Implement 'functions -d' to set function description

This was documented, but didn't actually work.

Fixes #5105

CHANGELOG.md

@@ -60,6 +60,7 @@ This section is for changes merged to the `major` branch that are not also merge
 - `alias` now has a `-s` and `--save` option to save the function generated by the alias using `funcsave` (#4878).
 - Path completions now support expansions, meaning expressions like `python ~/<TAB>` now provides file suggestions just like any other relative or absolute path. (This includes support for other expansions, too.)
 - The `string` builtin has new commands `split0` and `join0` for working with NUL-delimited output.
+- The `-d` option to `functions` to set the description of an existing function now works; before 3.0 it was documented but unimplemented. Note that the long form `--description` continues to work. (#5105)
 
 ## Other significant changes
 - Command substitution output is now limited to 10 MB by default (#3822).

src/builtin_functions.cpp

@@ -39,7 +39,7 @@ struct functions_cmd_opts_t {
     wchar_t *handlers_type = NULL;
     wchar_t *description = NULL;
 };
-static const wchar_t *short_options = L":HDacehnqv";
+static const wchar_t *short_options = L":HDacd:ehnqv";
 static const struct woption long_options[] = {
     {L"erase", no_argument, NULL, 'e'},   {L"description", required_argument, NULL, 'd'},
     {L"names", no_argument, NULL, 'n'},   {L"all", no_argument, NULL, 'a'},

tests/functions.in

@@ -60,3 +60,17 @@ set x (functions -v -D multiline_descr)
 if test $x[5] != 'line 1\\\\n\\nline 2 & more; way more'
     echo "Unexpected output for 'functions -v -D multiline_descr': $x" >&2
 end
+
+# ==========
+# Verify function description setting
+function test_func_desc ; end
+functions test_func_desc | string match --quiet '*description*'
+and echo "Unexpected description" >&2
+
+functions --description description1 test_func_desc
+functions test_func_desc | string match --quiet '*description1*'
+or echo "Failed to find description 1" >&2
+
+functions -d description2 test_func_desc
+functions test_func_desc | string match --quiet '*description2*'
+or echo "Failed to find description 2" >&2