cgen: add support for v -printfn main file.v

vlib/v/gen/cgen.v

@@ -60,6 +60,7 @@ struct Gen {
 mut:
 	file                 ast.File
 	fn_decl              &ast.FnDecl // pointer to the FnDecl we are currently inside otherwise 0
+	last_fn_c_name       string
 	tmp_count            int
 	variadic_args        map[string]int
 	is_c_call            bool // e.g. `C.printf("v")`
@@ -479,8 +480,12 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 			}
 		}
 		ast.FnDecl {
+			fn_start_pos := g.out.len
 			g.fn_decl = it // &it
 			g.gen_fn_decl(it)
+			if g.last_fn_c_name in g.pref.printfn_list {
+				println(g.out.after(fn_start_pos))
+			}
 			g.writeln('')
 		}
 		ast.ForCStmt {

vlib/v/gen/fn.v

@@ -19,12 +19,15 @@ fn (mut g Gen) gen_fn_decl(it ast.FnDecl) {
 			if g.is_gui_app() {
 				// GUI application
 				g.writeln('int WINAPI wWinMain(HINSTANCE instance, HINSTANCE prev_instance, LPWSTR cmd_line, int show_cmd')
+				g.last_fn_c_name = 'wWinMain'
 			} else {
 				// Console application
 				g.writeln('int wmain(int ___argc, wchar_t* ___argv[], wchar_t* ___envp[]')
+				g.last_fn_c_name = 'wmain'
 			}
 		} else {
 			g.write('int ${it.name}(int ___argc, char** ___argv')
+			g.last_fn_c_name = it.name
 		}
 	} else {
 		mut name := it.name
@@ -46,6 +49,7 @@ fn (mut g Gen) gen_fn_decl(it ast.FnDecl) {
 		// type_name := g.table.Type_to_str(it.return_type)
 		type_name := g.typ(it.return_type)
 		g.write('$type_name ${name}(')
+		g.last_fn_c_name = name
 		g.definitions.write('$type_name ${name}(')
 	}
 	// Receiver is the first argument

vlib/v/pref/pref.v

@@ -77,8 +77,8 @@ pub mut:
 	compile_defines_all []string // contains both: ['vfmt','another']
 
 	mod                 string
-	run_args []string  // `v run x.v 1 2 3` => `1 2 3`
-
+	run_args            []string // `v run x.v 1 2 3` => `1 2 3`
+	printfn_list        []string // a list of generated function names, whose source should be shown, for debugging
 }
 
 pub fn backend_from_string(s string) ?Backend {
@@ -97,4 +97,3 @@ pub fn backend_from_string(s string) ?Backend {
 		}
 	}
 }
-