cgen: panic with error message when go command fails (#10943)

vlib/builtin/builtin.c.v

@@ -3,6 +3,7 @@ module builtin
 type FnExitCb = fn ()
 
 fn C.atexit(f FnExitCb) int
+fn C.strerror(int) &char
 
 [noreturn]
 fn vhalt() {
@@ -100,6 +101,28 @@ pub fn panic(s string) {
 	vhalt()
 }
 
+// return a C-API error message matching to `errnum`
+pub fn c_error_number_str(errnum int) string {
+	mut err_msg := ''
+	$if freestanding {
+		err_msg = 'error $errnum'
+	} $else {
+		c_msg := C.strerror(errnum)
+		err_msg = string{
+			str: &byte(c_msg)
+			len: unsafe { C.strlen(c_msg) }
+			is_lit: 1
+		}
+	}
+	return err_msg
+}
+
+// panic with a C-API error message matching `errnum`
+[noreturn]
+pub fn panic_error_number(basestr string, errnum int) {
+	panic(basestr + c_error_number_str(errnum))
+}
+
 // eprintln prints a message with a line end, to stderr. Both stderr and stdout are flushed.
 pub fn eprintln(s string) {
 	if s.str == 0 {

vlib/builtin/builtin_nix.c.v

@@ -134,3 +134,11 @@ fn break_if_debugger_attached() {
 		_ = ptr
 	}
 }
+
+// These functions are Windows specific - provide dummys for *nix
+pub fn winapi_lasterr_str() string {
+	return ''
+}
+
+[noreturn]
+pub fn panic_lasterr() {}

vlib/builtin/builtin_windows.c.v

@@ -54,6 +54,8 @@ fn C.SymFromAddr(h_process voidptr, address u64, p_displacement voidptr, p_symbo
 
 fn C.SymGetLineFromAddr64(h_process voidptr, address u64, p_displacement voidptr, p_line &Line64) int
 
+fn C.FormatMessage(dwFlags u32, lpSource voidptr, dwMessageId u32, dwLanguageId u32, lpBuffer &voidptr, nSize u32, Arguments voidptr) u32
+
 // Ref - https://docs.microsoft.com/en-us/windows/win32/api/dbghelp/nf-dbghelp-symsetoptions
 const (
 	symopt_undname               = 0x00000002
@@ -277,3 +279,26 @@ fn break_if_debugger_attached() {
 		}
 	}
 }
+
+// return an error message generated from WinAPI's `LastError`
+pub fn winapi_lasterr_str() string {
+	mut msgbuf := &byte(0)
+	err_msg_id := C.GetLastError()
+	res := C.FormatMessage(C.FORMAT_MESSAGE_ALLOCATE_BUFFER | C.FORMAT_MESSAGE_FROM_SYSTEM,
+		C.NULL, err_msg_id, 0, &msgbuf, 0, C.NULL)
+	err_msg := if res == 0 {
+		'unknown error $err_msg_id'
+	} else {
+		string{
+			str: msgbuf
+			len: int(res)
+		}
+	}
+	return err_msg
+}
+
+// panic with an error message generated from WinAPI's `LastError`
+[noreturn]
+pub fn panic_lasterr(base string) {
+	panic(base + winapi_lasterr_str())
+}