Added cargo dev setup git-hook

clippy_dev/src/setup/git_hook.rs

@@ -0,0 +1,61 @@
+use std::fs;
+use std::path::Path;
+
+/// Rusts setup uses `git rev-parse --git-common-dir` to get the root directory of the repo.
+/// I've decided against this for the sake of simplicity and to make sure that it doesn't install
+/// the hook if `clippy_dev` would be used in the rust tree. The hook also references this tool
+/// for formatting and should therefor only be used in a normal clone of clippy
+const REPO_GIT_DIR: &str = ".git";
+const HOOK_SOURCE_PATH: &str = "util/etc/pre-commit.sh";
+const HOOK_TARGET_PATH: &str = ".git/hooks/pre-commit";
+
+pub fn run(force_override: bool) {
+    if let Err(_) = check_precondition(force_override) {
+        return;
+    }
+
+    // So a little bit of a funny story. Git on unix requires the pre-commit file
+    // to have the `execute` permission to be set. The Rust functions for modifying
+    // these flags doesn't seem to work when executed with normal user permissions.
+    //
+    // However, there is a little hack that is also being used by Rust itself in their
+    // setup script. Git saves the `execute` flag when syncing files. This means
+    // that we can check in a file with execution permissions and the sync it to create
+    // a file with the flag set. We then copy this file here. The copy function will also
+    // include the `execute` permission.
+    match fs::copy(HOOK_SOURCE_PATH, HOOK_TARGET_PATH) {
+        Ok(_) => println!("Git hook successfully installed :)"),
+        Err(err) => println!(
+            "error: unable to copy `{}` to `{}` ({})",
+            HOOK_SOURCE_PATH, HOOK_TARGET_PATH, err
+        ),
+    }
+}
+
+fn check_precondition(force_override: bool) -> Result<(), ()> {
+    // Make sure that we can find the git repository
+    let git_path = Path::new(REPO_GIT_DIR);
+    if !git_path.exists() || !git_path.is_dir() {
+        println!("error: clippy_dev was unable to find the `.git` directory");
+        return Err(());
+    }
+
+    // Make sure that we don't override an existing hook by accident
+    let path = Path::new(HOOK_TARGET_PATH);
+    if path.exists() {
+        if !force_override {
+            println!("warn: The found `.git` directory already has a commit hook");
+        }
+
+        if force_override || super::ask_yes_no_question("Do you want to override it?") {
+            if fs::remove_file(path).is_err() {
+                println!("error: unable to delete existing pre-commit git hook");
+                return Err(());
+            }
+        } else {
+            return Err(());
+        }
+    }
+
+    Ok(())
+}