strange rust_2018_idioms warning on code comment in clippy codebase

[matthiaskrgr]

git clone http://github.com/rust-lang-nursery/rust-clippy/
cd rust-clippy
git checkout 656b26ea4f2b330ea5b9cb3cb38545587eac8f7f
cargo check --all-targets --all-features 

=>

....
warning: `extern crate` is not idiomatic in the new edition
 --> src/lib.rs:1:1
  |
1 | // error-pattern:cargo-clippy
  | ^ help: convert it to a `use`
  |
note: lint level defined here
 --> src/lib.rs:8:9
  |
8 | #![warn(rust_2018_idioms)]
  |         ^^^^^^^^^^^^^^^^
  = note: #[warn(unused_extern_crates)] implied by #[warn(rust_2018_idioms)]

this is confusing because the highlighted line is a code comment and seems unrelated to the extern crate keyword.

rustc 1.28.0-nightly (cd494c1 2018-06-27)

link to file: https://github.com/rust-lang-nursery/rust-clippy/blob/656b26ea4f2b330ea5b9cb3cb38545587eac8f7f/src/lib.rs

[oli-obk]

I'd assume this is the 2015 prelude's extern crate std; item.

[najamelan]

I get this warning consistently from RLS-preview in crates that don't have extern crate. I have edition = 2018 in cargo.toml. I do not get this warning when compiling from the command line with cargo build.

[Mark-Simulacrum]

cc @nrc for the change between RLS and CLI behavior

[nikomatsakis]

Removing from the release milestone, as functional Idiom Lints are not part of our Rust 2018 release goals. But we oughta' fix this =)

[Enselic]

Triage: The reproducer does not work any longer with Rust 1.74, but I get a bunch of other errors, so hard to say if the reported problem has been fixed. Here is the current output:

Click to expand

$ cargo check --all-targets --all-features
warning: the cargo feature `edition` has been stabilized in the 1.31 release and is no longer necessary to be listed in the manifest
  See https://doc.rust-lang.org/cargo/reference/manifest.html#the-edition-field for more information about using this feature.
    Updating crates.io index
   Compiling proc-macro2 v1.0.69
   Compiling unicode-ident v1.0.12
   Compiling libc v0.2.150
   Compiling serde v1.0.192
    Checking memchr v2.6.4
   Compiling serde_json v1.0.108
    Checking gimli v0.28.0
    Checking adler v1.0.2
    Checking cfg-if v1.0.0
   Compiling semver-parser v0.7.0
    Checking rustc-demangle v0.1.23
   Compiling pulldown-cmark v0.1.2
    Checking ryu v1.0.15
    Checking itoa v1.0.9
   Compiling syn v1.0.109
    Checking tinyvec_macros v0.1.1
    Checking unicode-width v0.1.11
    Checking unicode-bidi v0.3.13
    Checking matches v0.1.10
   Compiling ansi_term v0.11.0
    Checking regex-syntax v0.8.2
   Compiling unicode-xid v0.2.4
    Checking percent-encoding v1.0.1
    Checking tinyvec v1.6.0
   Compiling failure_derive v0.1.8
    Checking miniz_oxide v0.7.1
    Checking either v1.9.0
    Checking bitflags v0.9.1
    Checking getopts v0.2.21
    Checking itertools v0.7.11
    Checking cfg-if v0.1.10
    Checking if_chain v0.1.3
    Checking lazy_static v1.4.0
    Checking regex-syntax v0.6.29
    Checking quine-mc_cluskey v0.2.4
    Checking log v0.4.20
   Compiling compiletest_rs v0.3.26
    Checking diff v0.1.13
   Compiling clippy-mini-macro-test v0.2.0 (/home/martin/src/rust-clippy/mini-macro)
error[E0557]: feature has been removed
 --> mini-macro/src/lib.rs:1:24
  |
1 | #![feature(proc_macro, proc_macro_non_items)]
  |                        ^^^^^^^^^^^^^^^^^^^^ feature has been removed
  |
  = note: subsumed by `#![feature(proc_macro_hygiene)]`

error[E0658]: use of unstable library feature 'proc_macro_quote'
 --> mini-macro/src/lib.rs:8:5
  |
8 |     quote!(
  |     ^^^^^
  |
  = note: see issue #54722 <https://github.com/rust-lang/rust/issues/54722> for more information

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> mini-macro/src/lib.rs:1:1
  |
1 | #![feature(proc_macro, proc_macro_non_items)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: the feature `proc_macro` has been stable since `1.29.0` and no longer requires an attribute to enable

error[E0658]: use of unstable library feature 'proc_macro_quote'
 --> mini-macro/src/lib.rs:4:31
  |
4 | use proc_macro::{TokenStream, quote};
  |                               ^^^^^
  |
  = note: see issue #54722 <https://github.com/rust-lang/rust/issues/54722> for more information

warning: the feature `proc_macro` has been stable since 1.29.0 and no longer requires an attribute to enable
 --> mini-macro/src/lib.rs:1:12
  |
1 | #![feature(proc_macro, proc_macro_non_items)]
  |            ^^^^^^^^^^
  |
  = note: `#[warn(stable_features)]` on by default

Some errors have detailed explanations: E0554, E0557, E0658.
For more information about an error, try `rustc --explain E0554`.
warning: `clippy-mini-macro-test` (lib) generated 1 warning
error: could not compile `clippy-mini-macro-test` (lib) due to 4 previous errors; 1 warning emitted
warning: build failed, waiting for other jobs to finish...