;;; flymake-rust.el -- Flymake integration for the Rust programming language  -*- lexical-binding: t; -*-

;; Copyright © 2021 Brian Cully <bjc@kublai.com>

;; Author: Brian Cully <bjc@kublai.com>
;; URL: https://github.com/bjc/flymake-rust
;; Keywords: tools, flymake, rust
;; Maintainer: Brian Cully <bjc@kublai.com>
;; Version: 0.1
;; Package-Requires: ((emacs "27.1"))

;;; License:

;; This program is free software; you can redistribute it and/or
;; modify it under the terms of the GNU General Public License as
;; published by the Free Software Foundation, either version 3 of the
;; License, or (at your option) any later version.

;; This program is distributed in the hope that it will be useful, but
;; WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;; General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with this program.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;;
;; Add support for the Rust programming language to Flymake.
;;
;; ## Usage
;;
;; Call ‘flymake-rust-setup’ in your Emacs initialization.
;;
;;     (add-hook ’rust-mode-hook ’flymake-rust-setup)
;;

;;; Code:

(require 'generator)

(defgroup flymake-rust nil
  "Flymake integration for the Rust programming language."
  :prefix "flymake-rust-"
  :group 'applications
  :link '(url-link :tag "Github" "https://github.com/bjc/flymake-rust")
  :link '(emacs-commentary-link :tag "Commentary" "flymake-rust"))

(defcustom flymake-rust-cargo-path "cargo"
  "Path to cargo executable."
  :package-version '(flymake-rust . "0.1")
  :type 'string
  :group 'flymake-rust)

(defcustom flymake-rust-rustc-path "rustc"
  "Path to rustc executable."
  :package-version '(flymake-rust . "0.1")
  :type 'string
  :group 'flymake-rust)

(defcustom flymake-rust-checker 'cargo
  "Which checker to use.

 * cargo (default) can only check on save, but is suitable
for all projects.

 * rustc can check between saves, but isn’t project aware, can’t
figure out your edition (or anything else from Cargo.toml) and
will complain about things like missing main functions."
  :package-version '(flymake-rust . "0.1")
  :type '(choice (const cargo)
                 (const rustc))
  :options '(cargo rustc)
  :group 'flymake-rust)

(defvar-local flymake-rust--process nil
  "Flymake process associated with the current buffer.")

(defvar flymake-rust--cargo-flags '("check" "--quiet" "--message-format=json")
  "Additional flags to pass to cargo.")

(defvar flymake-rust--rustc-flags '("--error-format=json" "-o" "/dev/null" "-")
  "Additional flags to pass to rustc.")

(defun flymake-rust--buffer-name (suffix)
  "What to name the flymake buffer for ‘SUFFIX’."
  (format "*flymake-rust[%s] %s*" suffix buffer-file-name))

(defun flymake-rust--extract-error-location (hash)
  "Extract file location data from ‘HASH’.

Returns a sequence of data in the form of (FILE-NAME (BYTE-START
. BYTE-END)).  If any datum is not available, nil will be used in
its place."
  (let* ((spans (or (gethash "spans" hash) [])))
    (mapcar (lambda (span)
              (let ((file-name (or (gethash "file_name" span) nil))
                    (byte-start (or (1+ (gethash "byte_start" span)) nil))
                    (byte-end (or (1+ (gethash "byte_end" span)) nil)))
                `(,file-name (,byte-start . ,byte-end))))
            spans)))

(defun flymake-rust--level-to-action (level)
  "Convert ‘LEVEL’ into a Flymake action."
  (pcase level
    ("error"   :error)
    ("warning" :warning)
    (_         :note)))

(defun flymake-rust--extract-msghash (hash)
  "Return the message hash from ‘HASH’.

Cargo and rustc have slightly different formats for this, which
this function attempts to account for."
  (pcase flymake-rust-checker
    ('cargo (when (string= (gethash "reason" hash) "compiler-message")
              (gethash "message" hash)))
    ('rustc hash)))

(defun flymake-rust--crate-local-path (crate)
  "Return the local path for ‘CRATE’."
  (string-match (rx "(path+file://" (group (1+ (not ")"))) ")")
                crate)
  (match-string-no-properties 1 crate))

(flymake-rust--crate-local-path "std-async 0.1.0 (path+file:///home/bjc/src/std-async)")


(defun flymake-rust--normalize-path (hash file-name)
  "Return full path to ‘FILE-NAME’.

Cargo only puts the relative path in there, so we need to add the
path to the workspace from ‘HASH’ to get the full path.
Unfortunately, cargo doesn’t include this information directly,
but instead appears to expect you to call it with the ‘metadata’
option to extract it.  Which would be fine, if everything were
local, but over TRAMP it may cause undue delay.

So, to avoid calling cargo twice every check, we make
the (probably safe) assumption that the thing we’re working on is
local, and thus the local crate will have a ‘path+file’ URL in
‘package_id’ section, from which we can extract the project
root."
  (pcase flymake-rust-checker
    ('cargo (expand-file-name file-name
                              (flymake-rust--crate-local-path
                               (gethash "package_id" hash))))
    ('rustc file-name)))

;; See https://doc.rust-lang.org/rustc/json.html for a description of
;; the JSON format.
(defun flymake-rust--make-diagnostics (source-buffer hash)
  "Extract diagnostic messages for Flymake from ‘HASH’.

‘HASH’ is the hash-table representation of the JSON output by the
checker for ‘SOURCE-BUFFER’"
  (when-let ((msghash (flymake-rust--extract-msghash hash)))
    (let ((message (gethash "message" msghash))
          (level (gethash "level" msghash))
          (errlocs (flymake-rust--extract-error-location msghash)))
      (mapcar (lambda (errloc)
                (pcase-let ((`(,file-name (,start . ,end)) errloc))
                  ;; Filter out errors that don’t relate to
                  ;; ‘source-buffer’.
                  (when (or (string= file-name "<anon>")
                            (string= (flymake-rust--normalize-path hash file-name)
                                     (file-local-name (buffer-file-name source-buffer))))
                    (flymake-make-diagnostic source-buffer
                                             start end
                                             (flymake-rust--level-to-action level)
                                             message))))
              errlocs))))

(iter-defun flymake-rust--json-generator ()
  "Return an iterator for JSON data from the current buffer.

The entire buffer is treated as JSON data, with the exception of
newlines, since that’s the only non-JSON data that the Rust
compiler suite currently emits.

This function parses the entire buffer from beginning to end, and
tramples all over point, so save that if you need to."
  (goto-char (point-min))
  (while (not (eobp))
    (iter-yield (json-parse-buffer))
    (search-forward "\n" nil t)))

(defun flymake-rust--parse-buffer (diag-buffer source-buffer report-fn)
  "Parse ‘DIAG-BUFFER’ as JSON compiler output for ‘REPORT-FN’.

‘DIAG-BUFFER’ is the process buffer associated with the checker
process for ‘SOURCE-BUFFER’, and is expected to contain JSON
output from the Rust compiler."
  (with-current-buffer diag-buffer
    (js-mode)
    (let ((diags))
      (iter-do (diag (flymake-rust--json-generator))
        (push (flymake-rust--make-diagnostics source-buffer diag) diags))
  
      (if diags
          (funcall report-fn (flatten-list diags))
        (funcall report-fn nil :explanation "no errors")))))

;; TODO: verify that idle timers don’t actually break anything, and
;; are, in fact, an improvement over just calling stuff in the
;; sentinel directly.
(defun flymake-rust--sentinel (source-buffer report-fn proc _event)
  "Handle events for ‘PROC’.

The only event currently being handled is the process finishing,
at which point a short-delay idle timer is set up to handle the
processing of compiler output for ‘SOURCE-BUFFER’ which is
reported to ‘REPORT-FN’."
  (when (eq 'exit (process-status proc))
    (let ((timer (run-with-idle-timer 0.1 nil
                                      'flymake-rust--parse-buffer
                                      (process-buffer proc) source-buffer report-fn)))
      (process-put proc 'timer timer))))

(defun flymake-rust--make-sentinel (source-buffer report-fn)
  "Create a sentinel for ‘SOURCE-BUFFER’ reporting to ‘REPORT-FN’."
  (lambda (proc event)
    (flymake-rust--sentinel source-buffer report-fn proc event)))

(defun flymake-rust--cleanup ()
  "Clean up processes and buffers associated with the current buffer."
  (when-let ((timer (and flymake-rust--process
                         (process-get flymake-rust--process 'timer))))
    (cancel-timer timer))

  (when (process-live-p flymake-rust--process)
    (flymake-log :warning "Killing out-of-date checker process.")
    (delete-process flymake-rust--process))

  (dolist (suffix '("stdout" "stderr"))
    (when-let ((buf (get-buffer (flymake-rust--buffer-name suffix))))
      (kill-buffer buf))))

(defun flymake-rust--cargo-command ()
  "Return a command line for cargo check."
  (cons (executable-find flymake-rust-cargo-path t)
        flymake-rust--cargo-flags))

(defun flymake-rust--rustc-command ()
  "Return a command line for rustc reading from standard input."
  (cons (executable-find flymake-rust-rustc-path t) flymake-rust--rustc-flags))

(defun flymake-rust--checker-command ()
  "Return a command line used to check the current buffer’s file.

This uses the value of ‘flymake-rust-checker’ to determine the
specific command line."
  (pcase flymake-rust-checker
    ('cargo (flymake-rust--cargo-command))
    ('rustc (flymake-rust--rustc-command))))

(defun flymake-rust--call (source-buffer report-fn)
  "Check ‘SOURCE-BUFFER’ for errors and report them to ‘REPORT-FN’.

‘REPORT-FN’ is a function normally created by Flymake which
expects a list of diagnostics created by
‘flymake-make-diagnostic’.  For further information, see the Info
node ‘(flymake)Backend functions’."
  (with-current-buffer source-buffer
    (flymake-rust--cleanup)

    (setq flymake-rust--process
          (make-process :name "flymake-rust"
                        :buffer (get-buffer-create (flymake-rust--buffer-name "stdout"))
                        :command (flymake-rust--checker-command)
                        :connection-type 'pipe
                        :noquery nil
                        :sentinel (flymake-rust--make-sentinel source-buffer report-fn)
                        :stderr (when (eq flymake-rust-checker 'cargo)
                                  (get-buffer-create (flymake-rust--buffer-name "stderr")))
                        :file-handler t))

    (when (eq flymake-rust-checker 'rustc)
      (process-send-region flymake-rust--process
                           (point-min) (point-max))
      (process-send-eof flymake-rust--process))))

(defun flymake-rust--backend (report-fn &rest args)
  "Send Flymake diagnostics to ‘REPORT-FN’.

For the value of ‘ARGS’, see the documentation for
‘flymake-diagnostic-functions’."
  ;; It seems like ‘:recent-changes’ is set when a temporary buffer
  ;; change happens between saves, but is nil on file save (or initial
  ;; check).
  (let ((rc (plist-member args :recent-changes)))
    (when (or (not rc) (and (eq flymake-rust-checker 'rustc) (cadr rc)))
      (flymake-rust--call (current-buffer) report-fn))))

;;;###autoload
(defun flymake-rust-setup ()
  "Provide Flymake support for the Rust programming language."
  (add-hook 'flymake-diagnostic-functions 'flymake-rust--backend nil t))

(provide 'flymake-rust)
;;; flymake-rust.el ends here