Add note when a function is called from a module like it's an object

[GuillaumeGomez]

The following code:

use std::io;

fn main() {
    io.stdin();
}

Which prints out:

test.rs:5:5: 5:7 error: unresolved name `io` [E0425]
test.rs:5     io.stdin();

Some beginners had issues with this error because they didn't understood it, so I added an explanation which helps them to understand it better.

r? @pnkfelix
cc @Manishearth
cc @nagisa

[nagisa]

Why not

To reference an item from the {module} module, use {module}::{next_path_component}.

instead? It is shorter and more-to-the-point. Also this obviously should become a span_help.

E.g. for your case it would then look like this:

To reference an item from the io module, use io::stdin.

[Manishearth]

Agreed

[nagisa]

Tests?

[Manishearth]

I guess the help msg should be passed down to resolve_error? I think the rest of the file does help messages this way

[pnkfelix]

I infer @Manishearth is referring to how a case like OnlyIrrefutablePatternsAllowedHere is handled; see linked code; following such a strategy would entail either changing the UnresolvedName variant, or adding a new variant that carries the necessary info... but nonetheless, it is probably a good suggestion, just to keep all the error reporting in one place...

[GuillaumeGomez]

I don't mind, it's the same for me. I'll just provide an Option as extra parameter and we're good to go.

[GuillaumeGomez]

@nagisa: Since it doesn't add a new error code, is it really needed ? I can add one, but if there is already one for this, it'll be useless...

[Manishearth]

There should always be tests. Error codes are orthogonal.

[Manishearth]

To be clearer, notes should have tests too. If there is a test, update it to mention the note, otherwise create your own.

[GuillaumeGomez]

PS: To add a "note" tested comment:

//~NOTE

[pnkfelix]

what is this?

[GuillaumeGomez]

A test of mine, wasn't intended to get committed. Just ignore it please.

[GuillaumeGomez]

@pnkfelix found a way to handle 2 of 4 cases with the following code. of this example:

mod a { mod b { fn f() -> i32 { 3 } } }

fn g1() -> i32 {
    a.b.f()
}

fn g2() -> i32 {
    a::b.f()
}

fn g3() -> i32 {
    a::b()
}

fn g4() -> i32 {
    a::b
}

fn main() {
    println!("g1: {} g2: {} g3: {}", g1(), g2(), g3());
}

I'll cherry-pick his work and try to handle the last case.

[pnkfelix]

closing in favor of PR #30413 (where @GuillaumeGomez still gets attribution)