Cannot run `cargo clippy` with --example

[kennytm]

When trying to run cargo clippy --example ..., the tool will error with:

error: extra arguments to rustc can only be passed to one target, consider filtering
the package by passing e.g. --lib or --bin NAME to specify a single target

---

Repro steps:

1. Create a cargo project with the following structure

   project/
       Cargo.toml
       src/
           ...
       examples/
           one.rs
   

2. Fill in one.rs with:

   /// BadComment.
   pub fn main() {
   }

3. Run clippy:

   $ cargo clippy --example one

4. Expecting a lint on /// BadComment. Got the error above instead.

---

Note that using the plugin, or running the lint via cargo rustc works fine:

$ cargo rustc --example one -- -L path/to/libclippy.dylib -Z extra-plugins=clippy
   Compiling test3 v0.1.0 (file:///path/to/project)
warning: you should put `BadComment` between ticks in the documentation, #[warn(doc_markdown)] on by default
 --> examples/one.rs:1:5
  |
1 | /// BadComment.
  |     ^^^^^^^^^^
  |
  = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#doc_markdown

    Finished debug [unoptimized + debuginfo] target(s) in 0.31 secs

Running cargo clippy without using any arguments will not lint the examples:

$ cargo clippy
   Compiling test3 v0.1.0 (file:///path/to/project)
    Finished debug [unoptimized + debuginfo] target(s) in 0.8 secs

---

Clippy version: v0.0.82

$ rustc -vV
rustc 1.12.0-nightly (197be89f3 2016-08-15)
binary: rustc
commit-hash: 197be89f367d1240d5f9cd9c4efd77812775354e
commit-date: 2016-08-15
host: x86_64-apple-darwin
release: 1.12.0-nightly

$ cargo -vV
cargo 0.13.0-nightly (cd8ad10 2016-08-15)

[Manishearth]

I guess this is a cargo bug? We run cargo rustc --example foo -- <some arguments> for that command. cargo rustc errors if you don't select a bin/lib for a multitarget project, but it should work with --example

[kennytm]

@Manishearth As I mentioned in the report, cargo rustc --example one -- -L path/to/libclippy.dylib -Z extra-plugins=clippy does work 😉 but cargo clippy --example one does not.

[oli-obk]

We don't even support cargo clippy --lib anymore, cargo clippy runs on all bin and lib targets. We probably need to add examples to the list of stuff that needs to be checked.

[oli-obk]

#1171 runs clippy on examples, too. But still no way to specify to only run on one example.

[phansch]

Looks like this was fixed with #2582, too:

$ cargo new repro && cd repro

$ echo >examples/one.rs '
fn main() {
    1111111111111111;
    ()
}
'

$ echo >examples/two.rs '
fn main() {
    2222222222222222;
    ()
}
'

Running cargo clippy --example one only lints example one and running cargo clippy --example two only lints example two.

To repro with the original example, it's now required to add #[warn(doc_markdown)] because the doc_markdown lint is now in the pedantic group which is allow by default.

I'm going ahead and close it, but feel free to re-open if it's still causing problems.