Only activate direct mode by default for local

docs/Local.md

@@ -30,7 +30,7 @@ Preprocessor cache mode will be disabled if any of the following holds:
 
 Configuration options and their default values:
 
-- `use_preprocessor_cache_mode`: `false`. Whether to use preprocessor cache mode entirely.
+- `use_preprocessor_cache_mode`: `true`. Whether to use preprocessor cache mode entirely.
 - `file_stat_matches`: `false`. If false, only compare header files by hashing their contents. If true, will use size + ctime + mtime to check whether a file has changed. See other flags below for more control over this behavior.
 - `use_ctime_for_stat`: `true`. If true, uses the ctime (file status change on UNIX, creation time on Windows) to check that a file has/hasn't changed. Can be useful to disable when backdating modification times in a controlled manner.
 

src/cache/cache.rs

@@ -366,7 +366,7 @@ pub trait Storage: Send + Sync {
 
     /// Return the config for preprocessor cache mode if applicable
     fn preprocessor_cache_mode_config(&self) -> PreprocessorCacheModeConfig {
-        // Disabled by default, only enabled in local mode
+        // Enable by default, only in local mode
         PreprocessorCacheModeConfig::default()
     }
     /// Return the preprocessor cache entry for a given preprocessor key,
@@ -431,6 +431,16 @@ impl Default for PreprocessorCacheModeConfig {
     }
 }
 
+impl PreprocessorCacheModeConfig {
+    /// Return a default [`Self`], but with the cache active.
+    pub fn activated() -> Self {
+        Self {
+            use_preprocessor_cache_mode: true,
+            ..Default::default()
+        }
+    }
+}
+
 /// Implement storage for operator.
 #[cfg(any(feature = "s3", feature = "azure", feature = "gcs", feature = "redis"))]
 #[async_trait]

src/config.rs

@@ -169,7 +169,7 @@ impl Default for DiskCacheConfig {
         DiskCacheConfig {
             dir: default_disk_cache_dir(),
             size: default_disk_cache_size(),
-            preprocessor_cache_mode: Default::default(),
+            preprocessor_cache_mode: PreprocessorCacheModeConfig::activated(),
         }
     }
 }
@@ -1296,7 +1296,7 @@ token = "webdavtoken"
                 disk: Some(DiskCacheConfig {
                     dir: PathBuf::from("/tmp/.cache/sccache"),
                     size: 7 * 1024 * 1024 * 1024,
-                    preprocessor_cache_mode: Default::default(),
+                    preprocessor_cache_mode: PreprocessorCacheModeConfig::activated(),
                 }),
                 gcs: Some(GCSCacheConfig {
                     bucket: "bucket".to_owned(),