Trace some basic I/O operations in bootstrap

Author: Kobzol

src/bootstrap/src/lib.rs

@@ -1773,6 +1773,10 @@ impl Build {
         if src == dst {
             return;
         }
+
+        #[cfg(feature = "tracing")]
+        let _span = tracing::trace_span!(target: "IO", "file-copy-link", ?src, ?dst).entered();
+
         if let Err(e) = fs::remove_file(dst)
             && cfg!(windows)
             && e.kind() != io::ErrorKind::NotFound
@@ -1908,13 +1912,21 @@ impl Build {
         if self.config.dry_run() {
             return;
         }
+
+        #[cfg(feature = "tracing")]
+        let _span = tracing::trace_span!(target: "IO", "dir-create", ?dir).entered();
+
         t!(fs::create_dir_all(dir))
     }
 
     fn remove_dir(&self, dir: &Path) {
         if self.config.dry_run() {
             return;
         }
+
+        #[cfg(feature = "tracing")]
+        let _span = tracing::trace_span!(target: "IO", "dir-remove", ?dir).entered();
+
         t!(fs::remove_dir_all(dir))
     }
 

src/doc/rustc-dev-guide/src/building/bootstrapping/debugging-bootstrap.md

@@ -81,9 +81,11 @@ There are two orthogonal ways to control which kind of tracing logs you want:
    - If you select a level, all events/spans with an equal or higher priority level will be shown.
 2. You can also control the log **target**, e.g. `bootstrap` or `bootstrap::core::config` or a custom target like `CONFIG_HANDLING` or `STEP`.
     - Custom targets are used to limit what kinds of spans you are interested in, as the `BOOTSTRAP_TRACING=trace` output can be quite verbose. Currently, you can use the following custom targets:
-        - `CONFIG_HANDLING`: show spans related to config handling
-        - `STEP`: show all executed steps. Note that executed commands have `info` event level.
-        - `COMMAND`: show all executed commands. Note that executed commands have `trace` event level.
+        - `CONFIG_HANDLING`: show spans related to config handling.
+        - `STEP`: show all executed steps. Executed commands have `info` event level.
+        - `COMMAND`: show all executed commands. Executed commands have `trace` event level.
+        - `IO`: show performed I/O operations. Executed commands have `trace` event level.
+            - Note that many I/O are currently not being traced.
 
 You can of course combine them (custom target logs are typically gated behind `TRACE` log level additionally):