warning, if not an error, because it's almost never what you want.

[emberian]

@Kimundi testcase please?

[Kimundi]

Well the problem is basically if you have written your source files like this:

crate.rs:

pub mod a;
pub mod b;

a.rs:

pub mod c;

b.rs:

pub mod c;

c.rs:

pub struct Foo;

A beginner not familiar with the module system might now think that a::c::Foo and b::c::Foo are aliases of each other because they refer to the same file, while in fact they are two distinct copies in your module hierarchy. Seeing how you almost never want this kind of module duplication to begin with, emitting at least a warning seems to be the right thing to do.

[huonw]

Triage: still occurs, having a lint would be useful.

[flaper87]

Triage: this is still an issue. I believe this will caught as part of the new resolve algorithm. ( #13449 )

[alexcrichton]

Closed by https://github.com/rust-lang/rfcs/blob/master/text/0063-module-file-system-hierarchy.md