The documentation for std::prelude states:

On a technical level, Rust inserts

extern crate std; 

into the crate root of every crate, and

use std::prelude::v1::*; 

into every module.

But I believe the second part of that is no longer a correct description of how the prelude works, as far as user-visible behaviour is concerned.

In particular, the following code compiles, but doesn't if I remove the use statement:

mod foo { use std::prelude::v1::*; type Bar = self::String; } 

And the following doesn't compile (saying «String is ambiguous»), but does if I remove the use for the prelude:

mod foo { use std::prelude::v1::*; use bar::*; mod bar { pub struct String {} } type Baz = String; } 

(Looking in libsyntax_ext/standard_library_imports.rs, I think it is still literally true that the compiler inserts a fake use directive, but the resolver treats it entirely differently to a normal one, so this doesn't seem to be something it makes sense to document.)