WIP: implement the fix described in the previous commit message

Author: ahartmetz

src/compiler/compiler.rs

@@ -503,8 +503,27 @@ where
                     stdout: entry.get_stdout(),
                     stderr: entry.get_stderr(),
                 };
+
+                let filtered_outputs =
+                if compilation.is_preprocessed_for_distribution() { // TODO probably rename that getter
+                    // In this mode, cache entries are exclusively distinguished by their preprocessed
+                    // source contents. But two files may differ in their names and /or the names of
+                    // included files while still producing the same preprocessed output, so they get
+                    // the same cache entry. That entry will have the names of dependencies wrong for
+                    // at least one of the source files that produced it.
+                    // Since we did local preprocessing, that should already have produced the dependency
+                    // file - just leave that one alone and don't overwrite it from the cache.
+                    outputs.iter().filter(|fobj_source| fobj_source.key != "d").cloned().collect()
+                } else {
+                    // In this mode, no local preprocessing was done, so the dependency file (if any)
+                    // has not been created. But in this mode, the cache key also includes a lot of
+                    // information about filenames (and less relevant here, file hashes), so it *is* safe
+                    // to restore the dependency file from the cache.
+                    outputs.clone()
+                };
+
                 let hit = CompileResult::CacheHit(duration);
-                match entry.extract_objects(outputs.clone(), &pool).await {
+                match entry.extract_objects(filtered_outputs, &pool).await {
                     Ok(()) => Ok(CacheLookupResult::Success(hit, output)),
                     Err(e) => {
                         if e.downcast_ref::<DecompressionFailure>().is_some() {