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Incorrect warning: type alias is never used #54234

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nitingupta910 opened this Issue Sep 14, 2018 · 2 comments

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@nitingupta910
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nitingupta910 commented Sep 14, 2018

With the following code, I'm getting an incorrect warning:

cargo run

   Compiling hellors v0.1.0 (file:///home/ngupta/temp/hellors)                                                                                                                                                
warning: type alias is never used: `Result`
 --> src/main.rs:3:1
  |
3 | type Result<T> = ::std::result::Result<(T, usize), usize>;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.44s
     Running `target/debug/hellors`

Code

// On success, return what was parsed and where to start parsing from next.
// On failure, return where the parsing failed.
type Result<T> = ::std::result::Result<(T, usize), usize>;

fn parse_until(terminator: &'static str) -> impl Fn(&str, usize) -> Result<&str> {
    move |s, location| {
        let s = &s[location..];
        match s.find(terminator) {
            Some(pos) => {
                let head = &s[..pos];
                Ok((head, location + pos + terminator.len()))
            },
            None => {
                Err(location)
            }
        }
    }
}

fn main() {
    let input = "hello\nworld";
    assert_eq!(Ok(("hello", 6)), parse_until("\n")(input, 0));
    assert_eq!(Err(6), parse_until("\n")(input, 6));
}

Version

rustc --version
rustc 1.29.0 (aa3ca1994 2018-09-11)
@Centril

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Centril commented Sep 14, 2018

minimized to:

type Alias = ();

fn fun() -> impl FnOnce() -> Alias {
    || ()
}

fn main() {
    fun()();
}
@RalfJung

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RalfJung commented Oct 16, 2018

Potentially related: #47131

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