Document tree-sitter features in the user manual

* lisp/progmodes/c-ts-mode.el (c-ts-mode-map): Bind "C-c .", for
consistency with CC mode.
* lisp/treesit.el (treesit-font-lock-level): Doc fix.

* doc/emacs/programs.texi (C Indent, Custom C Indent): Document
the indentation features of 'c-ts-mode'.
(Moving by Defuns): Document 'treesit-defun-tactic'.
* doc/emacs/files.texi (Visiting): Document
'treesit-max-buffer-size'.
* doc/emacs/display.texi (Traditional Font Lock)
(Parser-based Font Lock): New subsections.
* doc/emacs/emacs.texi (Top): Update top-level menu.

1 files changed, 111 insertions, 20 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index f77ab569483..97732b65e32 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -1024,17 +1024,65 @@ customize-group @key{RET} font-lock-faces @key{RET}}.  You can then
 use that customization buffer to customize the appearance of these
 faces.  @xref{Face Customization}.
 
+@cindex just-in-time (JIT) font-lock
+@cindex background syntax highlighting
+  Fontifying very large buffers can take a long time.  To avoid large
+delays when a file is visited, Emacs initially fontifies only the
+visible portion of a buffer.  As you scroll through the buffer, each
+portion that becomes visible is fontified as soon as it is displayed;
+this type of Font Lock is called @dfn{Just-In-Time} (or @dfn{JIT})
+Lock.  You can control how JIT Lock behaves, including telling it to
+perform fontification while idle, by customizing variables in the
+customization group @samp{jit-lock}.  @xref{Specific Customization}.
+
+  The information that major modes use for determining which parts of
+buffer text to fontify and what faces to use can be based on several
+different ways of analyzing the text:
+
+@itemize @bullet
+@item
+Search for keywords and other textual patterns based on regular
+expressions (@pxref{Regexp Search,, Regular Expression Search}).
+
+@item
+Find syntactically distinct parts of text based on built-in syntax
+tables (@pxref{Syntax Tables,,, elisp, The Emacs Lisp Reference
+Manual}).
+
+@item
+Use syntax tree produced by a full-blown parser, via a special-purpose
+library, such as the tree-sitter library (@pxref{Parsing Program
+Source,,, elisp, The Emacs Lisp Reference Manual}), or an external
+program.
+@end itemize
+
+@menu
+* Traditional Font Lock::  Font Lock based on regexps and syntax tables.
+* Parser-based Font Lock:: Font Lock based on external parser.
+@end menu
+
+@node Traditional Font Lock
+@subsection Traditional Font Lock
+@cindex traditional font-lock
+
+  ``Traditional'' methods of providing font-lock information are based
+on regular-expression search and on syntactic analysis using syntax
+tables built into Emacs.  This subsection describes the use and
+customization of font-lock for major modes which use these traditional
+methods.
+
 @vindex font-lock-maximum-decoration
-  You can customize the variable @code{font-lock-maximum-decoration}
-to alter the amount of fontification applied by Font Lock mode, for
-major modes that support this feature.  The value should be a number
-(with 1 representing a minimal amount of fontification; some modes
-support levels as high as 3); or @code{t}, meaning ``as high as
-possible'' (the default).  To be effective for a given file buffer,
-the customization of @code{font-lock-maximum-decoration} should be
-done @emph{before} the file is visited; if you already have the file
-visited in a buffer when you customize this variable, kill the buffer
-and visit the file again after the customization.
+  You can control the amount of fontification applied by Font Lock
+mode by customizing the variable @code{font-lock-maximum-decoration},
+for major modes that support this feature.  The value of this variable
+should be a number (with 1 representing a minimal amount of
+fontification; some modes support levels as high as 3); or @code{t},
+meaning ``as high as possible'' (the default).  To be effective for a
+given file buffer, the customization of
+@code{font-lock-maximum-decoration} should be done @emph{before} the
+file is visited; if you already have the file visited in a buffer when
+you customize this variable, kill the buffer and visit the file again
+after the customization.
 
 You can also specify different numbers for particular major modes; for
 example, to use level 1 for C/C++ modes, and the default level
@@ -1082,16 +1130,59 @@ keywords by customizing the @code{font-lock-ignore} option,
 @pxref{Customizing Keywords,,, elisp, The Emacs Lisp Reference
 Manual}.
 
-@cindex just-in-time (JIT) font-lock
-@cindex background syntax highlighting
-  Fontifying large buffers can take a long time.  To avoid large
-delays when a file is visited, Emacs initially fontifies only the
-visible portion of a buffer.  As you scroll through the buffer, each
-portion that becomes visible is fontified as soon as it is displayed;
-this type of Font Lock is called @dfn{Just-In-Time} (or @dfn{JIT})
-Lock.  You can control how JIT Lock behaves, including telling it to
-perform fontification while idle, by customizing variables in the
-customization group @samp{jit-lock}.  @xref{Specific Customization}.
+@node Parser-based Font Lock
+@subsection Parser-based Font Lock
+@cindex font-lock via tree-sitter
+@cindex parser-based font-lock
+  If your Emacs was built with the tree-sitter library, it can use the
+results of parsing the buffer text by that library for the purposes of
+fontification.  This is usually faster and more accurate than the
+``traditional'' methods described in the previous subsection, since
+the tree-sitter library provides full-blown parsers for programming
+languages and other kinds of formatted text which it supports.  Major
+modes which utilize the tree-sitter library are named
+@code{@var{foo}-ts-mode}, with the @samp{-ts-} part indicating the use
+of the library.  This subsection documents the Font Lock support based
+on the tree-sitter library.
+
+@vindex treesit-font-lock-level
+  You can control the amount of fontification applied by Font Lock
+mode of major modes based on tree-sitter by customizing the variable
+@code{treesit-font-lock-level}.  Its value is a number between 1 and
+4:
+
+@table @asis
+@item Level 1
+This level usually fontifies only comments and function names in
+function definitions.
+@item Level 2
+This level adds fontification of keywords, strings, and data types.
+@item Level 3
+This is the default level; it adds fontification of assignments,
+numbers, properties, etc.
+@item Level 4
+This level adds everything else that can be fontified: operators,
+delimiters, brackets, other punctuation, function names in function
+calls, variables, etc.
+@end table
+
+@vindex treesit-font-lock-feature-list
+@noindent
+What exactly constitutes each of the syntactical categories mentioned
+above depends on the major mode and the parser grammar used by
+tree-sitter for the major-mode's language.  However, in general the
+categories follow the conventions of the programming language or the
+file format supported by the major mode.  The buffer-local value of
+the variable @code{treesit-font-lock-feature-list} holds the
+fontification features supported by a tree-sitter based major mode,
+where each sub-list shows the features provided by the corresponding
+fontification level.
+
+  Once you change the value of @code{treesit-font-lock-level} via
+@w{@kbd{M-x customize-variable}} (@pxref{Specific Customization}), it
+takes effect immediately in all the existing buffers and for files you
+visit in the future in the same session.
+
 
 @node Highlight Interactively
 @section Interactive Highlighting