driver: Fix a bug that prevented subsequent, third-party zle-line-pre-redraw hooks from running.

Without this patch, `_zsh_highlight` was invoked by add-zle-hook-widget
with `$?` being non-zero (see add-zle-hook-widget:48-52).  Since
`_zsh_highlight` preserves `$?` from its caller's point of view,
add-zle-hook-widget saw a non-zero exit code from `_zsh_highlight` and
did not run any the remaining zle-line-pre-redraw hooks.

See https://github.com/zsh-users/zsh-syntax-highlighting/issues/579#issuecomment-623576907.

README.md

@@ -39,11 +39,23 @@ FAQ
 
 ### Why must `zsh-syntax-highlighting.zsh` be sourced at the end of the `.zshrc` file?
 
-`zsh-syntax-highlighting.zsh` wraps ZLE widgets.  It must be sourced after all
-custom widgets have been created (i.e., after all `zle -N` calls and after
-running `compinit`).  Widgets created later will work, but will not update the
+zsh-syntax-highlighting works by hooking into the Zsh Line Editor (ZLE) and
+computing syntax highlighting for the command-line buffer as it stands at the
+time z-sy-h's hook is invoked.
+
+In zsh 5.2 and older,
+`zsh-syntax-highlighting.zsh` hooks into ZLE by wrapping ZLE widgets.  It must
+be sourced after all custom widgets have been created (i.e., after all `zle -N`
+calls and after running `compinit`) in order to be able to wrap all of them.
+Widgets created after z-sy-h is sourced will work, but will not update the
 syntax highlighting.
 
+In zsh 5.3 and newer,
+zsh-syntax-highlighting uses the `add-zle-hook-widget` facility to install
+a `zle-line-pre-redraw` hook.  Hooks are run in order of registration,
+therefore, z-sy-h must be sourced (and register its hook) after anything else
+that adds hooks that modify the command-line buffer.
+
 ### Does syntax highlighting work during incremental history search?
 
 Highlighting the command line during an incremental history search (by default bound to

changelog.md

@@ -292,6 +292,28 @@ in this area.
   (0a9b347483ae)
 
 
+# Changes in version 0.5.0
+
+
+## Incompatible changes:
+
+- An unsuccessful completion (a <kbd>⮀ Tab</kbd> press that doesn't change the
+  command line) no longer causes highlighting to be lost.  Visual feedback can
+  alternatively be achieved by setting the `format` zstyle under the `warnings`
+  tag, for example,
+
+        zstyle ':completion:*:warnings' format '%F{red}No matches%f'
+
+    Refer to the [description of the `format` style in `zshcompsys(1)`]
+    [zshcompsys-Standard-Styles-format].
+
+    (#90, part of #245, XXXXXXXXXXXX)
+
+[zshcompsys-Standard-Styles]: http://zsh.sourceforge.net/Doc/Release/Completion-System.html#Standard-Styles
+[zshcompsys-Standard-Styles-format]: http://zsh.sourceforge.net/Doc/Release/Completion-System.html#index-format_002c-completion-style
+  
+
+
 # Changes in version 0.4.1
 
 ## Fixes:

tests/generate.zsh

@@ -31,6 +31,9 @@
 emulate -LR zsh
 setopt localoptions extendedglob
 
+# Required for add-zle-hook-widget.
+zmodload zsh/zle
+
 # Argument parsing.
 if (( $# != 3 )) || [[ $1 == -* ]]; then
   print -r -- >&2 "$0: usage: $0 BUFFER HIGHLIGHTER BASENAME"

tests/test-highlighting.zsh

@@ -31,6 +31,9 @@
 
 setopt NO_UNSET WARN_CREATE_GLOBAL
 
+# Required for add-zle-hook-widget.
+zmodload zsh/zle
+
 # Check an highlighter was given as argument.
 [[ -n "$1" ]] || {
   echo >&2 "Bail out! You must provide the name of a valid highlighter as argument."

tests/test-perfs.zsh

@@ -29,6 +29,9 @@
 # -------------------------------------------------------------------------------------------------
 
 
+# Required for add-zle-hook-widget.
+zmodload zsh/zle
+
 # Check an highlighter was given as argument.
 [[ -n "$1" ]] || {
   echo >&2 "Bail out! You must provide the name of a valid highlighter as argument."

zsh-syntax-highlighting.zsh

@@ -49,6 +49,52 @@ if true; then
   fi
 fi
 
+# This function takes a single argument F and returns True iff F is an autoload stub.
+_zsh_highlight__function_is_autoload_stub_p() {
+  if zmodload -e zsh/parameter; then
+    #(( ${+functions[$1]} )) &&
+    [[ "$functions[$1]" == *"builtin autoload -X"* ]]
+  else
+    #[[ $(type -wa -- "$1") == *'function'* ]] &&
+    [[ "${${(@f)"$(which -- "$1")"}[2]}" == $'\t'$histchars[3]' undefined' ]]
+  fi
+  # Do nothing here: return the exit code of the if.
+}
+
+# Return True iff the argument denotes a function name.
+_zsh_highlight__is_function_p() {
+  if zmodload -e zsh/parameter; then
+    (( ${+functions[$1]} ))
+  else
+    [[ $(type -wa -- "$1") == *'function'* ]]
+  fi
+}
+
+# This function takes a single argument F and returns True iff F denotes the
+# name of a callable function.  A function is callable if it is fully defined
+# or if it is marked for autoloading and autoloading it at the first call to it
+# will succeed.  In particular, if a function has been marked for autoloading
+# but is not available in $fpath, then this function will return False therefor.
+#
+# See users/21671 http://www.zsh.org/cgi-bin/mla/redirect?USERNUMBER=21671
+_zsh_highlight__function_callable_p() {
+  if _zsh_highlight__is_function_p "$1" &&
+     ! _zsh_highlight__function_is_autoload_stub_p "$1"
+  then
+    # Already fully loaded.
+    return 0 # true
+  else
+    # "$1" is either an autoload stub, or not a function at all.
+    #
+    # Use a subshell to avoid affecting the calling shell.
+    #
+    # We expect 'autoload +X' to return non-zero if it fails to fully load
+    # the function.
+    ( autoload -U +X -- "$1" 2>/dev/null )
+    return $?
+  fi
+}
+
 # -------------------------------------------------------------------------------------------------
 # Core highlighting update system
 # -------------------------------------------------------------------------------------------------
@@ -290,9 +336,35 @@ _zsh_highlight_call_widget()
   _zsh_highlight
 }
 
-# Rebind all ZLE widgets to make them invoke _zsh_highlights.
-_zsh_highlight_bind_widgets()
-{
+if _zsh_highlight__function_callable_p add-zle-hook-widget
+then
+  autoload -U add-zle-hook-widget
+  _zsh_highlight__zle-line-finish() {
+    # Reset $WIDGET since the 'main' highlighter depends on it.
+    #
+    # Since $WIDGET is declared by zle as read-only in this function's scope,
+    # a nested function is required in order to shadow its built-in value;
+    # see "User-defined widgets" in zshall.
+    () {
+      local -h -r WIDGET=zle-line-finish
+      _zsh_highlight
+    }
+  }
+  _zsh_highlight__zle-line-pre-redraw() {
+    # Set $? to 0 for _zsh_highlight.  Without this, subsequent
+    # zle-line-pre-redraw hooks won't run, since add-zle-hook-widget happens to
+    # call us with $? == 1 in the common case.
+    true && _zsh_highlight "$@"
+  }
+  _zsh_highlight_bind_widgets(){}
+  if [[ -o zle ]]; then
+    add-zle-hook-widget zle-line-pre-redraw _zsh_highlight__zle-line-pre-redraw
+    add-zle-hook-widget zle-line-finish _zsh_highlight__zle-line-finish
+  fi
+else
+  # Rebind all ZLE widgets to make them invoke _zsh_highlights.
+  _zsh_highlight_bind_widgets()
+  {
     setopt localoptions noksharrays
     typeset -F SECONDS
     local prefix=orig-s$SECONDS-r$RANDOM # unique each time, in case we're sourced more than once
@@ -355,7 +427,8 @@ _zsh_highlight_bind_widgets()
            fi
       esac
     done
-}
+  }
+fi
 
 # Load highlighters from directory.
 #