strange absolute paths warning if ed2018 crate is build with warn( rust_2018_compatibility)

[matthiaskrgr]

Cargo.toml:

[package]
name = "claptest"
version = "0.1.0"
authors = ["Matthias Krüger"]
edition = "2018"

[dependencies]
clap ="2.32.0"

src/main.rs

#![warn(rust_2018_compatibility)]

use clap::{App,value_t};


fn main() {
    println!("Hello, world!");

let matches = App::new("myapp")
              .arg_from_usage("[length] 'Set the length to use as a pos whole num, i.e. 20'")
              .get_matches();
let len = value_t!(matches.value_of("length"), u32)
                .unwrap_or_else(|e|{
                    println!("{}",e);
                    std::process::exit(1)
                });
}

This produces the warning

warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
  --> src/main.rs:12:11
   |
12 | let len = value_t!(matches.value_of("length"), u32)
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: lint level defined here
  --> src/main.rs:1:9
   |
1  | #![warn(rust_2018_compatibility)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^
   = note: #[warn(absolute_paths_not_starting_with_crate)] implied by #[warn(rust_2018_compatibility)]
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = note: for more information, see issue #53130 <https://github.com/rust-lang/rust/issues/53130>
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
  --> src/main.rs:12:11
   |
12 | let len = value_t!(matches.value_of("length"), u32)
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = note: for more information, see issue #53130 <https://github.com/rust-lang/rust/issues/53130>
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

but only if rust_2018_compatibility is warned about which is confusing.
Maybe rust_2018_compatibility should be disabled if the crate is already edition 2018?

This warning appeared first with

rustc 1.32.0-nightly (400c2bc5e 2018-11-27)
binary: rustc
commit-hash: 400c2bc5ed292f77c49693320f4eda37bb375e90
commit-date: 2018-11-27
host: x86_64-unknown-linux-gnu
release: 1.32.0-nightly
LLVM version: 8.0

Probably related to #56262

[petrochenkov]

Fixed in #56322

[pnkfelix]

triage: tagging as P-high. Has an r+'ed p=1 fix in PR #56322

[pnkfelix]

reopening issue to ensure we don't close it until beta backport is done.

[alexcrichton]

This is now tracked at #56352 and this won't get auto-closed when that merges because github only does that for the master branch, so I'm going to go ahead and close this in favor of that PR.