Purge mtime information from Fingerprint

This has proven to be a very unreliable piece of information to hash, so
let's not! Instead we track what files are supposed to be relative to,
and we check both mtimes when necessary.

Author: alexcrichton

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

@@ -166,8 +166,13 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
         }
     }
 
-    /// Returns the root of the build output tree.
+    /// Returns the root of the build output tree for the target
     pub fn target_root(&self) -> &Path {
+        self.target.as_ref().unwrap_or(&self.host).dest()
+    }
+
+    /// Returns the root of the build output tree for the host
+    pub fn host_root(&self) -> &Path {
         self.host.dest()
     }
 

src/cargo/core/compiler/custom_build.rs

@@ -235,7 +235,7 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
     let output_file = script_run_dir.join("output");
     let err_file = script_run_dir.join("stderr");
     let root_output_file = script_run_dir.join("root-output");
-    let host_target_root = cx.files().target_root().to_path_buf();
+    let host_target_root = cx.files().host_root().to_path_buf();
     let all = (
         id,
         pkg_name.clone(),