checker, cgen: add support for #insert, to simplify bootstrapping V

vlib/v/checker/checker.v

@@ -2260,7 +2260,8 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 		return
 	}
 	match node.kind {
-		'include' {
+		'include', 'insert' {
+			original_flag := node.main
 			mut flag := node.main
 			if flag.contains('@VROOT') {
 				// c.note(checker.vroot_is_deprecated_message, node.pos)
@@ -2268,13 +2269,13 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 					c.error(err.msg(), node.pos)
 					return
 				}
-				node.val = 'include $vroot'
+				node.val = '$node.kind $vroot'
 				node.main = vroot
 				flag = vroot
 			}
 			if flag.contains('@VEXEROOT') {
 				vroot := flag.replace('@VEXEROOT', os.dir(pref.vexe_path()))
-				node.val = 'include $vroot'
+				node.val = '$node.kind $vroot'
 				node.main = vroot
 				flag = vroot
 			}
@@ -2283,7 +2284,7 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 					c.error(err.msg(), node.pos)
 					return
 				}
-				node.val = 'include $vroot'
+				node.val = '$node.kind $vroot'
 				node.main = vroot
 				flag = vroot
 			}
@@ -2295,12 +2296,35 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 				node.main = env
 			}
 			flag_no_comment := flag.all_before('//').trim_space()
+			if node.kind == 'include' {
 				if !((flag_no_comment.starts_with('"') && flag_no_comment.ends_with('"'))
 					|| (flag_no_comment.starts_with('<') && flag_no_comment.ends_with('>'))) {
 					c.error('including C files should use either `"header_file.h"` or `<header_file.h>` quoting',
 						node.pos)
 				}
 			}
+			if node.kind == 'insert' {
+				if !(flag_no_comment.starts_with('"') && flag_no_comment.ends_with('"')) {
+					c.error('inserting .c or .h files, should use `"header_file.h"` quoting',
+						node.pos)
+				}
+				node.main = node.main.trim('"')
+				if fcontent := os.read_file(node.main) {
+					node.val = fcontent
+				} else {
+					mut missing_message := 'The file $original_flag, needed for insertion by module `$node.mod`,'
+					if os.is_file(node.main) {
+						missing_message += ' is not readable.'
+					} else {
+						missing_message += ' does not exist.'
+					}
+					if node.msg != '' {
+						missing_message += ' ${node.msg}.'
+					}
+					c.error(missing_message, node.pos)
+				}
+			}
+		}
 		'pkgconfig' {
 			args := if node.main.contains('--') {
 				node.main.split(' ')
@@ -2361,7 +2385,7 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 		}
 		else {
 			if node.kind != 'define' {
-				c.error('expected `#define`, `#flag`, `#include` or `#pkgconfig` not $node.val',
+				c.error('expected `#define`, `#flag`, `#include`, `#insert` or `#pkgconfig` not $node.val',
 					node.pos)
 			}
 		}

vlib/v/checker/tests/comptime_insert_err.cc

@@ -0,0 +1 @@
+// just some text

vlib/v/checker/tests/comptime_insert_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/comptime_insert_err.vv:6:1: error: The file "@VEXEROOT/vlib/v/checker/tests/Non_Existant_File.cc", needed for insertion by module `main`, does not exist.
+    4 | // some more comments
+    5 | 
+    6 | #insert "@VEXEROOT/vlib/v/checker/tests/Non_Existant_File.cc"
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    7 | 
+    8 | fn main() {

vlib/v/checker/tests/comptime_insert_err.vv

@@ -0,0 +1,10 @@
+// The following file exists, so there should not be a checker error for it:
+#insert "@VEXEROOT/vlib/v/checker/tests/comptime_insert_err.cc"
+
+// some more comments
+
+#insert "@VEXEROOT/vlib/v/checker/tests/Non_Existant_File.cc"
+
+fn main() {
+	println('hello')
+}

vlib/v/gen/c/cgen.v

@@ -1797,6 +1797,7 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 			g.writeln('goto ${c_name(node.name)};')
 		}
 		ast.HashStmt {
+			line_nr := node.pos.line_nr + 1
 			mut ct_condition := ''
 			if node.ct_conds.len > 0 {
 				ct_condition_start := g.out.len
@@ -1830,7 +1831,7 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 					}
 					// Objective C code import, include it after V types, so that e.g. `string` is
 					// available there
-					g.definitions.writeln('// added by module `$node.mod`')
+					g.definitions.writeln('// added by module `$node.mod`, file: ${os.file_name(node.source_file)}:$line_nr:')
 					g.definitions.writeln(guarded_include)
 					if ct_condition.len > 0 {
 						g.definitions.writeln('#endif // \$if $ct_condition')
@@ -1841,13 +1842,22 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 					if ct_condition.len > 0 {
 						g.includes.writeln('#if $ct_condition')
 					}
-					g.includes.writeln('// added by module `$node.mod`')
+					g.includes.writeln('// added by module `$node.mod`, file: ${os.file_name(node.source_file)}:$line_nr:')
 					g.includes.writeln(guarded_include)
 					if ct_condition.len > 0 {
 						g.includes.writeln('#endif // \$if $ct_condition')
 					}
 					g.includes.writeln('\n')
 				}
+			} else if node.kind == 'insert' {
+				if ct_condition.len > 0 {
+					g.includes.writeln('#if $ct_condition')
+				}
+				g.includes.writeln('// inserted by module `$node.mod`, file: ${os.file_name(node.source_file)}:$line_nr:')
+				g.includes.writeln(node.val)
+				if ct_condition.len > 0 {
+					g.includes.writeln('#endif // \$if $ct_condition')
+				}
 			} else if node.kind == 'define' {
 				if ct_condition.len > 0 {
 					g.includes.writeln('#if $ct_condition')