Incorrect span for unused import when nested

[Badel2]

Consider this import:

use std::io::{BufRead, BufReader, Read};

When BufRead is not used in the code, but BufReader and Read are used, one would expect this error:

warning: unused import: `BufRead`
 --> src/main.rs:3:15
  |
3 | use std::io::{BufRead, BufReader, Read};
  |               ^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

Which is what happens on stable. On nighly, however, that does not happen, sometimes we got this error instead:

warning: unused import: `std::io::{BufRead, BufReader, Read}`
 --> src/main.rs:3:5
  |
3 | use std::io::{BufRead, BufReader, Read};
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

But sometimes it shows the correct error, and sometimes it shows both errors.

Playground link with minimal example
If you comment out the first function it shows both errors, and if you comment out the second function it shows the stable error.

[shssoichiro]

Confirmed here on 2017-12-17 nightly.

[Badel2]

Run a bisect on this: 91ba8b4 is the first bad commit

[shssoichiro]

I'll take a shot at fixing this one. Thanks for bisecting the commit @Badel2!

[arielb1]

Hi @petrochenkov - could you try to help @shssoichiro debug this? This looks pretty bad.

[petrochenkov]

Slightly minimized:

use std::fs::File;
use std::io::{BufRead, BufReader, Read};

pub fn read_from_file(path: &str) {
    let file = File::open(&path).unwrap();
    let mut reader = BufReader::new(file);
    let mut s = String::new();
    reader.read_to_string(&mut s).unwrap();
    
    // comment out this line to solve bug
    s.lines();
}

fn main() {}

This is a fun issue. "Unused import" warnings are reported either during name resolution (normally) or during type checking (for traits that are potentially implicitly used through method calls).

Trait BufRead has method lines, so having any call to a method named lines in code shifts the "unused import" check from resolve to typeck.
All "unused import" warnings reported during typeck have bad spans because spans of imports in HIR are set to spans of whole import trees during lowering from AST to HIR, they need to use spans of final path segments instead.
@shssoichiro The span that needs to be changed is here

rust/src/librustc/hir/lowering.rs

Line 2124 in 4ce6b9a

span: use_tree.span,

[shssoichiro]

Thanks! I'm attempting to test a fix, however the compiler seems to be taking forever to compile stage1 since I rebased off of master. I'll let everyone know how it goes when (if?) compilation ever finishes.