refactor: Introduce crates folder (#2088)

This PR introduces a new `crates` directory and moves all "product" crates into that folder. 

Part of #2059.

crates/ruff_cli/src/cache.rs

@@ -0,0 +1,156 @@
+use std::collections::hash_map::DefaultHasher;
+use std::fs;
+use std::hash::{Hash, Hasher};
+use std::io::Write;
+use std::path::Path;
+
+use anyhow::Result;
+use filetime::FileTime;
+use log::error;
+use path_absolutize::Absolutize;
+use ruff::message::Message;
+use ruff::settings::{flags, AllSettings, Settings};
+use serde::{Deserialize, Serialize};
+
+const CARGO_PKG_VERSION: &str = env!("CARGO_PKG_VERSION");
+
+#[derive(Serialize, Deserialize)]
+struct CacheMetadata {
+    mtime: i64,
+}
+
+#[derive(Serialize)]
+struct CheckResultRef<'a> {
+    metadata: &'a CacheMetadata,
+    messages: &'a [Message],
+}
+
+#[derive(Deserialize)]
+struct CheckResult {
+    metadata: CacheMetadata,
+    messages: Vec<Message>,
+}
+
+fn content_dir() -> &'static Path {
+    Path::new("content")
+}
+
+fn cache_key<P: AsRef<Path>>(
+    path: P,
+    package: Option<&P>,
+    settings: &Settings,
+    autofix: flags::Autofix,
+) -> u64 {
+    let mut hasher = DefaultHasher::new();
+    CARGO_PKG_VERSION.hash(&mut hasher);
+    path.as_ref().absolutize().unwrap().hash(&mut hasher);
+    package
+        .as_ref()
+        .map(|path| path.as_ref().absolutize().unwrap())
+        .hash(&mut hasher);
+    settings.hash(&mut hasher);
+    autofix.hash(&mut hasher);
+    hasher.finish()
+}
+
+#[allow(dead_code)]
+/// Initialize the cache at the specified `Path`.
+pub fn init(path: &Path) -> Result<()> {
+    // Create the cache directories.
+    fs::create_dir_all(path.join(content_dir()))?;
+
+    // Add the CACHEDIR.TAG.
+    if !cachedir::is_tagged(path)? {
+        cachedir::add_tag(path)?;
+    }
+
+    // Add the .gitignore.
+    let gitignore_path = path.join(".gitignore");
+    if !gitignore_path.exists() {
+        let mut file = fs::File::create(gitignore_path)?;
+        file.write_all(b"*")?;
+    }
+
+    Ok(())
+}
+
+fn write_sync(cache_dir: &Path, key: u64, value: &[u8]) -> Result<(), std::io::Error> {
+    fs::write(
+        cache_dir.join(content_dir()).join(format!("{key:x}")),
+        value,
+    )
+}
+
+fn read_sync(cache_dir: &Path, key: u64) -> Result<Vec<u8>, std::io::Error> {
+    fs::read(cache_dir.join(content_dir()).join(format!("{key:x}")))
+}
+
+fn del_sync(cache_dir: &Path, key: u64) -> Result<(), std::io::Error> {
+    fs::remove_file(cache_dir.join(content_dir()).join(format!("{key:x}")))
+}
+
+/// Get a value from the cache.
+pub fn get<P: AsRef<Path>>(
+    path: P,
+    package: Option<&P>,
+    metadata: &fs::Metadata,
+    settings: &AllSettings,
+    autofix: flags::Autofix,
+) -> Option<Vec<Message>> {
+    let encoded = read_sync(
+        &settings.cli.cache_dir,
+        cache_key(path, package, &settings.lib, autofix),
+    )
+    .ok()?;
+    let (mtime, messages) = match bincode::deserialize::<CheckResult>(&encoded[..]) {
+        Ok(CheckResult {
+            metadata: CacheMetadata { mtime },
+            messages,
+        }) => (mtime, messages),
+        Err(e) => {
+            error!("Failed to deserialize encoded cache entry: {e:?}");
+            return None;
+        }
+    };
+    if FileTime::from_last_modification_time(metadata).unix_seconds() != mtime {
+        return None;
+    }
+    Some(messages)
+}
+
+/// Set a value in the cache.
+pub fn set<P: AsRef<Path>>(
+    path: P,
+    package: Option<&P>,
+    metadata: &fs::Metadata,
+    settings: &AllSettings,
+    autofix: flags::Autofix,
+    messages: &[Message],
+) {
+    let check_result = CheckResultRef {
+        metadata: &CacheMetadata {
+            mtime: FileTime::from_last_modification_time(metadata).unix_seconds(),
+        },
+        messages,
+    };
+    if let Err(e) = write_sync(
+        &settings.cli.cache_dir,
+        cache_key(path, package, &settings.lib, autofix),
+        &bincode::serialize(&check_result).unwrap(),
+    ) {
+        error!("Failed to write to cache: {e:?}");
+    }
+}
+
+/// Delete a value from the cache.
+pub fn del<P: AsRef<Path>>(
+    path: P,
+    package: Option<&P>,
+    settings: &AllSettings,
+    autofix: flags::Autofix,
+) {
+    drop(del_sync(
+        &settings.cli.cache_dir,
+        cache_key(path, package, &settings.lib, autofix),
+    ));
+}