Add nix-linter checker

doc/languages.rst

@@ -755,6 +755,12 @@ to view the docstring of the syntax checker.  Likewise, you may use
 
       .. _nix-instantiate: https://nixos.org/nix/manual/#sec-nix-instantiate
 
+   .. syntax-checker:: nix-linter
+
+      Check Nix with nix-linter_.
+
+      .. _nix-linter: https://github.com/Synthetica9/nix-linter
+
 .. supported-language:: Perl
 
    Flycheck checks Perl with `perl` and `perl-perlcritic`.

flycheck.el

@@ -211,6 +211,7 @@ attention to case differences."
     markdown-markdownlint-cli
     markdown-mdl
     nix
+    nix-linter
     perl
     perl-perlcritic
     php
@@ -9421,6 +9422,36 @@ See URL `https://nixos.org/nix/manual/#sec-nix-instantiate'."
   (lambda (errors)
     (flycheck-sanitize-errors
      (flycheck-remove-error-file-names "(string)" errors)))
+  :next-checkers ((warning . nix-linter))
+  :modes nix-mode)
+
+(defun flycheck-parse-nix-linter (output checker buffer)
+  "Parse nix-linter warnings from JSON OUTPUT.
+
+CHECKER and BUFFER denote the CHECKER that returned OUTPUT and
+the BUFFER that was checked respectively.
+
+See URL `https://github.com/Synthetica9/nix-linter' for more
+information about nix-linter."
+  (mapcar (lambda (err)
+            (let-alist err
+              (flycheck-error-new-at
+               .pos.spanBegin.sourceLine
+               .pos.spanBegin.sourceColumn
+               'warning
+               .description
+               :id .offense
+               :checker checker
+               :buffer buffer
+               :filename (buffer-file-name buffer))))
+          (flycheck-parse-json output)))
+
+(flycheck-define-checker nix-linter
+  "Nix checker using nix-linter.
+
+See URL `https://github.com/Synthetica9/nix-linter'."
+  :command ("nix-linter" "--json-stream" source)
+  :error-parser flycheck-parse-nix-linter
   :modes nix-mode)
 
 (defun flycheck-locate-sphinx-source-directory ()

test/flycheck-test.el

@@ -4062,9 +4062,16 @@ Why not:
 
 (flycheck-ert-def-checker-test nix nix nil
   (flycheck-ert-should-syntax-check
-   "language/nix.nix" 'nix-mode
+   "language/nix/syntax-error.nix" 'nix-mode
    '(3 1 error "syntax error, unexpected IN, expecting ';'," :checker nix)))
 
+(flycheck-ert-def-checker-test nix-linter nix nil
+  (flycheck-ert-should-syntax-check
+   "language/nix/warnings.nix" 'nix-mode
+   '(1 1 warning "LetInInheritRecset" :id "LetInInheritRecset" :checker nix-linter)
+   '(2 3 warning "Unused `let` bind `x`" :id "UnusedLetBind" :checker nix-linter)
+   '(3 4 warning "Unneeded `rec` on set" :id "UnneededRec" :checker nix-linter)))
+
 (ert-deftest flycheck-locate-sphinx-source-directory/not-in-a-sphinx-project ()
   :tags '(language-rst)
   (flycheck-ert-with-resource-buffer "language/rst/errors.rst"

test/resources/language/nix/warnings.nix

@@ -0,0 +1,5 @@
+let
+  x = 5;
+in rec {
+  inherit x;
+}