Meta/ShellCompletions: Complete binaries for run lagom

This completion only works if you have lagom already built in some
capacity, since it scans the build directory tree for binaries, removing
known false positives. However, that is both more accurate than asking
ninja for the targets and filtering those, and it also makes it
independent of the build system used.

Meta/ShellCompletions/zsh/_serenity

@@ -1,11 +1,25 @@
 #compdef serenity serenity.sh
 
+get_top_dir() {
+    git rev-parse --show-toplevel
+}
+
+get_lagom_executables() {
+    # Make completion work with user-defined source directory.
+    SERENITY_SOURCE_DIR="${SERENITY_SOURCE_DIR:-$(get_top_dir)}"
+    # If the Lagom binary directory is missing, this creates an empty list instead of erroring.
+    # Known false positives need to be filtered manually; please add new ones.
+    find "${SERENITY_SOURCE_DIR}/Build/lagom" -mindepth 1 -type f -executable -not -name '*.so*' \
+        -not \( -name 'SQLServer' -o -name 'a.out' -o -name 'CMake*.bin' \) \
+        -printf '%f\n' 2>/dev/null || true
+}
+
 _serenity() {
     local args
     args=(
         '1:command:->commands'
         '2:target:->targets'
-        '3:toolchain:->toolchains'
+        '3:toolchain_or_executable:->toolchains_or_executables'
         '*:: :->args'
     )
 
@@ -41,6 +55,12 @@ _serenity() {
         'Clang:Toolchain clang'
     )
 
+    local lagom_executables
+    # Prevent splitting array on spaces with IFS; bash would have `mapfile` for this.
+    IFS=$'\n'; set -f
+    lagom_executables=($(get_lagom_executables))
+    unset IFS; set +f
+
     _arguments -C -S "$args[@]"
 
     local command
@@ -69,10 +89,13 @@ _serenity() {
             esac
             _describe 'target' targets
             ;;
-        toolchains)
+        toolchains_or_executables)
             if [[ "$command" != help && "$target" != lagom ]]; then
                 # Toolchain-dependent invocations.
-                _describe 'toolchain' toolchains
+                _describe 'toolchain_or_executable' toolchains
+            elif [[ "$command" = run && "$target" = lagom ]]; then
+                # With `run lagom` this already is the executable.
+                _describe 'toolchain_or_executable' lagom_executables
             fi
             ;;
         args)