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, 99 insertions, 0 deletions

diff --git a/tests/test-coverage-elisp--detect.el b/tests/test-coverage-elisp--detect.el
new file mode 100644
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+++ b/tests/test-coverage-elisp--detect.el
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+;;; test-coverage-elisp--detect.el --- Tests for cj/--coverage-elisp-detect -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/--coverage-elisp-detect', the heuristic that
+;; decides whether a given project root is an Elisp project.
+;;
+;; Heuristic: requires BOTH a Makefile/Eask/Cask AND some .el files
+;; (at root or under modules/).
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'coverage-elisp)
+
+(defun test-coverage-elisp-detect--make-project (spec)
+  "Create a temp directory matching SPEC.
+SPEC is a list of relative paths to create.  Paths ending in `/' are
+directories; others are files.  Returns the temp directory path."
+  (let ((root (make-temp-file "test-elisp-project-" t)))
+	(dolist (path spec)
+	  (let ((full (expand-file-name path root)))
+		(if (string-suffix-p "/" path)
+			(make-directory full t)
+		  (make-directory (file-name-directory full) t)
+		  (write-region "" nil full))))
+	root))
+
+;;; Normal cases
+
+(ert-deftest test-coverage-elisp-detect-with-makefile-and-modules ()
+  "Normal: Makefile plus modules/foo.el is detected as elisp."
+  (let ((root (test-coverage-elisp-detect--make-project
+			   '("Makefile" "modules/foo.el"))))
+	(unwind-protect
+		(should (cj/--coverage-elisp-detect root))
+	  (delete-directory root t))))
+
+(ert-deftest test-coverage-elisp-detect-with-eask-and-root-el ()
+  "Normal: Eask plus root-level .el file is detected."
+  (let ((root (test-coverage-elisp-detect--make-project
+			   '("Eask" "main.el"))))
+	(unwind-protect
+		(should (cj/--coverage-elisp-detect root))
+	  (delete-directory root t))))
+
+(ert-deftest test-coverage-elisp-detect-with-cask-and-modules ()
+  "Normal: Cask plus modules/ directory is detected."
+  (let ((root (test-coverage-elisp-detect--make-project
+			   '("Cask" "modules/bar.el"))))
+	(unwind-protect
+		(should (cj/--coverage-elisp-detect root))
+	  (delete-directory root t))))
+
+;;; Boundary cases
+
+(ert-deftest test-coverage-elisp-detect-no-build-file ()
+  "Boundary: .el files without a Makefile/Eask/Cask is NOT detected."
+  (let ((root (test-coverage-elisp-detect--make-project
+			   '("main.el" "other.el"))))
+	(unwind-protect
+		(should-not (cj/--coverage-elisp-detect root))
+	  (delete-directory root t))))
+
+(ert-deftest test-coverage-elisp-detect-makefile-without-el ()
+  "Boundary: Makefile with no .el files is NOT detected."
+  (let ((root (test-coverage-elisp-detect--make-project
+			   '("Makefile" "README.md"))))
+	(unwind-protect
+		(should-not (cj/--coverage-elisp-detect root))
+	  (delete-directory root t))))
+
+(ert-deftest test-coverage-elisp-detect-empty-directory ()
+  "Boundary: an empty directory is not an elisp project."
+  (let ((root (make-temp-file "test-empty-" t)))
+	(unwind-protect
+		(should-not (cj/--coverage-elisp-detect root))
+	  (delete-directory root t))))
+
+;;; Error cases
+
+(ert-deftest test-coverage-elisp-detect-nonexistent-root ()
+  "Error: a nonexistent ROOT returns nil, not an error."
+  (should-not (cj/--coverage-elisp-detect "/nonexistent/path/for-test-12345")))
+
+;;; Registry integration
+
+(ert-deftest test-coverage-elisp-registered-on-load ()
+  "Normal: loading coverage-elisp registers the `elisp' backend."
+  (let ((backend (cj/--coverage-backend-by-name 'elisp)))
+	(should backend)
+	(should (eq 'elisp (plist-get backend :name)))
+	(should (functionp (plist-get backend :detect)))
+	(should (functionp (plist-get backend :run)))
+	(should (functionp (plist-get backend :lcov-path)))))
+
+(provide 'test-coverage-elisp--detect)
+;;; test-coverage-elisp--detect.el ends here