Fix error related to multiple binary targets

When both 'src/lib.rs' and 'src/main.rs' are present in the project, we
default to invoking `cargo rustc`.  This command requires an explicit
binary name with the `--bin` flag, otherwise it causes the error
described in GH-23.

This commit adds logic to parse the output of `cargo read-manifest` to
determine the target type to check, based on the buffer file name.  If
the file name matches one of the targets, this target is chosen.
Otherwise, the first target is chosen as a default.

If the target is a binary, we also extract the binary name and set a new
`flycheck-rust-binary-name` variable (see flycheck/flycheck#958) to
solve the error.

I've tested this work with 'src/main.rs' binaries as well as multiples
binaries under 'src/bin'.

Closes GH-25, fixes GH-23, and adds logic to tackle GH-8.

Author: fmdkdd

flycheck-rust.el

@@ -6,7 +6,7 @@
 ;; URL: https://github.com/flycheck/flycheck-rust
 ;; Keywords: tools, convenience
 ;; Version: 0.1-cvs
-;; Package-Requires: ((emacs "24.1") (flycheck "0.20") (dash "2.4.0"))
+;; Package-Requires: ((emacs "24.1") (flycheck "0.20") (dash "2.4.0") (seq "2.15"))
 
 ;; This file is not part of GNU Emacs.
 
@@ -44,6 +44,8 @@
 
 (require 'dash)
 (require 'flycheck)
+(require 'seq)
+(require 'json)
 
 (defun flycheck-rust-executable-p (rel-name)
   "Whether REL-NAME denotes an executable.
@@ -94,6 +96,30 @@ Return non-nil if PROJECT-ROOT is a binary crate, nil otherwise."
   (let ((root-dir (file-name-directory project-root)))
     (file-exists-p (expand-file-name "src/main.rs" root-dir))))
 
+(defun flycheck-rust-find-target (file-name)
+  "Find and return the cargo target associated with the given file.
+
+FILE-NAME is the name of the file that is matched against the
+`src_path' value in the list `targets' returned by `cargo
+read-manifest'.  If there is no match, the first target is
+returned by default.
+
+Return a cons cell (TYPE . NAME), where TYPE is the target
+type (lib or bin), and NAME the target name (usually, the crate
+name)."
+  (let ((json-array-type 'list))
+    (let-alist (with-temp-buffer
+                 (call-process "cargo" nil t nil "read-manifest")
+                 (goto-char (point-min))
+                 (json-read))
+      (let ((targets .targets))
+        ;; If there is a target that matches the file-name exactly, pick that
+        ;; one.  Otherwise, just pick the first target.
+        (let-alist (seq-find (lambda (target)
+                               (let-alist target (string= file-name .src_path)))
+                             targets (car targets))
+          (cons (car .kind) .name))))))
+
 ;;;###autoload
 (defun flycheck-rust-setup ()
   "Setup Rust in Flycheck.
@@ -103,7 +129,9 @@ Flycheck according to the Cargo project layout."
   (interactive)
   (when (buffer-file-name)
     (-when-let (root (flycheck-rust-project-root))
-      (let ((rel-name (file-relative-name (buffer-file-name) root)))
+      (pcase-let ((rel-name (file-relative-name (buffer-file-name) root))
+                  (`(,target-type . ,target-name) (flycheck-rust-find-target
+                                                     (buffer-file-name))))
         ;; These are valid crate roots as by Cargo's layout
         (if (or (flycheck-rust-executable-p rel-name)
                 (flycheck-rust-test-p rel-name)
@@ -117,9 +145,15 @@ Flycheck according to the Cargo project layout."
         ;; Check tests in libraries and integration tests
         (setq-local flycheck-rust-check-tests
                     (not (flycheck-rust-executable-p rel-name)))
+        ;; Set the crate type
         (setq-local flycheck-rust-crate-type
-                    (if (flycheck-rust-binary-crate-p root)
-                        "bin" "lib"))        ;; Set the crate type
+                    (if (string= target-type "bin")
+                        (progn
+                          ;; If it's binary target, we need to pass the binary
+                          ;; name
+                          (setq-local flycheck-rust-binary-name target-name)
+                          "bin")
+                      "lib"))
         ;; Find build libraries
         (setq-local flycheck-rust-library-path
                     (list (expand-file-name "target/debug" root)