feat(coverage): add elisp backend

First of the pluggable coverage backends. Registers itself with coverage-core on load.

- :name is elisp
- :detect returns non-nil when the project root has a Makefile, Eask, or Cask alongside .el files at root or under modules/. The heuristic is deliberately loose. For anything unusual, .dir-locals.el can pin the backend with cj/coverage-backend.
- :run invokes make coverage in a compilation buffer. On success the callback fires with the LCOV path. On failure the buffer stays visible so the user can read the error.
- :lcov-path resolves to <project-root>/.coverage/lcov.info.

undercover is declared via use-package with :defer t so it's installed but not loaded at Emacs startup. The make coverage target will require it explicitly.

Tests cover Normal (Makefile + modules/, Eask + root .el, Cask + modules/), Boundary (no build file, Makefile without .el, empty directory), and Error (nonexistent root returns nil). The registration-on-load case is also verified.

The Makefile coverage target and the cj/coverage-report user command arrive in follow-up commits.

1 files changed, 74 insertions, 0 deletions

diff --git a/modules/coverage-elisp.el b/modules/coverage-elisp.el
new file mode 100644
index 00000000..63d89906
--- /dev/null
+++ b/modules/coverage-elisp.el
@@ -0,0 +1,74 @@
+;;; coverage-elisp.el --- Elisp coverage backend for coverage-core -*- lexical-binding: t; coding: utf-8; -*-
+;; author: Craig Jennings <c@cjennings.net>
+
+;;; Commentary:
+;; Registers the `elisp' coverage backend with `coverage-core'.
+;;
+;; Detection: a project root with a Makefile / Eask / Cask plus any
+;; .el files (either at root or under modules/).  Loose on purpose —
+;; `.dir-locals.el' can pin the backend explicitly when the heuristic
+;; guesses wrong.
+;;
+;; :run invokes `make coverage' in a compilation buffer.  On success,
+;; the callback is invoked with the LCOV path; on failure, the buffer
+;; stays visible for the user to inspect.
+;;
+;; :lcov-path resolves to `<project-root>/.coverage/lcov.info', which
+;; matches the path the Makefile's coverage target writes to.
+
+;;; Code:
+
+(require 'coverage-core)
+
+(use-package undercover
+  :defer t)
+
+(defconst cj/--coverage-elisp-lcov-relative-path
+  ".coverage/lcov.info"
+  "Project-relative path to the LCOV file produced by `make coverage'.")
+
+(defun cj/--coverage-elisp-project-root (&optional root)
+  "Return ROOT or fall back to projectile's root or `default-directory'."
+  (or root
+	  (and (fboundp 'projectile-project-root)
+		   (projectile-project-root))
+	  default-directory))
+
+(defun cj/--coverage-elisp-detect (root)
+  "Return non-nil if ROOT looks like an Elisp project.
+The heuristic needs both (a) a Makefile, Eask, or Cask at ROOT and
+\(b) any .el files at ROOT or under modules/."
+  (and (or (file-exists-p (expand-file-name "Makefile" root))
+		   (file-exists-p (expand-file-name "Eask" root))
+		   (file-exists-p (expand-file-name "Cask" root)))
+	   (or (file-expand-wildcards (expand-file-name "modules/*.el" root))
+		   (file-expand-wildcards (expand-file-name "*.el" root)))))
+
+(defun cj/--coverage-elisp-lcov-path (&optional root)
+  "Return the absolute path to the LCOV file for ROOT."
+  (expand-file-name cj/--coverage-elisp-lcov-relative-path
+					(cj/--coverage-elisp-project-root root)))
+
+(defun cj/--coverage-elisp-run (callback)
+  "Run `make coverage' asynchronously.
+CALLBACK is invoked with the LCOV path when the build finishes
+successfully.  On failure, no callback is invoked and the compilation
+buffer stays visible so the user can read the error."
+  (let* ((default-directory (cj/--coverage-elisp-project-root))
+		 (buffer (compilation-start "make coverage" nil
+									(lambda (_mode) "*coverage-run*"))))
+	(with-current-buffer buffer
+	  (add-hook 'compilation-finish-functions
+				(lambda (_buf status)
+				  (when (string-match-p "^finished" status)
+					(funcall callback (cj/--coverage-elisp-lcov-path))))
+				nil t))))
+
+(cj/coverage-register-backend
+ (list :name 'elisp
+	   :detect #'cj/--coverage-elisp-detect
+	   :run #'cj/--coverage-elisp-run
+	   :lcov-path #'cj/--coverage-elisp-lcov-path))
+
+(provide 'coverage-elisp)
+;;; coverage-elisp.el ends here