refactor(fingerprint): Track the intent for each use of UnitHash

We have
- `-C metadata`
- `-C extra-filename`
- Uniquifying build scripts
- Uniquifying scraped documentation

The last two I'm tracking as a `unit_id`.  As we evolve the first two
hashes, we can decide which would be a better fit for backing this.
For scraping,  we could treat this as
`-C extra-filename` but then we'd have a lot of `.expect()`s.

src/cargo/core/compiler/build_runner/compilation_files.rs

@@ -91,17 +91,19 @@ pub struct Metadata {
 }
 
 impl Metadata {
-    /// The symbol hash to use.
-    pub fn meta_hash(&self) -> UnitHash {
+    /// A hash to identify a given [`Unit`] in the build graph
+    pub fn unit_id(&self) -> UnitHash {
         self.meta_hash
     }
 
-    /// Whether or not the `-C extra-filename` flag is used to generate unique
-    /// output filenames for this `Unit`.
-    ///
-    /// If this is `true`, the `meta_hash` is used for the filename.
-    pub fn use_extra_filename(&self) -> bool {
-        self.use_extra_filename
+    /// A hash to add to symbol naming through `-C metadata`
+    pub fn c_metadata(&self) -> UnitHash {
+        self.meta_hash
+    }
+
+    /// A hash to add to file names through `-C extra-filename`
+    pub fn c_extra_filename(&self) -> Option<UnitHash> {
+        self.use_extra_filename.then_some(self.meta_hash)
     }
 }
 
@@ -230,8 +232,8 @@ impl<'a, 'gctx: 'a> CompilationFiles<'a, 'gctx> {
     fn pkg_dir(&self, unit: &Unit) -> String {
         let name = unit.pkg.package_id().name();
         let meta = self.metas[unit];
-        if meta.use_extra_filename() {
-            format!("{}-{}", name, meta.meta_hash())
+        if let Some(c_extra_filename) = meta.c_extra_filename() {
+            format!("{}-{}", name, c_extra_filename)
         } else {
             format!("{}-{}", name, self.target_short_hash(unit))
         }
@@ -475,7 +477,7 @@ impl<'a, 'gctx: 'a> CompilationFiles<'a, 'gctx> {
                 let file_name = format!(
                     "{}-{}.examples",
                     unit.pkg.name(),
-                    self.metadata(unit).meta_hash()
+                    self.metadata(unit).unit_id()
                 );
                 let path = self.deps_dir(unit).join(file_name);
                 vec![OutputFile {
@@ -533,9 +535,7 @@ impl<'a, 'gctx: 'a> CompilationFiles<'a, 'gctx> {
         let mut outputs = Vec::new();
         for file_type in file_types {
             let meta = self.metas[unit];
-            let meta_opt = meta
-                .use_extra_filename()
-                .then(|| meta.meta_hash().to_string());
+            let meta_opt = meta.c_extra_filename().map(|h| h.to_string());
             let path = out_dir.join(file_type.output_filename(&unit.target, meta_opt.as_deref()));
 
             // If, the `different_binary_name` feature is enabled, the name of the hardlink will

src/cargo/core/compiler/build_runner/mod.rs

@@ -443,7 +443,7 @@ impl<'a, 'gctx> BuildRunner<'a, 'gctx> {
     /// Returns the metadata hash for a `RunCustomBuild` unit.
     pub fn get_run_build_script_metadata(&self, unit: &Unit) -> UnitHash {
         assert!(unit.mode.is_run_custom_build());
-        self.files().metadata(unit).meta_hash()
+        self.files().metadata(unit).unit_id()
     }
 
     pub fn is_primary_package(&self, unit: &Unit) -> bool {

src/cargo/core/compiler/job_queue/mod.rs

@@ -685,7 +685,7 @@ impl<'gctx> DrainState<'gctx> {
                             .failed_scrape_units
                             .lock()
                             .unwrap()
-                            .insert(build_runner.files().metadata(&unit).meta_hash());
+                            .insert(build_runner.files().metadata(&unit).unit_id());
                         self.queue.finish(&unit, &artifact);
                     }
                     Err(error) => {

src/cargo/core/compiler/mod.rs

@@ -279,15 +279,12 @@ fn rustc(
     // don't pass the `-l` flags.
     let pass_l_flag = unit.target.is_lib() || !unit.pkg.targets().iter().any(|t| t.is_lib());
 
-    let dep_info_name = if build_runner.files().metadata(unit).use_extra_filename() {
-        format!(
-            "{}-{}.d",
-            unit.target.crate_name(),
-            build_runner.files().metadata(unit).meta_hash()
-        )
-    } else {
-        format!("{}.d", unit.target.crate_name())
-    };
+    let dep_info_name =
+        if let Some(c_extra_filename) = build_runner.files().metadata(unit).c_extra_filename() {
+            format!("{}-{}.d", unit.target.crate_name(), c_extra_filename)
+        } else {
+            format!("{}.d", unit.target.crate_name())
+        };
     let rustc_dep_info_loc = root.join(dep_info_name);
     let dep_info_loc = fingerprint::dep_info_loc(build_runner, unit);
 
@@ -766,7 +763,7 @@ fn prepare_rustdoc(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResu
     let metadata = build_runner.metadata_for_doc_units[unit];
     rustdoc
         .arg("-C")
-        .arg(format!("metadata={}", metadata.meta_hash()));
+        .arg(format!("metadata={}", metadata.c_metadata()));
 
     if unit.mode.is_doc_scrape() {
         debug_assert!(build_runner.bcx.scrape_units.contains(unit));
@@ -844,7 +841,7 @@ fn rustdoc(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResult<W
                 .iter()
                 .map(|unit| {
                     Ok((
-                        build_runner.files().metadata(unit).meta_hash(),
+                        build_runner.files().metadata(unit).unit_id(),
                         scrape_output_path(build_runner, unit)?,
                     ))
                 })
@@ -1158,10 +1155,11 @@ fn build_base_args(
     cmd.args(&check_cfg_args(unit));
 
     let meta = build_runner.files().metadata(unit);
-    cmd.arg("-C").arg(&format!("metadata={}", meta.meta_hash()));
-    if meta.use_extra_filename() {
+    cmd.arg("-C")
+        .arg(&format!("metadata={}", meta.c_metadata()));
+    if let Some(c_extra_filename) = meta.c_extra_filename() {
         cmd.arg("-C")
-            .arg(&format!("extra-filename=-{}", meta.meta_hash()));
+            .arg(&format!("extra-filename=-{c_extra_filename}"));
     }
 
     if rpath {