Add more doc comments for gc changes.

I missed adding these in rust-lang#12634.

src/cargo/core/global_cache_tracker.rs

@@ -154,43 +154,53 @@ type Timestamp = u64;
 /// The key for a registry index entry stored in the database.
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub struct RegistryIndex {
+    /// A unique name of the registry source.
     pub encoded_registry_name: InternedString,
 }
 
 /// The key for a registry `.crate` entry stored in the database.
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub struct RegistryCrate {
+    /// A unique name of the registry source.
     pub encoded_registry_name: InternedString,
+    /// The filename of the compressed crate, like `foo-1.2.3.crate`.
     pub crate_filename: InternedString,
+    /// The size of the `.crate` file.
     pub size: u64,
 }
 
 /// The key for a registry src directory entry stored in the database.
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub struct RegistrySrc {
+    /// A unique name of the registry source.
     pub encoded_registry_name: InternedString,
+    /// The directory name of the extracted source, like `foo-1.2.3`.
     pub package_dir: InternedString,
-    // Total size of the src directory in bytes.
-    //
-    // This can be None when the size is unknown. For example, when the src
-    // directory already exists on disk, and we just want to update the
-    // last-use timestamp. We don't want to take the expense of computing disk
-    // usage unless necessary. `populate_untracked_src` will handle any actual
-    // NULL values in the database, which can happen when the src directory is
-    // created by an older version of cargo that did not track sizes.
+    /// Total size of the src directory in bytes.
+    ///
+    /// This can be None when the size is unknown. For example, when the src
+    /// directory already exists on disk, and we just want to update the
+    /// last-use timestamp. We don't want to take the expense of computing disk
+    /// usage unless necessary. [`GlobalCacheTracker::populate_untracked`]
+    /// will handle any actual NULL values in the database, which can happen
+    /// when the src directory is created by an older version of cargo that
+    /// did not track sizes.
     pub size: Option<u64>,
 }
 
 /// The key for a git db entry stored in the database.
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub struct GitDb {
+    /// A unique name of the git database.
     pub encoded_git_name: InternedString,
 }
 
 /// The key for a git checkout entry stored in the database.
 #[derive(Clone, Debug, Hash, Eq, PartialEq)]
 pub struct GitCheckout {
+    /// A unique name of the git database.
     pub encoded_git_name: InternedString,
+    /// A unique name of the checkout without the database.
     pub short_name: InternedString,
     /// Total size of the checkout directory.
     ///

src/cargo/sources/git/source.rs

@@ -76,7 +76,17 @@ pub struct GitSource<'cfg> {
     /// The unique identifier of this source.
     source_id: SourceId,
     /// The underlying path source to discover packages inside the Git repository.
+    ///
+    /// This gets set to `Some` after the git repo has been checked out
+    /// (automatically handled via [`GitSource::block_until_ready`]).
     path_source: Option<PathSource<'cfg>>,
+    /// A short string that uniquely identifies the version of the checkout.
+    ///
+    /// This is typically a 7-character string of the OID hash, automatically
+    /// increasing in size if it is ambiguous.
+    ///
+    /// This is set to `Some` after the git repo has been checked out
+    /// (automatically handled via [`GitSource::block_until_ready`]).
     short_id: Option<InternedString>,
     /// The identifier of this source for Cargo's Git cache directory.
     /// See [`ident`] for more.

src/cargo/sources/registry/http_remote.rs

@@ -54,6 +54,8 @@ const UNKNOWN: &'static str = "Unknown";
 ///
 /// [RFC 2789]: https://github.com/rust-lang/rfcs/pull/2789
 pub struct HttpRegistry<'cfg> {
+    /// The name of this source, a unique string (across all sources) used as
+    /// the directory name where its cached content is stored.
     name: InternedString,
     /// Path to the registry index (`$CARGO_HOME/registry/index/$REG-HASH`).
     ///

src/cargo/sources/registry/mod.rs

@@ -240,6 +240,8 @@ struct LockMetadata {
 ///
 /// For general concepts of registries, see the [module-level documentation](crate::sources::registry).
 pub struct RegistrySource<'cfg> {
+    /// A unique name of the source (typically used as the directory name
+    /// where its cached content is stored).
     name: InternedString,
     /// The unique identifier of this source.
     source_id: SourceId,
@@ -444,6 +446,11 @@ mod remote;
 /// Generates a unique name for [`SourceId`] to have a unique path to put their
 /// index files.
 fn short_name(id: SourceId, is_shallow: bool) -> String {
+    // CAUTION: This should not change between versions. If you change how
+    // this is computed, it will orphan previously cached data, forcing the
+    // cache to be rebuilt and potentially wasting significant disk space. If
+    // you change it, be cautious of the impact. See `test_cratesio_hash` for
+    // a similar discussion.
     let hash = hex::short_hash(&id);
     let ident = id.url().host_str().unwrap_or("").to_string();
     let mut name = format!("{}-{}", ident, hash);

src/cargo/sources/registry/remote.rs

@@ -48,6 +48,8 @@ use tracing::{debug, trace};
 ///
 /// [`HttpRegistry`]: super::http_remote::HttpRegistry
 pub struct RemoteRegistry<'cfg> {
+    /// The name of this source, a unique string (across all sources) used as
+    /// the directory name where its cached content is stored.
     name: InternedString,
     /// Path to the registry index (`$CARGO_HOME/registry/index/$REG-HASH`).
     index_path: Filesystem,

src/cargo/util/config/mod.rs

@@ -245,7 +245,10 @@ pub struct Config {
     pub nightly_features_allowed: bool,
     /// WorkspaceRootConfigs that have been found
     pub ws_roots: RefCell<HashMap<PathBuf, WorkspaceRootConfig>>,
+    /// The global cache tracker is a database used to track disk cache usage.
     global_cache_tracker: LazyCell<RefCell<GlobalCacheTracker>>,
+    /// A cache of modifications to make to [`Config::global_cache_tracker`],
+    /// saved to disk in a batch to improve performance.
     deferred_global_last_use: LazyCell<RefCell<DeferredGlobalLastUse>>,
 }