diff --git a/cmd/tools/vbin2v.v b/cmd/tools/vbin2v.v
index c1ab6e09da..ed1b776db2 100644
--- a/cmd/tools/vbin2v.v
+++ b/cmd/tools/vbin2v.v
@@ -77,7 +77,7 @@ fn (context Context) bname_and_bytes(file string) ?(string, []byte) {
 	fname := os.file_name(file)
 	fname_escaped := fname.replace_each(['.', '_', '-', '_'])
 	byte_name := '$context.prefix$fname_escaped'.to_lower()
-	fbytes := os.read_bytes(file) or { return error('Error: $err.msg') }
+	fbytes := os.read_bytes(file) or { return error('Error: $err.msg()') }
 	return byte_name, fbytes
 }
 
@@ -108,7 +108,7 @@ fn main() {
 		exit(0)
 	}
 	files := fp.finalize() or {
-		eprintln('Error: $err.msg')
+		eprintln('Error: $err.msg()')
 		exit(1)
 	}
 	real_files := files.filter(it != 'bin2v')
@@ -123,7 +123,7 @@ fn main() {
 	mut file_byte_map := map[string][]byte{}
 	for file in real_files {
 		bname, fbytes := context.bname_and_bytes(file) or {
-			eprintln(err.msg)
+			eprintln(err.msg())
 			exit(1)
 		}
 		file_byte_map[bname] = fbytes
diff --git a/cmd/tools/vbuild-tools.v b/cmd/tools/vbuild-tools.v
index b13f1a1ad2..6138612b9d 100644
--- a/cmd/tools/vbuild-tools.v
+++ b/cmd/tools/vbuild-tools.v
@@ -66,8 +66,9 @@ fn main() {
 		}
 		target_path := os.join_path(tfolder, texe)
 		os.mv_by_cp(tpath, target_path) or {
-			if !err.msg.contains('vbuild-tools') && !err.msg.contains('vtest-all') {
-				eprintln('error while moving $tpath to $target_path: $err.msg')
+			emsg := err.msg()
+			if !emsg.contains('vbuild-tools') && !emsg.contains('vtest-all') {
+				eprintln('error while moving $tpath to $target_path: $emsg')
 			}
 			continue
 		}
diff --git a/cmd/tools/vdoc/vdoc.v b/cmd/tools/vdoc/vdoc.v
index bdc02589bc..db1e4d39b4 100644
--- a/cmd/tools/vdoc/vdoc.v
+++ b/cmd/tools/vdoc/vdoc.v
@@ -244,8 +244,8 @@ fn (vd VDoc) get_readme(path string) string {
 
 fn (vd VDoc) emit_generate_err(err IError) {
 	cfg := vd.cfg
-	mut err_msg := err.msg
-	if err.code == 1 {
+	mut err_msg := err.msg()
+	if err.code() == 1 {
 		mod_list := get_modules_list(cfg.input_path, []string{})
 		println('Available modules:\n==================')
 		for mod in mod_list {
@@ -467,7 +467,7 @@ fn parse_arguments(args []string) Config {
 					exit(1)
 				}
 				selected_platform := doc.platform_from_string(platform_str) or {
-					eprintln(err.msg)
+					eprintln(err.msg())
 					exit(1)
 				}
 				cfg.platform = selected_platform
diff --git a/cmd/tools/vfmt.v b/cmd/tools/vfmt.v
index fbf22de893..932651fbf6 100644
--- a/cmd/tools/vfmt.v
+++ b/cmd/tools/vfmt.v
@@ -82,7 +82,7 @@ fn main() {
 		eprintln('vfmt possible_files: ' + possible_files.str())
 	}
 	files := util.find_all_v_files(possible_files) or {
-		verror(err.msg)
+		verror(err.msg())
 		return
 	}
 	if os.is_atty(0) == 0 && files.len == 0 {
diff --git a/cmd/tools/vgret.v b/cmd/tools/vgret.v
index b9f575ec62..fbeceeb220 100644
--- a/cmd/tools/vgret.v
+++ b/cmd/tools/vgret.v
@@ -211,7 +211,7 @@ fn generate_screenshots(mut opt Options, output_path string) ? {
 
 		app_config.screenshots_path = screenshot_path
 		app_config.screenshots = take_screenshots(opt, app_config) or {
-			return error('Failed taking screenshots of `$app_path`:\n$err.msg')
+			return error('Failed taking screenshots of `$app_path`:\n$err.msg()')
 		}
 	}
 }
diff --git a/cmd/tools/vpm.v b/cmd/tools/vpm.v
index 260a24e283..b116ad91ed 100644
--- a/cmd/tools/vpm.v
+++ b/cmd/tools/vpm.v
@@ -485,7 +485,7 @@ fn vpm_remove(module_names []string) {
 		println('Removing module "$name" ...')
 		verbose_println('removing folder $final_module_path')
 		os.rmdir_all(final_module_path) or {
-			verbose_println('error while removing "$final_module_path": $err.msg')
+			verbose_println('error while removing "$final_module_path": $err.msg()')
 		}
 		// delete author directory if it is empty
 		author := name.split('.')[0]
@@ -496,7 +496,7 @@ fn vpm_remove(module_names []string) {
 		if os.is_dir_empty(author_dir) {
 			verbose_println('removing author folder $author_dir')
 			os.rmdir(author_dir) or {
-				verbose_println('error while removing "$author_dir": $err.msg')
+				verbose_println('error while removing "$author_dir": $err.msg()')
 			}
 		}
 	}
diff --git a/cmd/tools/vself.v b/cmd/tools/vself.v
index 6f6f8f0991..7774fc6ff4 100644
--- a/cmd/tools/vself.v
+++ b/cmd/tools/vself.v
@@ -30,7 +30,7 @@ fn main() {
 		// The user just wants an independent copy of v, and so we are done.
 		return
 	}
-	backup_old_version_and_rename_newer(short_v_name) or { panic(err.msg) }
+	backup_old_version_and_rename_newer(short_v_name) or { panic(err.msg()) }
 	println('V built successfully as executable "$vexe_name".')
 }
 
@@ -71,17 +71,17 @@ fn backup_old_version_and_rename_newer(short_v_name string) ?bool {
 
 	list_folder(short_v_name, 'before:', 'removing $bak_file ...')
 	if os.exists(bak_file) {
-		os.rm(bak_file) or { errors << 'failed removing $bak_file: $err.msg' }
+		os.rm(bak_file) or { errors << 'failed removing $bak_file: $err.msg()' }
 	}
 
 	list_folder(short_v_name, '', 'moving $v_file to $bak_file ...')
-	os.mv(v_file, bak_file) or { errors << err.msg }
+	os.mv(v_file, bak_file) or { errors << err.msg() }
 
 	list_folder(short_v_name, '', 'removing $v_file ...')
 	os.rm(v_file) or {}
 
 	list_folder(short_v_name, '', 'moving $v2_file to $v_file ...')
-	os.mv_by_cp(v2_file, v_file) or { panic(err.msg) }
+	os.mv_by_cp(v2_file, v_file) or { panic(err.msg()) }
 
 	list_folder(short_v_name, 'after:', '')
 
diff --git a/cmd/tools/vsymlink.v b/cmd/tools/vsymlink.v
index a45c120070..dba5b26959 100644
--- a/cmd/tools/vsymlink.v
+++ b/cmd/tools/vsymlink.v
@@ -106,7 +106,7 @@ fn setup_symlink_windows(vexe string) {
 		println('Symlink $vsymlink to $vexe created.')
 		println('Checking system %PATH%...')
 		reg_sys_env_handle := get_reg_sys_env_handle() or {
-			warn_and_exit(err.msg)
+			warn_and_exit(err.msg())
 			return
 		}
 		// TODO: Fix defers inside ifs
@@ -133,7 +133,7 @@ fn setup_symlink_windows(vexe string) {
 			println('Adding symlink directory to system %PATH%...')
 			set_reg_value(reg_sys_env_handle, 'Path', new_sys_env_path) or {
 				C.RegCloseKey(reg_sys_env_handle)
-				warn_and_exit(err.msg)
+				warn_and_exit(err.msg())
 			}
 			println('Done.')
 		}
diff --git a/cmd/tools/vtest-parser.v b/cmd/tools/vtest-parser.v
index e3650f653c..0e41a98013 100644
--- a/cmd/tools/vtest-parser.v
+++ b/cmd/tools/vtest-parser.v
@@ -121,7 +121,7 @@ fn process_cli_args() &Context {
 		exit(0)
 	}
 	context.all_paths = fp.finalize() or {
-		context.error(err.msg)
+		context.error(err.msg())
 		exit(1)
 	}
 	if !context.is_worker && context.all_paths.len == 0 {
diff --git a/cmd/tools/vup.v b/cmd/tools/vup.v
index ba1ad1b8cf..89bda387f9 100644
--- a/cmd/tools/vup.v
+++ b/cmd/tools/vup.v
@@ -124,9 +124,9 @@ fn (app App) show_current_v_version() {
 fn (app App) backup(file string) {
 	backup_file := '${file}_old.exe'
 	if os.exists(backup_file) {
-		os.rm(backup_file) or { eprintln('failed removing $backup_file: $err.msg') }
+		os.rm(backup_file) or { eprintln('failed removing $backup_file: $err.msg()') }
 	}
-	os.mv(file, backup_file) or { eprintln('failed moving $file: $err.msg') }
+	os.mv(file, backup_file) or { eprintln('failed moving $file: $err.msg()') }
 }
 
 fn (app App) git_command(command string) {
diff --git a/doc/docs.md b/doc/docs.md
index c07bd51d1b..89cea74cbe 100644
--- a/doc/docs.md
+++ b/doc/docs.md
@@ -1863,7 +1863,7 @@ import os
 
 fn read_log() {
 	mut ok := false
-	mut f := os.open('log.txt') or { panic(err.msg) }
+	mut f := os.open('log.txt') or { panic(err) }
 	defer {
 		f.close()
 	}
@@ -3452,7 +3452,7 @@ to break from the current block.
 Note that `break` and `continue` can only be used inside a `for` loop.
 
 V does not have a way to forcibly "unwrap" an optional (as other languages do,
-for instance Rust's `unwrap()` or Swift's `!`). To do this, use `or { panic(err.msg) }` instead.
+for instance Rust's `unwrap()` or Swift's `!`). To do this, use `or { panic(err) }` instead.
 
 ---
 The third method is to provide a default value at the end of the `or` block.
@@ -5536,7 +5536,7 @@ eprintln('file: ' + @FILE + ' | line: ' + @LINE + ' | fn: ' + @MOD + '.' + @FN)
 Another example, is if you want to embed the version/name from v.mod *inside* your executable:
 ```v ignore
 import v.vmod
-vm := vmod.decode( @VMOD_FILE ) or { panic(err.msg) }
+vm := vmod.decode( @VMOD_FILE ) or { panic(err) }
 eprintln('$vm.name $vm.version\n $vm.description')
 ```
 
diff --git a/examples/v_script.vsh b/examples/v_script.vsh
index fb76d9fa34..89f033b779 100755
--- a/examples/v_script.vsh
+++ b/examples/v_script.vsh
@@ -13,14 +13,14 @@ mkdir('v_script_dir') ?
 
 println("\nEntering into v_script_dir and listing it's files.")
 chdir('v_script_dir') ?
-files := ls('.') or { panic(err.msg) }
+files := ls('.') or { panic(err) }
 println(files)
 
 println('\nCreating foo.txt')
 create('foo.txt') ?
 
 println('\nFiles:')
-again_ls := ls('.') or { panic(err.msg) }
+again_ls := ls('.') or { panic(err) }
 println(again_ls)
 
 println('\nRemoving foo.txt and v_script_dir')
diff --git a/vlib/crypto/ed25519/ed25519.v b/vlib/crypto/ed25519/ed25519.v
index 9a3c7d415c..b9643c46c8 100644
--- a/vlib/crypto/ed25519/ed25519.v
+++ b/vlib/crypto/ed25519/ed25519.v
@@ -170,7 +170,7 @@ fn new_key_from_seed_generic(mut privatekey []byte, seed []byte) {
 
 	mut h := sha512.sum512(seed)
 	mut s := edwards25519.new_scalar()
-	s.set_bytes_with_clamping(h[..32]) or { panic(err.msg) }
+	s.set_bytes_with_clamping(h[..32]) or { panic(err) }
 	mut aa := edwards25519.Point{}
 	aa.scalar_base_mult(mut s)
 
diff --git a/vlib/crypto/ed25519/examples/example.v b/vlib/crypto/ed25519/examples/example.v
index 32a358ee12..02919accaf 100644
--- a/vlib/crypto/ed25519/examples/example.v
+++ b/vlib/crypto/ed25519/examples/example.v
@@ -8,11 +8,11 @@ import crypto.ed25519
 fn main() {
 	msg := 'Hello Girl'
 
-	publ, priv := ed25519.generate_key() or { panic(err.msg) }
+	publ, priv := ed25519.generate_key() ?
 
 	m := msg.bytes()
 
-	sig := ed25519.sign(priv, m) or { panic(err.msg) }
+	sig := ed25519.sign(priv, m) ?
 
 	println('=== Message ===')
 	println('Msg: $msg \nHash: $m')
@@ -31,7 +31,7 @@ fn main() {
 	println('signature: R=${sig[0..32].hex()} s=${sig[32..64].hex()}')
 	println('   signature (Base64)=${base64.encode(sig)}')
 
-	rtn := ed25519.verify(publ, m, sig) or { panic(err.msg) }
+	rtn := ed25519.verify(publ, m, sig) ?
 
 	if rtn {
 		println('Signature verified :$rtn')
diff --git a/vlib/crypto/ed25519/internal/ed25519_test.v b/vlib/crypto/ed25519/internal/ed25519_test.v
index 412bb0a776..af154656b0 100644
--- a/vlib/crypto/ed25519/internal/ed25519_test.v
+++ b/vlib/crypto/ed25519/internal/ed25519_test.v
@@ -85,10 +85,10 @@ fn works_check_on_sign_input_string(item string) bool {
 		return false
 	}
 	// assert parts.len == 5
-	privbytes := hex.decode(parts[0]) or { panic(err.msg) }
-	pubkey := hex.decode(parts[1]) or { panic(err.msg) }
-	msg := hex.decode(parts[2]) or { panic(err.msg) }
-	mut sig := hex.decode(parts[3]) or { panic(err.msg) }
+	privbytes := hex.decode(parts[0]) or { panic(err) }
+	pubkey := hex.decode(parts[1]) or { panic(err) }
+	msg := hex.decode(parts[2]) or { panic(err) }
+	mut sig := hex.decode(parts[3]) or { panic(err) }
 
 	if pubkey.len != ed25519.public_key_size {
 		return false
@@ -100,12 +100,12 @@ fn works_check_on_sign_input_string(item string) bool {
 	copy(mut priv[..], privbytes)
 	copy(mut priv[32..], pubkey)
 
-	sig2 := ed25519.sign(priv[..], msg) or { panic(err.msg) }
+	sig2 := ed25519.sign(priv[..], msg) or { panic(err) }
 	if sig != sig2[..] {
 		return false
 	}
 
-	res := ed25519.verify(pubkey, msg, sig2) or { panic(err.msg) }
+	res := ed25519.verify(pubkey, msg, sig2) or { panic(err) }
 	// assert res == true
 	if !res {
 		return false
@@ -150,7 +150,7 @@ mut:
 // so, maybe need a long time to finish.
 // be quiet and patient
 fn test_input_from_djb_ed25519_crypto_sign_input_with_syncpool() ? {
-	// contents := os.read_lines('testdata/sign.input') or { panic(err.msg) } //[]string
+	// contents := os.read_lines('testdata/sign.input') or { panic(err) } //[]string
 	mut pool_s := pool.new_pool_processor(
 		callback: worker_for_string_content
 		maxjobs: 4
@@ -165,7 +165,7 @@ fn test_input_from_djb_ed25519_crypto_sign_input_with_syncpool() ? {
 // same as above, but without sync.pool
 /*
 fn test_input_from_djb_ed25519_crypto_sign_input_without_syncpool() ? {
-	// contents := os.read_lines('testdata/sign.input') or { panic(err.msg) } //[]string
+	// contents := os.read_lines('testdata/sign.input') or { panic(err) } //[]string
 	for i, item in ed25519.contents {
 		parts := item.split(':') // []string
 		// println(parts)
diff --git a/vlib/crypto/ed25519/internal/edwards25519/element_test.v b/vlib/crypto/ed25519/internal/edwards25519/element_test.v
index a0dbf43b68..3036a4fa88 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/element_test.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/element_test.v
@@ -387,11 +387,11 @@ fn test_bytes_big_equivalence() ? {
 	mut fe := el.generate_element()
 	mut fe1 := el.generate_element()
 
-	fe.set_bytes(inp) or { panic(err.msg) }
+	fe.set_bytes(inp) or { panic(err) }
 	inp[inp.len - 1] &= (1 << 7) - 1 // mask the most significant bit
 
 	mut b := big.integer_from_bytes(swap_endianness(mut inp)) // need swap_endianness
-	fe1.from_big_integer(b) or { panic(err.msg) } // do swap_endianness internally
+	fe1.from_big_integer(b) or { panic(err) } // do swap_endianness internally
 
 	assert fe == fe1
 
diff --git a/vlib/crypto/ed25519/internal/edwards25519/extra_test.v b/vlib/crypto/ed25519/internal/edwards25519/extra_test.v
index 957f01f63b..f6548f3c10 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/extra_test.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/extra_test.v
@@ -67,20 +67,20 @@ fn test_bytes_montgomery_infinity() {
 
 const (
 	loworder_string = '26e8958fc2b227b045c3f489f2ef98f0d5dfac05d3c63339b13802886d53fc85'
-	loworder_bytes  = hex.decode(loworder_string) or { panic(err.msg) }
+	loworder_bytes  = hex.decode(loworder_string) or { panic(err) }
 )
 
 fn fn_cofactor(mut data []byte) bool {
 	if data.len != 64 {
-		panic('err.msg')
+		panic('data.len should be 64')
 	}
 	mut loworder := Point{}
-	loworder.set_bytes(edwards25519.loworder_bytes) or { panic(err.msg) }
+	loworder.set_bytes(edwards25519.loworder_bytes) or { panic(err) }
 
 	mut s := new_scalar()
 	mut p := Point{}
 	mut p8 := Point{}
-	s.set_uniform_bytes(data) or { panic(err.msg) }
+	s.set_uniform_bytes(data) or { panic(err) }
 	p.scalar_base_mult(mut s)
 	p8.mult_by_cofactor(p)
 
@@ -128,8 +128,8 @@ fn invert_works(mut xinv Scalar, x NotZeroScalar) bool {
 }
 
 fn test_scalar_invert() {
-	nsc := generate_notzero_scalar(5) or { panic(err.msg) }
-	mut xsc := generate_scalar(5) or { panic(err.msg) }
+	nsc := generate_notzero_scalar(5) or { panic(err) }
+	mut xsc := generate_scalar(5) or { panic(err) }
 	assert invert_works(mut xsc, nsc) == true
 
 	mut zero := new_scalar()
@@ -140,9 +140,9 @@ fn test_scalar_invert() {
 
 fn test_multiscalarmultmatchesbasemult() {
 	for i in 0 .. 6 {
-		x := generate_scalar(100) or { panic(err.msg) }
-		y := generate_scalar(5) or { panic(err.msg) }
-		z := generate_scalar(2) or { panic(err.msg) }
+		x := generate_scalar(100) or { panic(err) }
+		y := generate_scalar(5) or { panic(err) }
+		z := generate_scalar(2) or { panic(err) }
 		assert multiscalarmultmatchesbasemult(x, y, z) == true
 	}
 }
@@ -196,9 +196,9 @@ fn vartime_multiscala_rmultmatches_basemult(xx Scalar, yy Scalar, zz Scalar) boo
 
 fn test_vartimemultiscalarmultmatchesbasemult() {
 	for i in 0 .. 5 {
-		x := generate_scalar(100) or { panic(err.msg) }
-		y := generate_scalar(5) or { panic(err.msg) }
-		z := generate_scalar(2) or { panic(err.msg) }
+		x := generate_scalar(100) or { panic(err) }
+		y := generate_scalar(5) or { panic(err) }
+		z := generate_scalar(2) or { panic(err) }
 		assert vartime_multiscala_rmultmatches_basemult(x, y, z) == true
 	}
 }
diff --git a/vlib/crypto/ed25519/internal/edwards25519/point.v b/vlib/crypto/ed25519/internal/edwards25519/point.v
index 4bf085d87e..ba815668e2 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/point.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/point.v
@@ -10,12 +10,12 @@ const (
 	gen_bytes = [byte(0x58), 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66,
 		0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66,
 		0x66, 0x66, 0x66, 0x66, 0x66]
-	d_const   = d_const_generate() or { panic(err.msg) }
-	d2_const  = d2_const_generate() or { panic(err.msg) }
+	d_const   = d_const_generate() or { panic(err) }
+	d2_const  = d2_const_generate() or { panic(err) }
 	// id_point is the point at infinity.
-	id_point  = id_point_generate() or { panic(err.msg) }
+	id_point  = id_point_generate() or { panic(err) }
 	// generator point
-	gen_point = generator() or { panic(err.msg) }
+	gen_point = generator() or { panic(err) }
 )
 
 fn d_const_generate() ?Element {
diff --git a/vlib/crypto/ed25519/internal/edwards25519/scalar_test.v b/vlib/crypto/ed25519/internal/edwards25519/scalar_test.v
index 12d1c6fcf6..9a5ab90eed 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/scalar_test.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/scalar_test.v
@@ -58,8 +58,8 @@ fn addlike_subneg(x Scalar, y Scalar) bool {
 
 fn test_scalar_add_like_subneg() {
 	for i in 0 .. 15 {
-		x := generate_scalar(1000) or { panic(err.msg) }
-		y := generate_scalar(1000) or { panic(err.msg) }
+		x := generate_scalar(1000) or { panic(err) }
+		y := generate_scalar(1000) or { panic(err) }
 		assert addlike_subneg(x, y) == true
 	}
 }
@@ -70,7 +70,7 @@ fn fg(sc Scalar) bool {
 
 fn test_scalar_generate() ? {
 	for i in 0 .. 15 {
-		sc := generate_scalar(1000) or { panic(err.msg) }
+		sc := generate_scalar(1000) or { panic(err) }
 
 		assert fg(sc) == true
 	}
@@ -79,10 +79,10 @@ fn test_scalar_generate() ? {
 //
 fn test_scalar_set_canonical_bytes() ? {
 	for i in 0 .. 10 {
-		mut buf := rand.bytes(32) or { panic(err.msg) }
-		mut sc := generate_scalar(1000) or { panic(err.msg) }
+		mut buf := rand.bytes(32) or { panic(err) }
+		mut sc := generate_scalar(1000) or { panic(err) }
 		buf[buf.len - 1] &= (1 << 4) - 1
-		sc = sc.set_canonical_bytes(buf) or { panic(err.msg) }
+		sc = sc.set_canonical_bytes(buf) or { panic(err) }
 
 		assert buf[..] == sc.bytes()
 		assert is_reduced(sc)
@@ -93,7 +93,7 @@ fn test_scalar_set_canonical_bytes_round_trip() ? {
 	for i in 0 .. 10 {
 		mut sc1 := generate_scalar(2) ?
 		mut sc2 := generate_scalar(6) ?
-		sc2.set_canonical_bytes(sc1.bytes()) or { panic(err.msg) }
+		sc2.set_canonical_bytes(sc1.bytes()) or { panic(err) }
 
 		assert sc1 == sc2
 	}
@@ -152,10 +152,10 @@ fn test_scalar_set_bytes_with_clamping() {
 		t.Errorf("random: got %q, want %q", got, want)
 	}*/
 	random := '633d368491364dc9cd4c1bf891b1d59460face1644813240a313e61f2c88216e'
-	random_bytes := hex.decode(random) or { panic(err.msg) }
+	random_bytes := hex.decode(random) or { panic(err) }
 
 	mut s0 := Scalar{}
-	s0.set_bytes_with_clamping(random_bytes) or { panic(err.msg) }
+	s0.set_bytes_with_clamping(random_bytes) or { panic(err) }
 
 	mut p0 := Point{}
 	p0.scalar_base_mult(mut s0)
@@ -167,8 +167,8 @@ fn test_scalar_set_bytes_with_clamping() {
 
 	zero := '0000000000000000000000000000000000000000000000000000000000000000'
 	mut s1 := Scalar{}
-	zero_bytes := hex.decode(zero) or { panic(err.msg) }
-	s1.set_bytes_with_clamping(zero_bytes) or { panic(err.msg) }
+	zero_bytes := hex.decode(zero) or { panic(err) }
+	s1.set_bytes_with_clamping(zero_bytes) or { panic(err) }
 	mut p1 := Point{}
 	p1.scalar_base_mult(mut s1)
 
@@ -178,8 +178,8 @@ fn test_scalar_set_bytes_with_clamping() {
 
 	one := 'ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff'
 	mut s2 := Scalar{}
-	mut one_bytes := hex.decode(one) or { panic(err.msg) }
-	s2.set_bytes_with_clamping(one_bytes) or { panic(err.msg) }
+	mut one_bytes := hex.decode(one) or { panic(err) }
+	s2.set_bytes_with_clamping(one_bytes) or { panic(err) }
 	mut p2 := Point{}
 	p2.scalar_base_mult(mut s2)
 
diff --git a/vlib/crypto/ed25519/internal/edwards25519/scalarmult_test.v b/vlib/crypto/ed25519/internal/edwards25519/scalarmult_test.v
index a68bfe58b7..e976141dd7 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/scalarmult_test.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/scalarmult_test.v
@@ -10,7 +10,7 @@ const (
 
 fn dalek_scalar_basepoint() Point {
 	mut p := Point{}
-	p.set_bytes(edwards25519.dsc_basepoint) or { panic(err.msg) }
+	p.set_bytes(edwards25519.dsc_basepoint) or { panic(err) }
 	return p
 }
 
@@ -70,8 +70,8 @@ fn test_vartime_double_basemult_vs_dalek() {
 }
 
 fn test_scalar_mult_distributes_over_add() {
-	mut x := generate_scalar(100) or { panic(err.msg) }
-	mut y := generate_scalar(100) or { panic(err.msg) }
+	mut x := generate_scalar(100) or { panic(err) }
+	mut y := generate_scalar(100) or { panic(err) }
 	mut z := Scalar{}
 
 	z.add(x, y)
@@ -142,7 +142,7 @@ fn test_basepoint_table_generation() {
 }
 
 fn test_scalar_mult_matches_base_mult() {
-	mut x := generate_scalar(100) or { panic(err.msg) }
+	mut x := generate_scalar(100) or { panic(err) }
 	b := new_generator_point()
 	mut p := Point{}
 	mut q := Point{}
@@ -165,8 +165,8 @@ fn test_basepoint_naf_table_generation() {
 }
 
 fn test_vartime_double_scalar_base_mult() {
-	mut x := generate_scalar(100) or { panic(err.msg) }
-	mut y := generate_scalar(100) or { panic(err.msg) }
+	mut x := generate_scalar(100) or { panic(err) }
+	mut y := generate_scalar(100) or { panic(err) }
 	b := new_generator_point()
 
 	mut p := Point{}
diff --git a/vlib/net/common.v b/vlib/net/common.v
index aab8f166b4..99dd1bcff9 100644
--- a/vlib/net/common.v
+++ b/vlib/net/common.v
@@ -74,7 +74,7 @@ fn select_with_retry(handle int, test Select, timeout time.Duration) ?bool {
 	mut retries := 10
 	for retries > 0 {
 		ready := @select(handle, test, timeout) or {
-			if err.code == 4 {
+			if err.code() == 4 {
 				// signal! lets retry max 10 times
 				// suspend thread with sleep to let the gc get
 				// cycles in the case the Bohem gc is interupting
diff --git a/vlib/net/websocket/io.v b/vlib/net/websocket/io.v
index 2ef7870a80..3d0f5fc832 100644
--- a/vlib/net/websocket/io.v
+++ b/vlib/net/websocket/io.v
@@ -15,7 +15,7 @@ fn (mut ws Client) socket_read(mut buffer []byte) ?int {
 		} else {
 			for {
 				r := ws.conn.read(mut buffer) or {
-					if err.code == net.err_timed_out_code {
+					if err.code() == net.err_timed_out_code {
 						continue
 					}
 					return err
@@ -39,7 +39,7 @@ fn (mut ws Client) socket_read_ptr(buf_ptr &byte, len int) ?int {
 		} else {
 			for {
 				r := ws.conn.read_ptr(buf_ptr, len) or {
-					if err.code == net.err_timed_out_code {
+					if err.code() == net.err_timed_out_code {
 						continue
 					}
 					return err
@@ -63,7 +63,7 @@ fn (mut ws Client) socket_write(bytes []byte) ?int {
 		} else {
 			for {
 				n := ws.conn.write(bytes) or {
-					if err.code == net.err_timed_out_code {
+					if err.code() == net.err_timed_out_code {
 						continue
 					}
 					return err
diff --git a/vlib/os/signal_test.v b/vlib/os/signal_test.v
index 87f5b9b2aa..5e6158fc85 100644
--- a/vlib/os/signal_test.v
+++ b/vlib/os/signal_test.v
@@ -21,7 +21,7 @@ fn test_signal_opt_invalid_argument() {
 	}
 	os.signal_opt(.kill, default_handler) or {
 		assert err.msg() == 'Invalid argument'
-		assert err.code == 22
+		assert err.code() == 22
 	}
 }
 
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 75f99cf3cb..44c413cf44 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -82,6 +82,7 @@ pub mut:
 	returns                   bool
 	scope_returns             bool
 	is_builtin_mod            bool // true inside the 'builtin', 'os' or 'strconv' modules; TODO: remove the need for special casing this
+	is_just_builtin_mod       bool // true only inside 'builtin'
 	is_generated              bool // true for `[generated] module xyz` .v files
 	inside_unsafe             bool // true inside `unsafe {}` blocks
 	inside_const              bool // true inside `const ( ... )` blocks
@@ -149,6 +150,7 @@ fn (mut c Checker) reset_checker_state_at_start_of_new_file() {
 	c.scope_returns = false
 	c.mod = ''
 	c.is_builtin_mod = false
+	c.is_just_builtin_mod = false
 	c.inside_unsafe = false
 	c.inside_const = false
 	c.inside_anon_fn = false
@@ -1721,6 +1723,21 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 			}
 		}
 	}
+
+	// >> Hack to allow old style custom error implementations
+	// TODO: remove once deprecation period for `IError` methods has ended
+	if sym.idx == ast.error_type_idx && !c.is_just_builtin_mod
+		&& (field_name == 'msg' || field_name == 'code') {
+		method := c.table.find_method(sym, field_name) or {
+			c.error('invalid `IError` interface implementation: $err', node.pos)
+			return ast.void_type
+		}
+		c.note('the `.$field_name` field on `IError` is deprecated, and will be removed after 2022-06-01, use `.${field_name}()` instead.',
+			node.pos)
+		return method.return_type
+	}
+	// <<<
+
 	if has_field {
 		if sym.mod != c.mod && !field.is_pub && sym.language != .c {
 			unwrapped_sym := c.table.sym(c.unwrap_generic(typ))
@@ -1758,19 +1775,6 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 			c.error(suggestion.say(unknown_field_msg), node.pos)
 			return ast.void_type
 		}
-
-		// >> Hack to allow old style custom error implementations
-		// TODO: remove once deprecation period for `IError` methods has ended
-		if sym.name == 'IError' && (field_name == 'msg' || field_name == 'code') {
-			method := c.table.find_method(sym, field_name) or {
-				c.error('invalid `IError` interface implementation: $err', node.pos)
-				return ast.void_type
-			}
-			c.note('the `.$field_name` field on `IError` is deprecated, use `.${field_name}()` instead.',
-				node.pos)
-			return method.return_type
-		}
-		// <<<
 		if c.smartcast_mut_pos != token.Pos{} {
 			c.note('smartcasting requires either an immutable value, or an explicit mut keyword before the value',
 				c.smartcast_mut_pos)
@@ -2041,7 +2045,8 @@ fn (mut c Checker) stmt(node_ ast.Stmt) {
 		}
 		ast.Module {
 			c.mod = node.name
-			c.is_builtin_mod = node.name in ['builtin', 'os', 'strconv']
+			c.is_just_builtin_mod = node.name == 'builtin'
+			c.is_builtin_mod = c.is_just_builtin_mod || node.name in ['os', 'strconv']
 			c.check_valid_snake_case(node.name, 'module name', node.pos)
 		}
 		ast.Return {
diff --git a/vlib/v/tests/option_print_errors_test.v b/vlib/v/tests/option_print_errors_test.v
index 42fbfec1a1..8e22e97332 100644
--- a/vlib/v/tests/option_print_errors_test.v
+++ b/vlib/v/tests/option_print_errors_test.v
@@ -4,7 +4,7 @@ fn test_error_can_be_converted_to_string() {
 
 fn test_error_can_be_assigned_to_a_variable() {
 	f := error('an error')
-	assert 'an error' == f.msg
+	assert 'an error' == f.msg()
 }
 
 fn test_error_can_be_printed() {
diff --git a/vlib/v/tests/option_test.v b/vlib/v/tests/option_test.v
index 3d3126793d..dfa0c5e26a 100644
--- a/vlib/v/tests/option_test.v
+++ b/vlib/v/tests/option_test.v
@@ -8,11 +8,11 @@ fn test_err_with_code() {
 		_ := w
 	} else {
 		assert err.msg() == 'hi'
-		assert err.code == 137
+		assert err.code() == 137
 	}
 	v := opt_err_with_code() or {
 		assert err.msg() == 'hi'
-		assert err.code == 137
+		assert err.code() == 137
 		return
 	}
 	assert false
diff --git a/vlib/v/tests/or_expr_with_nested_match_expr_test.v b/vlib/v/tests/or_expr_with_nested_match_expr_test.v
index b28f386d65..3dacc7e456 100644
--- a/vlib/v/tests/or_expr_with_nested_match_expr_test.v
+++ b/vlib/v/tests/or_expr_with_nested_match_expr_test.v
@@ -5,7 +5,7 @@ pub mut:
 
 fn delete_secret_v1() API_error {
 	response := req_do() or {
-		match err.msg {
+		match err.msg() {
 			'dial_tcp failed' {
 				return API_error{
 					errors: ['Vault server not started']
@@ -13,7 +13,7 @@ fn delete_secret_v1() API_error {
 			}
 			else {
 				return API_error{
-					errors: [err.msg]
+					errors: [err.msg()]
 				}
 			}
 		}
diff --git a/vlib/vweb/vweb.v b/vlib/vweb/vweb.v
index 69f3b998a6..8c131f6e81 100644
--- a/vlib/vweb/vweb.v
+++ b/vlib/vweb/vweb.v
@@ -393,7 +393,8 @@ pub struct RunParams {
 [manualfree]
 pub fn run_at<T>(global_app &T, params RunParams) ? {
 	mut l := net.listen_tcp(params.family, '$params.host:$params.port') or {
-		return error('failed to listen $err.code $err')
+		ecode := err.code()
+		return error('failed to listen $ecode $err')
 	}
 
 	// Parsing methods attributes