lintcheck: fix --fix and document it in the readme.

also hook lintcheck into clippy-dev so that `clippy dev fmt` formats it.

clippy_dev/src/fmt.rs

@@ -54,6 +54,7 @@ pub fn run(check: bool, verbose: bool) {
         success &= cargo_fmt(context, project_root.as_path())?;
         success &= cargo_fmt(context, &project_root.join("clippy_dev"))?;
         success &= cargo_fmt(context, &project_root.join("rustc_tools_util"))?;
+        success &= cargo_fmt(context, &project_root.join("lintcheck"))?;
 
         for entry in WalkDir::new(project_root.join("tests")) {
             let entry = entry?;

lintcheck/README.md

@@ -67,3 +67,9 @@ is checked.
 
 **Note:** `-Wclippy::all` is always enabled by default, unless `-Aclippy::all`
 is explicitly specified in the options.
+
+### Fix mode
+You can run `./lintcheck/target/debug/lintcheck --fix` which will run Clippy with `-Zunstable-options --fix` and print a warning if Clippys suggestions fail to apply (if the resulting code does not build).
+This lets us spot bad suggestions or false positives automatically in some cases.  
+
+Please note that the target dir should be cleaned afterwards since clippy will modify the downloaded sources which can lead to unexpected results when running lintcheck again afterwards.

lintcheck/src/main.rs

@@ -862,7 +862,11 @@ fn get_clap_config<'a>() -> ArgMatches<'a> {
                 .long("jobs")
                 .help("number of threads to use, 0 automatic choice"),
         )
-        .arg(Arg::with_name("fix").help("runs cargo clippy --fix and checks if all suggestions apply"))
+        .arg(
+            Arg::with_name("fix")
+                .long("--fix")
+                .help("runs cargo clippy --fix and checks if all suggestions apply"),
+        )
         .get_matches()
 }