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flycheck-rust.el
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;;; flycheck-rust.el --- Flycheck: Rust additions and Cargo support -*- lexical-binding: t; -*-
;; Copyright (C) 2016, 2017 fmdkdd
;; Copyright (C) 2014, 2015 Sebastian Wiesner <swiesner@lunaryorn.com>
;; Copyright (C) 2024-2025 Shen, Jen-Chieh
;; Author: Sebastian Wiesner <swiesner@lunaryorn.com>
;; URL: https://github.com/flycheck/flycheck-rust
;; Keywords: tools, convenience
;; Version: 1.1
;; Package-Requires: ((emacs "24.1") (flycheck "28") (dash "2.13.0") (seq "2.3") (let-alist "1.0.4"))
;; This file is not part of GNU Emacs.
;; 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:
;; This Flycheck extension configures Flycheck automatically for the current
;; Cargo project.
;;
;; # Setup
;;
;; (with-eval-after-load 'rust-mode
;; (add-hook 'flycheck-mode-hook #'flycheck-rust-setup))
;;
;; # Usage
;;
;; Just use Flycheck as usual in your Rust/Cargo projects.
;;; Code:
(eval-when-compile
(require 'pcase)
(require 'let-alist))
(require 'dash)
(require 'flycheck)
(require 'seq)
(require 'json)
(defun flycheck-rust-find-manifest (file-name)
"Get the Cargo.toml manifest for FILE-NAME.
FILE-NAME is the path of a file in a cargo project given as a
string.
See http://doc.crates.io/guide.html for an introduction to the
Cargo.toml manifest.
Return the path to the Cargo.toml manifest file, or nil if the
manifest could not be located."
(-when-let (root-dir (locate-dominating-file file-name "Cargo.toml"))
(expand-file-name "Cargo.toml" root-dir)))
(defun flycheck-rust-dirs-list (start end)
"Return a list of directories from START (inclusive) to END (exclusive).
E.g., if START is '/a/b/c/d' and END is '/a', return the list
'(/a/b/c/d /a/b/c /a/b) in this order.
START and END are strings representing file paths. END should be
above START in the file hierarchy; if not, the list stops at the
root of the file hierarchy."
(let ((dirlist)
(dir (expand-file-name start))
(end (expand-file-name end)))
(while (not (or (equal dir (car dirlist)) ; avoid infinite loop
(file-equal-p dir end)))
(push dir dirlist)
(setq dir (directory-file-name (file-name-directory dir))))
(nreverse dirlist)))
(defun flycheck-rust-get-cargo-targets (manifest)
"Return the list of available Cargo targets for the given project.
MANIFEST is the path to the Cargo.toml file of the project.
Calls `cargo metadata --no-deps --manifest-path MANIFEST
--format-version 1', parses and collects the targets for the
current workspace, and returns them in a list, or nil if no
targets could be found."
(let ((cargo (funcall flycheck-executable-find "cargo")))
(unless cargo
(user-error "flycheck-rust cannot find `cargo'. Please \
make sure that cargo is installed and on your PATH. See \
http://www.flycheck.org/en/latest/user/troubleshooting.html for \
more information on setting your PATH with Emacs."))
;; metadata contains a list of packages, and each package has a list of
;; targets. We concatenate all targets, regardless of the package.
(-when-let (packages (let-alist
(with-temp-buffer
(call-process cargo nil '(t nil) nil
"metadata" "--no-deps"
"--manifest-path" manifest
"--format-version" "1")
(goto-char (point-min))
(let ((json-array-type 'list)
(json-object-type 'alist))
(json-read)))
.packages))
(seq-map (lambda (pkg)
(let-alist pkg .targets))
packages))))
(defun flycheck-rust-find-cargo-target (file-name)
"Return the Cargo build target associated with FILE-NAME.
FILE-NAME is the path of the file that is matched against the
`src_path' value in the list of `targets' returned by `cargo
read-manifest'.
Return an alist ((KIND . k) (NAME . n) (REQUIRED-FEATURES . rf))
where KIND is the target kind (lib, bin, test, example or bench),
NAME the target name (usually, the crate name), and REQUIRED-FEATURES is the
optional list of features required to build the selected target. If FILE-NAME
exactly matches a target `src-path', this target is returned. Otherwise, return
the closest matching target, or nil if no targets could be found.
See http://doc.crates.io/manifest.html#the-project-layout for a
description of the conventional Cargo project layout."
(-when-let* ((manifest (flycheck-rust-find-manifest file-name))
(packages (flycheck-rust-get-cargo-targets manifest))
(targets (-flatten-n 1 packages)))
(let ((target
(or
;; If there is a target that matches the file-name exactly, pick
;; that one
(seq-find (lambda (target)
(let-alist target (string= file-name .src_path)))
targets)
;; Otherwise find the closest matching target by walking up the tree
;; from FILE-NAME and looking for targets in each directory. E.g.,
;; the file 'tests/common/a.rs' will look for a target in
;; 'tests/common', then in 'tests/', etc.
(car (seq-find
(lambda (pair)
(-let [((&alist 'src_path target-path) . dir) pair]
(file-equal-p dir (file-name-directory target-path))))
;; build a list of (target . dir) candidates
(-table-flat
'cons targets
(flycheck-rust-dirs-list file-name
(file-name-directory manifest)))))
;; If all else fails, just pick the first target
(car targets))))
;; If target is 'custom-build', we pick another target from the same package (see GH-62)
(when (string= "custom-build" (let-alist target (car .kind)))
(setq target (->> packages
;; find the same package as current build-script buffer
(--find (--any? (equal target it) it))
(--find (not (equal target it))))))
(when target
(let-alist target
(seq-filter (lambda (kv) (cdr kv))
(list (cons 'kind (flycheck-rust-normalize-target-kind .kind))
(cons 'name .name)
(cons 'required-features .required-features))))))))
(defun flycheck-rust-normalize-target-kind (kinds)
"Return the normalized target name from KIND.
KIND is a list of target name as returned by `cargo metadata',
which do not necessarily correspond to to target names that can
be passed as argument to `cargo rustc'.
The normalization returns a valid cargo target based on KINDS."
;; Assumption: we only care about the first kind name. Multiple kinds only
;; seem to happen for library crate types, and those all maps to the same
;; `lib' target.
(pcase (car kinds)
(`"dylib" "lib")
(`"rlib" "lib")
(`"staticlib" "lib")
(`"cdylib" "lib")
(`"proc-macro" "lib")
(_ (car kinds))))
;;;###autoload
(defun flycheck-rust-setup ()
"Setup Rust in Flycheck.
If the current file is part of a Cargo project, configure
Flycheck according to the Cargo project layout."
(interactive)
;; We should avoid raising any error in this function, as in combination
;; with `global-flycheck-mode' it will render Emacs unusable (see
;; https://github.com/flycheck/flycheck-rust/issues/40#issuecomment-253760883).
(with-demoted-errors "Error in flycheck-rust-setup: %S"
(-when-let* ((file-name (buffer-file-name))
(target (flycheck-rust-find-cargo-target file-name)))
(let-alist target
(setq-local flycheck-rust-features .required-features)
(setq-local flycheck-rust-crate-type .kind)
(setq-local flycheck-rust-binary-name .name)))))
(provide 'flycheck-rust)
;;; flycheck-rust.el ends here