Auto merge of #9831 - ehuss:fix-debug, r=Eh2406

Add some debug logging for `cargo fix`

This adds some debug logging to the `cargo fix` command to give better insight into what it is actually doing. I've needed to try to debug some things recently where this would have been useful, since cargo runs rustc many times with different arguments.  I think this will be useful if other people report problems, we can ask them to run with `CARGO_LOG=cargo::ops::fix=trace` to gather more information.

src/cargo/ops/fix.rs

@@ -364,6 +364,7 @@ pub fn fix_maybe_exec_rustc(config: &Config) -> CargoResult<bool> {
         let mut cmd = rustc.build_command();
         args.apply(&mut cmd, config);
         cmd.arg("--error-format=json");
+        debug!("calling rustc for final verification: {:?}", cmd);
         let output = cmd.output().context("failed to spawn rustc")?;
 
         if output.status.success() {
@@ -389,6 +390,7 @@ pub fn fix_maybe_exec_rustc(config: &Config) -> CargoResult<bool> {
         if !output.status.success() {
             if env::var_os(BROKEN_CODE_ENV).is_none() {
                 for (path, file) in fixes.files.iter() {
+                    debug!("reverting {:?} due to errors", path);
                     paths::write(path, &file.original_code)?;
                 }
             }
@@ -407,6 +409,7 @@ pub fn fix_maybe_exec_rustc(config: &Config) -> CargoResult<bool> {
         // things like colored output to work correctly.
         cmd.arg(arg);
     }
+    debug!("calling rustc to display remaining diagnostics: {:?}", cmd);
     exit_with(cmd.status().context("failed to spawn rustc")?);
 }
 
@@ -545,6 +548,10 @@ fn rustfix_and_fix(
     let mut cmd = rustc.build_command();
     cmd.arg("--error-format=json");
     args.apply(&mut cmd, config);
+    debug!(
+        "calling rustc to collect suggestions and validate previous fixes: {:?}",
+        cmd
+    );
     let output = cmd.output().with_context(|| {
         format!(
             "failed to execute `{}`",
@@ -609,6 +616,7 @@ fn rustfix_and_fix(
             continue;
         }
 
+        trace!("adding suggestion for {:?}: {:?}", file_name, suggestion);
         file_map
             .entry(file_name)
             .or_insert_with(Vec::new)