windows: preserve external executable manifests (#31012)

doc/windows_long_paths.md

@@ -12,6 +12,8 @@ such an executable can create, check and open files past `MAX_PATH`.
 
 * An executable that its linker already gave a manifest keeps that manifest unchanged.
   MinGW GCC links one that declares `longPathAware` already.
+* If `<output>.manifest` exists beside the final executable, V leaves that external manifest
+  in control. Include `longPathAware` there if the program also needs long paths.
 * Executables linked by MSVC get no manifest: they load `dbghelp.dll` at startup, and its
   initialization would hang on a long `TMP` (see below) before any V code runs.
 * Executables built with the FastC backend (`-b fastc`), shared libraries, and Windows

vlib/v/driver/driver.v

@@ -16892,7 +16892,7 @@ Please install the corresponding development package/libraries and make sure the
 			// in a longPathAware process that loops forever on a TMP past MAX_PATH,
 			// before any V code can shorten it, so they stay as the linker made them.
 			if prefs.normalized_target_os() == 'windows' && !is_o && !is_shared
-				&& !c_compiler_is_msvc(c_compiler) {
+				&& !c_compiler_is_msvc(c_compiler) && !os.is_file('${bin_file}.manifest') {
 				add_windows_long_path_manifest(staged_binary) or {
 					// Long build paths need the manifest, so say so when it is missing for them.
 					if verbose || show_cc || cc_dir_moved {

vlib/v/driver/long_path_manifest_windows_test.c.v

@@ -66,3 +66,25 @@ fn test_v_builds_get_the_manifest() ! {
 	assert os.read_file(exe)!.contains('longPathAware')
 	assert cmdexec.run(exe, []).output.contains('Hello, World!')
 }
+
+fn test_side_by_side_manifest_at_final_output_is_kept() ! {
+	root := manifest_test_root('v3_external_manifest_build')!
+	defer { os.rmdir_all(root) or {} }
+	src := os.join_path(@VEXEROOT, 'examples', 'hello_world.v')
+	exe := os.join_path(root, 'hello_external.exe')
+	manifest := '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+' +
+		'<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">' +
+		'<dependency><dependentAssembly><assemblyIdentity type="win32" ' +
+		'name="Microsoft.Windows.Common-Controls" version="6.0.0.0" ' +
+		'processorArchitecture="*" publicKeyToken="6595b64144ccf1df" language="*"/>' +
+		'</dependentAssembly></dependency></assembly>'
+	os.write_file('${exe}.manifest', manifest)!
+	for flags in [[]string{}, ['-nocache']] {
+		res := cmdexec.run(@VEXE, ['-cc', 'tcc', ...flags, '-o', exe, src])
+		assert res.exit_code == 0, res.output
+		assert !windows_executable_has_manifest(exe)!
+		assert os.read_file('${exe}.manifest')! == manifest
+		assert cmdexec.run(exe, []).output.contains('Hello, World!')
+	}
+}