feat(bash_completion.d): load from one relative to the main script

Primarily for run-in-place-from-git-clone setups.

bash_completion

@@ -2610,14 +2610,31 @@ _comp_xfunc()
 }
 
 # source compat completion directory definitions
-_comp__init_compat_dir=${BASH_COMPLETION_COMPAT_DIR:-/etc/bash_completion.d}
-if [[ -d $_comp__init_compat_dir && -r $_comp__init_compat_dir && -x $_comp__init_compat_dir ]]; then
+_comp__init_compat_dirs=()
+if [[ -n $BASH_COMPLETION_COMPAT_DIR ]]; then
+    _comp__init_compat_dirs+=("$BASH_COMPLETION_COMPAT_DIR")
+else
+    _comp__init_compat_dirs+=(/etc/bash_completion.d)
+    # Similarly as for the "completions" dir, look up from relative to
+    # bash_completion, primarily for run-in-place-from-git-clone setups.
+    # Notably we do it after the system location here, in order to
+    # prefer in-tree variables and functions.
+    if [[ $BASH_SOURCE == */* ]]; then
+        _comp__init_compat_dir="${BASH_SOURCE%/*}/bash_completion.d"
+        [[ ${_comp__init_compat_dirs[0]} == "$_comp__init_compat_dir" ]] ||
+            _comp__init_compat_dirs+=("$_comp__init_compat_dir")
+    else
+        _comp__init_compat_dirs+=(./bash_completion.d)
+    fi
+fi
+for _comp__init_compat_dir in "${_comp__init_compat_dirs[@]}"; do
+    [[ -d $_comp__init_compat_dir && -r $_comp__init_compat_dir && -x $_comp__init_compat_dir ]] || continue
     for _comp__init_file in "$_comp__init_compat_dir"/*; do
         [[ ${_comp__init_file##*/} != @($_comp_backup_glob|Makefile*|${BASH_COMPLETION_COMPAT_IGNORE-}) &&
             -f $_comp__init_file && -r $_comp__init_file ]] && . "$_comp__init_file"
     done
-fi
-unset -v _comp__init_compat_dir _comp__init_file
+done
+unset -v _comp__init_compat_dirs _comp__init_compat_dir _comp__init_file
 
 # source user completion file
 #

doc/configuration.md

@@ -20,10 +20,11 @@ used instead of it.
 
 ### `BASH_COMPLETION_COMPAT_DIR`
 
-Directory for pre-dynamic loading era (pre-2.0) backwards compatibility
-completion files that are loaded eagerly from `bash_completion` when it is
-loaded. If unset or null, the default compatibility directory to use is
-`/etc/bash_completion.d`.
+Directory for pre-dynamic loading era (pre-2.0) and other backwards
+compatibility completion files that are loaded eagerly from `bash_completion`
+when it is loaded. If unset or null, the default compatibility directories to
+use are `/etc/bash_completion.d`, and `bash_completion.d` relative to
+`bash_completion` location.
 
 ### `BASH_COMPLETION_FILEDIR_FALLBACK`