Trac #21793: Document whitespace policy, provide editor configuration…

…s for non-Python source files

This is a follow-up on #21792.

URL: https://trac.sagemath.org/21793
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Jeroen Demeyer

.dir-locals.el

@@ -0,0 +1,9 @@
+;;; Directory Local Variables
+;;; For more information see (info "(emacs) Directory Variables")
+
+((nil
+  ;; Use space instead of tabs for indentation
+  (indent-tabs-mode . nil))
+ (makefile-mode
+  ;; But use tabs in Makefiles
+  (indent-tabs-mode . t)))

build/make/deps

@@ -1,4 +1,4 @@
-###############################################################################
+## -*- Makefile -*- ###########################################################
 # This file ($SAGE_ROOT/build/make/deps) will be copied into
 # $SAGE_ROOT/build/make/Makefile by $SAGE_ROOT/build/make/install
 ###############################################################################

src/doc/en/developer/coding_basics.rst

@@ -1073,6 +1073,27 @@ See the files in ``SAGE_ROOT/src/doc/en/tutorial/`` for many
 examples of how to include automated testing in ReST documentation for
 Sage.
 
+
+.. _section-coding-general-whitespace:
+
+General Coding Style Regarding Whitespace
+=========================================
+
+Use spaces instead of tabs for indentation.  The only exception is for
+makefiles, in which tabs have a syntactic meaning different from
+spaces.
+
+Do not add trailing whitespace.
+
+Sage provides editor configuration for Emacs, using the file
+``.dir-locals.el``, to use spaces instead of tabs.  Regarding trailing
+whitespace, see https://www.emacswiki.org/emacs/DeletingWhitespace
+for various solutions.
+
+If you use another editor, we recommend to configure it so you do not
+add tabs to files.
+
+
 .. _chapter-picklejar:
 
 The Pickle Jar