Changed flycheck-rust-manifest-directory to use 'cargo metadata'

This commit fixes flycheck#1397.
By the recent change of cargo(rust-lang/cargo#4788),
flycheck can't detect the error file.
So I changed to use 'workspace_root' in 'cargo metadata's outputs,
as working directory.
If the user's cargo is a bit old and there's no 'workspace_root',
flycheck-rust-manifest-directory search a directory with 'Cargo.toml',
as ever.

flycheck.el

@@ -9471,14 +9471,38 @@ Relative paths are relative to the file being checked."
               errors))
 
 (defun flycheck-rust-manifest-directory ()
-  "Return the nearest directory holding the Cargo manifest.
+  "Return the working directory of Rust project.
 
-Return the nearest directory containing the `Cargo.toml' manifest
-file, starting from the current buffer and using
+If we can get workspace root by 'cargo metadata' command, return it.
+If not, return the nearest directory containing the `Cargo.toml'
+manifest file, starting from the current buffer and using
 `locate-dominating-file'.  Return nil if there is no such file,
 or if the current buffer has no file name."
-  (and buffer-file-name
-       (locate-dominating-file buffer-file-name "Cargo.toml")))
+  (let ((workspace-root (flycheck-rust-workspace-root)))
+    (or workspace-root
+        (and buffer-file-name
+             (locate-dominating-file buffer-file-name "Cargo.toml")))))
+
+(defun flycheck-rust-get-metadata ()
+  "Exec 'cargo metadata' and return result as json.
+
+Return nil if command doesn't exist or a parse error occurs."
+  (let ((metadata-list
+         (ignore-errors
+           (process-lines "cargo" "metadata" "--no-deps" "--format-version" "1"))))
+    (and metadata-list
+         (json-read-from-string (mapconcat 'identity metadata-list "")))))
+
+(defun flycheck-rust-workspace-root ()
+  "Return the workspace root of Rust project.
+
+This function depends on the 'workspace_root' field of 'cargo metadata' command.
+This field is add on Dec 13, 2017 by  https://github.com/rust-lang/cargo/pull/4788.
+Return nil if 'workspace_root' doesn't exist."
+  (let ((metadata
+         (flycheck-rust-get-metadata)))
+    (and metadata
+         (alist-get 'workspace_root metadata))))
 
 (defun flycheck-rust-cargo-has-command-p (command)
   "Whether Cargo has COMMAND in its list of commands.