Add brew style --changed

This emulates `brew tests --changed` but for style checks. Should make
things a bit quicker for the case of wanting to quickly fix up
local files.

Library/Homebrew/dev-cmd/style.rb

@@ -24,6 +24,8 @@ module Homebrew
                hidden:      true
         switch "--reset-cache",
                description: "Reset the RuboCop cache."
+        switch "--changed",
+               description: "Check files that were changed from the `main` branch."
         switch "--formula", "--formulae",
                description: "Treat all named arguments as formulae."
         switch "--cask", "--casks",
@@ -45,12 +47,23 @@ module Homebrew
       def run
         Homebrew.install_bundler_gems!(groups: ["style"])
 
-        target = if args.no_named?
+        if args.changed? && !args.no_named?
+          raise UsageError, "`--changed` and named arguments are mutually exclusive!"
+        end
+
+        target = if args.changed?
+          changed_ruby_or_shell_files
+        elsif args.no_named?
           nil
         else
           args.named.to_paths
         end
 
+        if target.blank? && args.changed?
+          opoo "No style checks are available for the changed files!"
+          return
+        end
+
         only_cops = args.only_cops
         except_cops = args.except_cops
 
@@ -70,6 +83,19 @@ module Homebrew
 
         Homebrew.failed = !Style.check_style_and_print(target, **options)
       end
+
+      sig { returns(T::Array[String]) }
+      def changed_ruby_or_shell_files
+        changed_files = Utils.popen_read("git", "diff", "--name-only", "main")
+
+        raise UsageError, "No files have been changed from the `main` branch!" if changed_files.blank?
+
+        changed_files.split("\n").filter_map do |file|
+          next if !file.end_with?(".rb", ".sh", ".yml", ".rbi") && file != "bin/brew"
+
+          Pathname(file)
+        end.select(&:exist?)
+      end
     end
   end
 end

Library/Homebrew/sorbet/rbi/dsl/homebrew/dev_cmd/style_cmd.rbi

@@ -17,6 +17,9 @@ class Homebrew::DevCmd::StyleCmd::Args < Homebrew::CLI::Args
   sig { returns(T::Boolean) }
   def casks?; end
 
+  sig { returns(T::Boolean) }
+  def changed?; end
+
   sig { returns(T::Boolean) }
   def display_cop_names?; end
 

completions/bash/brew

@@ -2429,6 +2429,7 @@ _brew_style() {
     -*)
       __brewcomp "
       --cask
+      --changed
       --debug
       --except-cops
       --fix

completions/fish/brew.fish

@@ -1598,6 +1598,7 @@ __fish_brew_complete_arg 'shellenv' -l verbose -d 'Make some output more verbose
 
 __fish_brew_complete_cmd 'style' 'Check formulae or files for conformance to Homebrew style guidelines'
 __fish_brew_complete_arg 'style' -l cask -d 'Treat all named arguments as casks'
+__fish_brew_complete_arg 'style' -l changed -d 'Check files that were changed from the `main` branch'
 __fish_brew_complete_arg 'style' -l debug -d 'Display any debugging information'
 __fish_brew_complete_arg 'style' -l except-cops -d 'Specify a comma-separated cops list to skip checking for violations of the listed RuboCop cops'
 __fish_brew_complete_arg 'style' -l fix -d 'Fix style violations automatically using RuboCop\'s auto-correct feature'

completions/zsh/_brew

@@ -1960,6 +1960,7 @@ _brew_shellenv() {
 # brew style
 _brew_style() {
   _arguments \
+    '--changed[Check files that were changed from the `main` branch]' \
     '--debug[Display any debugging information]' \
     '(--only-cops)--except-cops[Specify a comma-separated cops list to skip checking for violations of the listed RuboCop cops]' \
     '--fix[Fix style violations automatically using RuboCop'\''s auto-correct feature]' \

docs/Manpage.md

@@ -3121,6 +3121,10 @@ core code and all formulae.
 
 : Reset the RuboCop cache.
 
+`--changed`
+
+: Check files that were changed from the `main` branch.
+
 `--formula`
 
 : Treat all named arguments as formulae.

manpages/brew.1

@@ -1977,6 +1977,9 @@ Fix style violations automatically using RuboCop\[u2019]s auto\-correct feature\
 \fB\-\-reset\-cache\fP
 Reset the RuboCop cache\.
 .TP
+\fB\-\-changed\fP
+Check files that were changed from the \fBmain\fP branch\.
+.TP
 \fB\-\-formula\fP
 Treat all named arguments as formulae\.
 .TP