Fix byte-compile warnings and correct minimum Emacs version

.github/workflows/byte-compile.yml

@@ -0,0 +1,44 @@
+name: Byte-compile
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  byte-compile:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        emacs-version:
+          - "26.1"
+          - "30.1"
+          - snapshot
+    steps:
+      - uses: actions/checkout@v5
+      - uses: purcell/setup-emacs@master
+        with:
+          version: ${{ matrix.emacs-version }}
+      - name: Byte-compile with warnings as errors
+        run: |
+          emacs -Q --batch -L . \
+            --eval '(setq byte-compile-error-on-warn t)' \
+            --eval '(let ((files (list "rust-cargo.el" "rust-common.el"
+                                       "rust-compile.el" "rust-mode.el"
+                                       "rust-playpen.el" "rust-prog-mode.el"
+                                       "rust-rustfmt.el" "rust-utils.el"))
+                          (ok t))
+                      (when (version<= "29.1" emacs-version)
+                        (setq files
+                              (append files (list "rust-mode-treesitter.el"))))
+                      (dolist (f files)
+                        (unless (byte-compile-file f)
+                          (setq ok nil)))
+                      (unless ok (kill-emacs 1)))'

rust-cargo.el

@@ -6,6 +6,7 @@
 ;;; Code:
 
 (require 'json)
+(require 'subr-x)
 
 ;;; Options
 

rust-mode.el

@@ -4,7 +4,7 @@
 ;; Author: Mozilla <rust-mode@noreply.github.com>
 ;; Url: https://github.com/rust-lang/rust-mode
 ;; Keywords: languages
-;; Package-Requires: ((emacs "25.1"))
+;; Package-Requires: ((emacs "26.1"))
 
 ;; This file is distributed under the terms of both the MIT license and the
 ;; Apache License (version 2.0).

rust-prog-mode.el

@@ -922,11 +922,12 @@ and end."
 (defun rust-string-interpolation-matcher (limit)
   "Match next Rust interpolation marker before LIMIT and set match data if found.
 Returns nil if not within a Rust string."
-  (when-let (((rust-in-str))
-             (match (rust-next-string-interpolation limit)))
-    (set-match-data match)
-    (goto-char (cadr match))
-    match))
+  (when (rust-in-str)
+    (let ((match (rust-next-string-interpolation limit)))
+      (when match
+        (set-match-data match)
+        (goto-char (cadr match))
+        match))))
 
 (defun rust-syntax-class-before-point ()
   (when (> (point) 1)
@@ -950,18 +951,19 @@ Returns nil if not within a Rust string."
    ;; we're looking back at this, we want to end up just after "Fn".
    ((member (char-before) '(?\] ?\) ))
     (let ((is-paren (rust-looking-back-str ")")))
-      (when-let ((dest (save-excursion
+      (let ((dest (save-excursion
+                    (backward-sexp)
+                    (rust-rewind-irrelevant)
+                    (or
+                     (when (rust-looking-back-str "->")
+                       (backward-char 2)
+                       (rust-rewind-irrelevant)
+                       (when (rust-looking-back-str ")")
                          (backward-sexp)
-                         (rust-rewind-irrelevant)
-                         (or
-                          (when (rust-looking-back-str "->")
-                            (backward-char 2)
-                            (rust-rewind-irrelevant)
-                            (when (rust-looking-back-str ")")
-                              (backward-sexp)
-                              (point)))
-                          (and is-paren (point))))))
-        (goto-char dest))))))
+                         (point)))
+                     (and is-paren (point))))))
+        (when dest
+          (goto-char dest)))))))
 
 (defun rust-rewind-to-decl-name ()
   "Return the point at the beginning of the name in a declaration.
@@ -1548,8 +1550,9 @@ whichever comes first."
 (defun rust-syntax-propertize (start end)
   "A `syntax-propertize-function' to apply properties from START to END."
   (goto-char start)
-  (when-let ((str-start (rust-in-str-or-cmnt)))
-    (rust--syntax-propertize-raw-string str-start end))
+  (let ((str-start (rust-in-str-or-cmnt)))
+    (when str-start
+      (rust--syntax-propertize-raw-string str-start end)))
   (funcall
    (syntax-propertize-rules
     ;; Character literals.