v3: suggest a Windows target for MSVC C generated off Windows

Off Windows, the missing-`cl` diagnostic suggested `-cc msvc -o file.c`,
which still generates C for the host target. Suggest `-os windows` (and
`-arch amd64` when the architecture would follow a non-x64 host) unless
the build already targets Windows.

Author: medvednikov

vlib/v/driver/driver.v

@@ -12654,7 +12654,7 @@ pub fn run(args []string) {
 			b.step('MSVC C compatibility')
 		}
 		if effective_c_compiler == 'msvc' && !c_only {
-			msvc_require_cl(c_compiler, host_os)
+			msvc_require_cl(c_compiler, host_os, prefs.target)
 		}
 		pic_flag := shared_pic_flag(is_shared || use_cached_dev_dylib, prefs.normalized_target_os())
 		mut linux_cross_sysroot := ''

vlib/v/driver/msvc.v

@@ -302,13 +302,29 @@ fn msvc_lower_c_file(path string) ! {
 }
 
 // msvc_require_cl exits with an explanation when MSVC's compiler cannot be run.
-fn msvc_require_cl(c_compiler string, host_os string) {
+fn msvc_require_cl(c_compiler string, host_os string, target pref.Target) {
 	os.find_abs_path_of_executable(c_compiler) or {
-		if host_os == 'windows' {
-			eprintln('`-cc msvc` could not find `${c_compiler}`. Run V from a Visual Studio Developer Command Prompt (or after `vcvars64.bat`), so that `cl` and its INCLUDE/LIB environment are available.')
-		} else {
-			eprintln('`-cc msvc` can only compile on Windows; use `-o file.c` to generate C for MSVC on ${host_os}.')
-		}
+		eprintln(msvc_missing_cl_message(c_compiler, host_os, target))
 		exit(1)
 	}
 }
+
+// msvc_missing_cl_message explains why `cl` cannot run. Off Windows, it suggests generating
+// the C file instead, for a Windows target: C generated for the host target contains the
+// host's platform code, which MSVC cannot compile.
+fn msvc_missing_cl_message(c_compiler string, host_os string, target pref.Target) string {
+	if host_os == 'windows' {
+		return '`-cc msvc` could not find `${c_compiler}`. Run V from a Visual Studio Developer Command Prompt (or after `vcvars64.bat`), so that `cl` and its INCLUDE/LIB environment are available.'
+	}
+	mut flags := []string{}
+	if target.os != 'windows' {
+		flags << '-os windows'
+		// The target architecture follows the host otherwise. V's MSVC code paths (for
+		// example the `math.bits` intrinsics) target x64.
+		if target.arch != 'amd64' {
+			flags << '-arch amd64'
+		}
+	}
+	flags << ['-cc msvc', '-o file.c']
+	return '`-cc msvc` can only compile on Windows. To generate C for MSVC on ${host_os}, use `${flags.join(' ')}`, and compile `file.c` with `cl` on Windows.'
+}

vlib/v/driver/msvc_test.v

@@ -1,5 +1,7 @@
 module driver
 
+import v.pref
+
 fn test_msvc_cl_args_translate_an_executable_build() {
 	args := msvc_cl_args(['-std=gnu11', '-w', '-fwrapv', '-Wno-int-conversion', '-O3', '-I',
 		'C:/v/thirdparty/include', '-DGC_THREADS=1', '-Wl,/STACK:33554432', '-o', 'out.exe', 'src.c',
@@ -53,3 +55,27 @@ fn test_c_compiler_is_msvc() {
 	assert !c_compiler_is_msvc('clang')
 	assert !c_compiler_is_msvc('cc')
 }
+
+fn test_msvc_missing_cl_message_targets_windows_off_windows() {
+	macos_arm := pref.Target{
+		os:   'macos'
+		arch: 'arm64'
+	}
+	assert msvc_missing_cl_message('msvc', 'macos', macos_arm).contains('use `-os windows -arch amd64 -cc msvc -o file.c`')
+	linux_x64 := pref.Target{
+		os:   'linux'
+		arch: 'amd64'
+	}
+	assert msvc_missing_cl_message('msvc', 'linux', linux_x64).contains('use `-os windows -cc msvc -o file.c`')
+	// An explicit Windows target, including its architecture, is kept.
+	windows_arm := pref.Target{
+		os:   'windows'
+		arch: 'arm64'
+	}
+	assert msvc_missing_cl_message('msvc', 'linux', windows_arm).contains('use `-cc msvc -o file.c`')
+	windows_x64 := pref.Target{
+		os:   'windows'
+		arch: 'amd64'
+	}
+	assert msvc_missing_cl_message('cl', 'windows', windows_x64).contains('Developer Command Prompt')
+}