Introduce the Coccinelle linter

For mistakes that are easy to fix automatically, Coccinelle patches may
be written. If a patch finds a mistake, the linter will ask the human to
fix either the code or the patch.

.ci/linters/c/cocci_linter.R

@@ -0,0 +1,20 @@
+cocci_linter = if (!nzchar(Sys.which("spatch"))) function(...) {} else function(c_obj) {
+  bad <- FALSE
+  for (spfile in list.files(".ci/linters/cocci", full.names = TRUE)) {
+    # Coccinelle parser gets confused sometimes, so ignore stderr and the exit code
+    out = suppressWarnings(system2(
+      "spatch",
+      shQuote(c(
+        "--sp-file", spfile, c_obj$path, "--recursive-includes",
+        "-I", R.home("include"), "-I", "src"
+      )),
+      stdout = TRUE, stderr = FALSE
+    ))
+    if (length(out) > 0) {
+      cat(sprintf("In file '%s', Coccinelle patch '%s' recommends the following changes:\n", c_obj$path, spfile))
+      writeLines(out)
+      bad <- TRUE
+    }
+  }
+  if (bad) stop("Please apply the changes above or fix the linter")
+}

.ci/linters/cocci/redundant_cast_to_same_pointer_type.cocci

@@ -0,0 +1,6 @@
+@@
+type T;
+T* E;
+@@
+- (T*)
+  E

.github/workflows/code-quality.yaml

@@ -35,6 +35,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: r-lib/actions/setup-r@v2
+      - name: Install Coccinelle
+        run: |
+          apt-get update
+          apt-get -y install coccinelle
       - name: Lint
         run: |
           linter_env = new.env()