Comment toggling
M-; comments or uncomments the line or selection using the language's syntax (//, #, <!-- -->, /* */, --, …).
M-; (Emacs comment-dwim) toggles comments using the language’s own syntax. A single line toggles a line comment; a multi-line selection toggles a block/region comment, // and /* */ for Java and C-likes, # for Python/shell/YAML, <!-- --> for XML/HTML/Markdown, -- for SQL, and so on.
It preserves indentation, falls back gracefully for line-only or block-only languages, and is a no-op for languages without comments.