compile-mode integration: add link to code references. (#391)

This allows to jump to code references such as lines 82 and 132 in the
following snippet error message.

error[E0061]: this function takes 1 parameter but 2 parameters were supplied
  --> file6.rs:132:34
    |
82  | fn duration_ms_since(time: &Option<SystemTime>) -> u128 {
    | ------------------------------------------------------- defined here
...
132 |             self.total_time_ms = duration_ms_since(&self.program_start, 2);
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 files changed, 9 insertions, 0 deletions

diff --git a/rust-mode.el b/rust-mode.el
index d5e0b93..9711a79 100644
--- a/rust-mode.el
+++ b/rust-mode.el
@@ -1947,6 +1947,12 @@ See `compilation-error-regexp-alist' for help on their format.")
   "Specifications for matching `:::` hints in rustc invocations.
 See `compilation-error-regexp-alist' for help on their format.")
 
+(defvar rustc-refs-compilation-regexps
+  (let ((re "^\\([0-9]+\\)[[:space:]]*|"))
+    (cons re '(nil 1 nil 0 1)))
+  "Specifications for matching code references in rustc invocations.
+See `compilation-error-regexp-alist' for help on their format.")
+
 ;; Match test run failures and panics during compilation as
 ;; compilation warnings
 (defvar cargo-compilation-regexps
@@ -1978,6 +1984,9 @@ the compilation window until the top of the error is visible."
 (eval-after-load 'compile
   '(progn
      (add-to-list 'compilation-error-regexp-alist-alist
+                  (cons 'rustc-refs rustc-refs-compilation-regexps))
+     (add-to-list 'compilation-error-regexp-alist 'rustc-refs)
+     (add-to-list 'compilation-error-regexp-alist-alist
                   (cons 'rustc rustc-compilation-regexps))
      (add-to-list 'compilation-error-regexp-alist 'rustc)
      (add-to-list 'compilation-error-regexp-alist-alist