Clarify public type aliasing behavior #26953
Description
Compile:
// Example #1
mod mod_a {
mod sub_a {
pub struct InnerA;
}
pub type A = sub_a::InnerA;
}
use mod_a::A;
fn main() {
let a=A;
}error[E0425]: unresolved name `A`
--> test.rs:12:11
|
12 | let a=A;
| ^ unresolved name
Yet A is quite obviously imported. The same error appears in a different place when compiling:
// Example #2
mod mod_a {
mod sub_a {
pub struct InnerA;
}
pub type A = sub_a::InnerA;
pub fn make_a() -> A {
A
}
}
use mod_a::{A,make_a};
fn main() {
let a: A=make_a();
}error[E0425]: unresolved name `A`
--> test.rs:9:9
|
9 | A
| ^ unresolved name
This compiles:
// Example #3
mod mod_a {
mod sub_a {
pub struct InnerA;
}
pub type A = sub_a::InnerA;
pub fn make_a() -> A {
sub_a::InnerA
}
}
use mod_a::{A,make_a};
fn main() {
let a: A=make_a();
}But wait, I thought in example no. 1, we couldn't resolve A from main()?
Using pub use instead makes example no. 1 work:
// Example #4
mod mod_a {
mod sub_a {
pub struct InnerA;
}
pub use self::sub_a::InnerA as A;
}
use mod_a::A;
fn main() {
let a=A;
}For completeness, here is the tuple-struct version:
// Example #5
mod mod_a {
mod sub_a {
pub struct InnerA(pub u8);
}
pub type A = sub_a::InnerA;
}
use mod_a::A;
fn main() {
let a=A(0);
}error[E0425]: unresolved name `A`
--> test.rs:12:11
|
12 | let a=A(0);
| ^ unresolved name
And the regular struct version (compiles without errors):
// Example #6
mod mod_a {
mod sub_a {
pub struct InnerA {pub i:u8}
}
pub type A = sub_a::InnerA;
}
use mod_a::A;
fn main() {
let a=A{i:0};
}It seems like structure expressionsconstructors are handled differently from other uses of types with regards to aliases. Also, the phrase pub type does not appear in the Rust Book or the Rust Reference.
I propose the following:
- Improve the error message in cases 1, 2 and 5 above
- Improve documentation on public type aliases
- Improve documentation on structure
expressionsconstructors involving type aliases