diff --git a/vlib/arrays/arrays.v b/vlib/arrays/arrays.v
index 23b90c7ef8..3a9eaeb778 100644
--- a/vlib/arrays/arrays.v
+++ b/vlib/arrays/arrays.v
@@ -544,7 +544,7 @@ fn swap_nonoverlapping<T>(x_ &T, y_ &T, count int) {
 // copy copies the `src` array elements to the `dst` array.
 // The number of the elements copied is the minimum of the length of both arrays.
 // Returns the number of elements copied.
-pub fn copy<T>(dst []T, src []T) int {
+pub fn copy<T>(mut dst []T, src []T) int {
 	min := if dst.len < src.len { dst.len } else { src.len }
 	if min > 0 {
 		blen := min * int(sizeof(T))
diff --git a/vlib/arrays/arrays_test.v b/vlib/arrays/arrays_test.v
index dfb3f364ff..0245e8d4e6 100644
--- a/vlib/arrays/arrays_test.v
+++ b/vlib/arrays/arrays_test.v
@@ -268,16 +268,16 @@ fn test_rotate_left_string() {
 fn test_copy() {
 	mut a := [1, 2, 3]
 	mut b := [4, 5, 6]
-	assert copy(b, a) == 3
+	assert copy(mut b, a) == 3
 	assert b == [1, 2, 3]
 	// check independent copies
 	b[0] = 99
 	assert a[0] == 1
 	// check longer src
 	b << 7
-	assert copy(a, b) == 3
+	assert copy(mut a, b) == 3
 	assert a == [99, 2, 3]
 	// check longer dst
-	assert copy(b, [8, 9]) == 2
+	assert copy(mut b, [8, 9]) == 2
 	assert b == [8, 9, 3, 7]
 }
diff --git a/vlib/builtin/array.v b/vlib/builtin/array.v
index 4c7620251a..3b8bc040ca 100644
--- a/vlib/builtin/array.v
+++ b/vlib/builtin/array.v
@@ -816,7 +816,7 @@ pub fn (b []byte) hex() string {
 // Returns the number of elements copied.
 // NOTE: This is not an `array` method. It is a function that takes two arrays of bytes.
 // See also: `arrays.copy`.
-pub fn copy(dst []byte, src []byte) int {
+pub fn copy(mut dst []byte, src []byte) int {
 	min := if dst.len < src.len { dst.len } else { src.len }
 	if min > 0 {
 		unsafe { vmemmove(&byte(dst.data), src.data, min) }
diff --git a/vlib/crypto/bcrypt/bcrypt.v b/vlib/crypto/bcrypt/bcrypt.v
index 4a8320d89f..8f1b13389f 100644
--- a/vlib/crypto/bcrypt/bcrypt.v
+++ b/vlib/crypto/bcrypt/bcrypt.v
@@ -151,13 +151,13 @@ fn (mut h Hashed) hash_byte() []byte {
 	}
 	arr[n] = `$`
 	n++
-	copy(arr[n..], '${int(h.cost):02}'.bytes())
+	copy(mut arr[n..], '${int(h.cost):02}'.bytes())
 	n += 2
 	arr[n] = `$`
 	n++
-	copy(arr[n..], h.salt)
+	copy(mut arr[n..], h.salt)
 	n += bcrypt.encoded_salt_size
-	copy(arr[n..], h.hash)
+	copy(mut arr[n..], h.hash)
 	n += bcrypt.encoded_hash_size
 	res := arr[..n].clone()
 	return res
diff --git a/vlib/crypto/cipher/aes_cbc_test.v b/vlib/crypto/cipher/aes_cbc_test.v
index 1c5071e7c4..7c37ac5603 100644
--- a/vlib/crypto/cipher/aes_cbc_test.v
+++ b/vlib/crypto/cipher/aes_cbc_test.v
@@ -21,7 +21,7 @@ fn test_aes_cbc() {
 
 fn aes_cbc_en(mut src []byte, key []byte, iv []byte) {
 	block := aes.new_cipher(key)
-	mode := cipher.new_cbc(block, iv)
+	mut mode := cipher.new_cbc(block, iv)
 	mode.encrypt_blocks(mut src, src.clone())
 }
 
diff --git a/vlib/crypto/cipher/aes_cfb_test.v b/vlib/crypto/cipher/aes_cfb_test.v
index db30e27b41..1fa7a437b2 100644
--- a/vlib/crypto/cipher/aes_cfb_test.v
+++ b/vlib/crypto/cipher/aes_cfb_test.v
@@ -18,7 +18,7 @@ fn test_aes_cfb() {
 
 fn aes_cfb_en(mut src []byte, key []byte, iv []byte) {
 	block := aes.new_cipher(key)
-	mode := cipher.new_cfb_encrypter(block, iv)
+	mut mode := cipher.new_cfb_encrypter(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
diff --git a/vlib/crypto/cipher/aes_ofb_test.v b/vlib/crypto/cipher/aes_ofb_test.v
index d5bd2bf380..40e32a51f6 100644
--- a/vlib/crypto/cipher/aes_ofb_test.v
+++ b/vlib/crypto/cipher/aes_ofb_test.v
@@ -20,7 +20,7 @@ fn test_aes_ofb() {
 
 fn aes_ofb_en(mut src []byte, key []byte, iv []byte) {
 	block := aes.new_cipher(key)
-	mode := cipher.new_ofb(block, iv)
+	mut mode := cipher.new_ofb(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
diff --git a/vlib/crypto/cipher/cbc.v b/vlib/crypto/cipher/cbc.v
index 45b2e87f5f..694a69923f 100644
--- a/vlib/crypto/cipher/cbc.v
+++ b/vlib/crypto/cipher/cbc.v
@@ -41,7 +41,7 @@ pub fn new_cbc(b Block, iv []byte) Cbc {
 
 // encrypt_blocks encrypts the blocks in `src_` to `dst_`.
 // Please note: `dst_` is mutable for performance reasons.
-pub fn (x &Cbc) encrypt_blocks(mut dst_ []byte, src_ []byte) {
+pub fn (mut x Cbc) encrypt_blocks(mut dst_ []byte, src_ []byte) {
 	unsafe {
 		mut dst := *dst_
 		mut src := src_
@@ -69,7 +69,7 @@ pub fn (x &Cbc) encrypt_blocks(mut dst_ []byte, src_ []byte) {
 			dst = dst[x.block_size..]
 		}
 		// Save the iv for the next crypt_blocks call.
-		copy(x.iv, iv)
+		copy(mut x.iv, iv)
 	}
 }
 
@@ -94,7 +94,7 @@ pub fn (mut x Cbc) decrypt_blocks(mut dst []byte, src []byte) {
 	mut start := end - x.block_size
 	mut prev := start - x.block_size
 	// Copy the last block of ciphertext in preparation as the new iv.
-	copy(x.tmp, src[start..end])
+	copy(mut x.tmp, src[start..end])
 	// Loop over all but the first block.
 	for start > 0 {
 		src_chunk := src[start..end]
@@ -113,9 +113,9 @@ pub fn (mut x Cbc) decrypt_blocks(mut dst []byte, src []byte) {
 	x.tmp = x.iv
 }
 
-fn (x &Cbc) set_iv(iv []byte) {
+fn (mut x Cbc) set_iv(iv []byte) {
 	if iv.len != x.iv.len {
 		panic('cipher: incorrect length IV')
 	}
-	copy(x.iv, iv)
+	copy(mut x.iv, iv)
 }
diff --git a/vlib/crypto/cipher/cfb.v b/vlib/crypto/cipher/cfb.v
index 6f041c6683..2c4747cd81 100644
--- a/vlib/crypto/cipher/cfb.v
+++ b/vlib/crypto/cipher/cfb.v
@@ -39,20 +39,18 @@ fn new_cfb(b Block, iv []byte, decrypt bool) Cfb {
 	if iv.len != block_size {
 		panic('cipher.new_cfb: IV length must be equal block size')
 	}
-	x := Cfb{
+	mut x := Cfb{
 		b: b
 		out: []byte{len: b.block_size}
 		next: []byte{len: b.block_size}
 		out_used: block_size
 		decrypt: decrypt
 	}
-
-	copy(x.next, iv)
-
+	copy(mut x.next, iv)
 	return x
 }
 
-pub fn (x &Cfb) xor_key_stream(mut dst_ []byte, src_ []byte) {
+pub fn (mut x Cfb) xor_key_stream(mut dst_ []byte, src_ []byte) {
 	unsafe {
 		mut dst := *dst_
 		mut src := src_
@@ -71,12 +69,12 @@ pub fn (x &Cfb) xor_key_stream(mut dst_ []byte, src_ []byte) {
 			}
 
 			if x.decrypt {
-				copy(x.next[x.out_used..], src)
+				copy(mut x.next[x.out_used..], src)
 			}
 
 			n := xor_bytes(mut dst, src, x.out[x.out_used..])
 			if !x.decrypt {
-				copy(x.next[x.out_used..], dst)
+				copy(mut x.next[x.out_used..], dst)
 			}
 			dst = dst[n..]
 			src = src[n..]
diff --git a/vlib/crypto/cipher/cipher.v b/vlib/crypto/cipher/cipher.v
index 81db26ee71..97de4448df 100644
--- a/vlib/crypto/cipher/cipher.v
+++ b/vlib/crypto/cipher/cipher.v
@@ -50,6 +50,6 @@ interface BlockMode {
 
 // fn dup(p []byte) []byte {
 // 	q := make([]byte, p.len)
-// 	copy(q, p)
+// 	copy(mut q, p)
 // 	return q
 // }
diff --git a/vlib/crypto/cipher/des_cbc_test.v b/vlib/crypto/cipher/des_cbc_test.v
index 1640cd985f..bdc6d62852 100644
--- a/vlib/crypto/cipher/des_cbc_test.v
+++ b/vlib/crypto/cipher/des_cbc_test.v
@@ -31,7 +31,7 @@ fn test_des_cbc() {
 
 fn des_cbc_en(mut src []byte, key []byte, iv []byte) {
 	block := des.new_cipher(key)
-	mode := cipher.new_cbc(block, iv)
+	mut mode := cipher.new_cbc(block, iv)
 	mode.encrypt_blocks(mut src, src.clone())
 }
 
@@ -43,7 +43,7 @@ fn des_cbc_de(mut src []byte, key []byte, iv []byte) {
 
 fn triple_des_cbc_en(mut src []byte, key []byte, iv []byte) {
 	block := des.new_triple_des_cipher(key)
-	mode := cipher.new_cbc(block, iv)
+	mut mode := cipher.new_cbc(block, iv)
 	mode.encrypt_blocks(mut src, src.clone())
 }
 
diff --git a/vlib/crypto/cipher/des_cfb_test.v b/vlib/crypto/cipher/des_cfb_test.v
index 0aa172b360..b5143e86e0 100644
--- a/vlib/crypto/cipher/des_cfb_test.v
+++ b/vlib/crypto/cipher/des_cfb_test.v
@@ -31,7 +31,7 @@ fn test_des_cfb() {
 
 fn des_cfb_en(mut src []byte, key []byte, iv []byte) {
 	block := des.new_cipher(key)
-	mode := cipher.new_cfb_encrypter(block, iv)
+	mut mode := cipher.new_cfb_encrypter(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
@@ -43,12 +43,12 @@ fn des_cfb_de(mut src []byte, key []byte, iv []byte) {
 
 fn triple_des_cfb_en(mut src []byte, key []byte, iv []byte) {
 	block := des.new_triple_des_cipher(key)
-	mode := cipher.new_cfb_encrypter(block, iv)
+	mut mode := cipher.new_cfb_encrypter(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
 fn triple_des_cfb_de(mut src []byte, key []byte, iv []byte) {
 	block := des.new_triple_des_cipher(key)
-	mode := cipher.new_cfb_decrypter(block, iv)
+	mut mode := cipher.new_cfb_decrypter(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
diff --git a/vlib/crypto/cipher/des_ofb_test.v b/vlib/crypto/cipher/des_ofb_test.v
index f01e471d85..966cfb4c11 100644
--- a/vlib/crypto/cipher/des_ofb_test.v
+++ b/vlib/crypto/cipher/des_ofb_test.v
@@ -31,7 +31,7 @@ fn test_des_ofb() {
 
 fn des_ofb_en(mut src []byte, key []byte, iv []byte) {
 	block := des.new_cipher(key)
-	mode := cipher.new_ofb(block, iv)
+	mut mode := cipher.new_ofb(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
@@ -43,7 +43,7 @@ fn des_ofb_de(mut src []byte, key []byte, iv []byte) {
 
 fn triple_des_ofb_en(mut src []byte, key []byte, iv []byte) {
 	block := des.new_triple_des_cipher(key)
-	mode := cipher.new_ofb(block, iv)
+	mut mode := cipher.new_ofb(block, iv)
 	mode.xor_key_stream(mut src, src.clone())
 }
 
diff --git a/vlib/crypto/cipher/ofb.v b/vlib/crypto/cipher/ofb.v
index d60d064e72..eaaa1259e5 100644
--- a/vlib/crypto/cipher/ofb.v
+++ b/vlib/crypto/cipher/ofb.v
@@ -25,19 +25,17 @@ pub fn new_ofb(b Block, iv []byte) Ofb {
 	if iv.len != block_size {
 		panic('cipher.new_ofb: IV length must be equal block size')
 	}
-	x := Ofb{
+	mut x := Ofb{
 		b: b
 		out: []byte{len: b.block_size}
 		next: []byte{len: b.block_size}
 		out_used: block_size
 	}
-
-	copy(x.next, iv)
-
+	copy(mut x.next, iv)
 	return x
 }
 
-pub fn (x &Ofb) xor_key_stream(mut dst_ []byte, src_ []byte) {
+pub fn (mut x Ofb) xor_key_stream(mut dst_ []byte, src_ []byte) {
 	unsafe {
 		mut dst := *dst_
 		mut src := src_
@@ -55,7 +53,7 @@ pub fn (x &Ofb) xor_key_stream(mut dst_ []byte, src_ []byte) {
 				x.out_used = 0
 			}
 
-			copy(x.next, x.out)
+			copy(mut x.next, x.out)
 
 			n := xor_bytes(mut dst, src, x.out)
 			dst = dst[n..]
diff --git a/vlib/crypto/ed25519/ed25519.v b/vlib/crypto/ed25519/ed25519.v
index 4827c717a7..9a3c7d415c 100644
--- a/vlib/crypto/ed25519/ed25519.v
+++ b/vlib/crypto/ed25519/ed25519.v
@@ -32,7 +32,7 @@ pub type PrivateKey = []byte
 // RFC 8032's private keys correspond to seeds in this module.
 pub fn (priv PrivateKey) seed() []byte {
 	mut seed := []byte{len: ed25519.seed_size}
-	copy(seed, priv[..32])
+	copy(mut seed, priv[..32])
 	return seed
 }
 
@@ -40,7 +40,7 @@ pub fn (priv PrivateKey) seed() []byte {
 pub fn (priv PrivateKey) public_key() PublicKey {
 	assert priv.len == ed25519.private_key_size
 	mut publickey := []byte{len: ed25519.public_key_size}
-	copy(publickey, priv[32..])
+	copy(mut publickey, priv[32..])
 	return PublicKey(publickey)
 }
 
@@ -62,11 +62,11 @@ pub fn (priv PrivateKey) sign(message []byte) ?[]byte {
 // sign`signs the message with privatekey and returns a signature
 pub fn sign(privatekey PrivateKey, message []byte) ?[]byte {
 	mut signature := []byte{len: ed25519.signature_size}
-	sign_generic(signature, privatekey, message) ?
+	sign_generic(mut signature, privatekey, message) ?
 	return signature
 }
 
-fn sign_generic(signature []byte, privatekey []byte, message []byte) ? {
+fn sign_generic(mut signature []byte, privatekey []byte, message []byte) ? {
 	if privatekey.len != ed25519.private_key_size {
 		panic('ed25519: bad private key length: $privatekey.len')
 	}
@@ -103,8 +103,8 @@ fn sign_generic(signature []byte, privatekey []byte, message []byte) ? {
 	mut ss := edwards25519.new_scalar()
 	ss.multiply_add(k, s, r)
 
-	copy(signature[..32], rr.bytes())
-	copy(signature[32..], ss.bytes())
+	copy(mut signature[..32], rr.bytes())
+	copy(mut signature[32..], ss.bytes())
 }
 
 // verify reports whether sig is a valid signature of message by publickey.
@@ -148,8 +148,8 @@ pub fn generate_key() ?(PublicKey, PrivateKey) {
 	mut seed := rand.bytes(ed25519.seed_size) ?
 
 	privatekey := new_key_from_seed(seed)
-	publickey := []byte{len: ed25519.public_key_size}
-	copy(publickey, privatekey[32..])
+	mut publickey := []byte{len: ed25519.public_key_size}
+	copy(mut publickey, privatekey[32..])
 
 	return publickey, privatekey
 }
@@ -158,12 +158,12 @@ pub fn generate_key() ?(PublicKey, PrivateKey) {
 // correspond to seeds in this module
 pub fn new_key_from_seed(seed []byte) PrivateKey {
 	// Outline the function body so that the returned key can be stack-allocated.
-	privatekey := []byte{len: ed25519.private_key_size}
-	new_key_from_seed_generic(privatekey, seed)
+	mut privatekey := []byte{len: ed25519.private_key_size}
+	new_key_from_seed_generic(mut privatekey, seed)
 	return PrivateKey(privatekey)
 }
 
-fn new_key_from_seed_generic(privatekey []byte, seed []byte) {
+fn new_key_from_seed_generic(mut privatekey []byte, seed []byte) {
 	if seed.len != ed25519.seed_size {
 		panic('ed25519: bad seed length: $seed.len')
 	}
@@ -176,6 +176,6 @@ fn new_key_from_seed_generic(privatekey []byte, seed []byte) {
 
 	mut publickey := aa.bytes()
 
-	copy(privatekey, seed)
-	copy(privatekey[32..], publickey)
+	copy(mut privatekey, seed)
+	copy(mut privatekey[32..], publickey)
 }
diff --git a/vlib/crypto/ed25519/internal/ed25519_test.v b/vlib/crypto/ed25519/internal/ed25519_test.v
index 98e3d38c77..412bb0a776 100644
--- a/vlib/crypto/ed25519/internal/ed25519_test.v
+++ b/vlib/crypto/ed25519/internal/ed25519_test.v
@@ -97,8 +97,8 @@ fn works_check_on_sign_input_string(item string) bool {
 
 	sig = sig[..ed25519.signature_size]
 	mut priv := []byte{len: ed25519.private_key_size}
-	copy(priv[..], privbytes)
-	copy(priv[32..], pubkey)
+	copy(mut priv[..], privbytes)
+	copy(mut priv[32..], pubkey)
 
 	sig2 := ed25519.sign(priv[..], msg) or { panic(err.msg) }
 	if sig != sig2[..] {
@@ -182,8 +182,8 @@ fn test_input_from_djb_ed25519_crypto_sign_input_without_syncpool() ? {
 
 		sig = sig[..signature_size]
 		mut priv := []byte{len: ed25519.private_key_size}
-		copy(priv[..], privbytes)
-		copy(priv[32..], pubkey)
+		copy(mut priv[..], privbytes)
+		copy(mut priv[32..], pubkey)
 
 		sig2 := ed25519.sign(priv[..], msg) ?
 		assert sig == sig2[..]
diff --git a/vlib/crypto/ed25519/internal/edwards25519/element_test.v b/vlib/crypto/ed25519/internal/edwards25519/element_test.v
index 7fae269467..a0dbf43b68 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/element_test.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/element_test.v
@@ -398,7 +398,7 @@ fn test_bytes_big_equivalence() ? {
 	mut buf := []byte{len: 32} // pad with zeroes
 	fedtobig := fe1.to_big_integer()
 	mut fedbig_bytes, _ := fedtobig.bytes()
-	copy(buf, fedbig_bytes) // does not need to do swap_endianness
+	copy(mut buf, fedbig_bytes) // does not need to do swap_endianness
 
 	assert fe.bytes() == buf && is_in_bounds(fe) && is_in_bounds(fe1)
 	// assert big_equivalence(inp, fe, fe1) == true
diff --git a/vlib/crypto/ed25519/internal/edwards25519/point.v b/vlib/crypto/ed25519/internal/edwards25519/point.v
index 3e9222dda7..4bf085d87e 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/point.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/point.v
@@ -229,7 +229,7 @@ fn (mut v Point) bytes_generic(mut buf [32]byte) []byte {
 fn copy_field_element(mut buf [32]byte, mut v Element) []byte {
 	// this fail in test
 	/*
-	copy(buf[..], v.bytes())
+	copy(mut buf[..], v.bytes())
 	return buf[..]
 	*/
 
diff --git a/vlib/crypto/ed25519/internal/edwards25519/scalar.v b/vlib/crypto/ed25519/internal/edwards25519/scalar.v
index aa1f3cc8e6..500d0c2525 100644
--- a/vlib/crypto/ed25519/internal/edwards25519/scalar.v
+++ b/vlib/crypto/ed25519/internal/edwards25519/scalar.v
@@ -91,7 +91,7 @@ pub fn (mut s Scalar) set_uniform_bytes(x []byte) ?Scalar {
 		return error('edwards25519: invalid set_uniform_bytes input length')
 	}
 	mut wide_bytes := []byte{len: 64}
-	copy(wide_bytes, x)
+	copy(mut wide_bytes, x)
 	// for i, item in x {
 	//	wide_bytes[i] = item
 	//}
@@ -112,7 +112,7 @@ pub fn (mut s Scalar) set_canonical_bytes(x []byte) ?Scalar {
 		ss.s[i] = item
 	}
 
-	//_ := copy(ss.s[..], x) //its not working
+	//_ := copy(mut ss.s[..], x) //its not working
 	if !is_reduced(ss) {
 		return error('invalid scalar encoding')
 	}
@@ -162,7 +162,7 @@ pub fn (mut s Scalar) set_bytes_with_clamping(x []byte) ?Scalar {
 	}
 
 	mut wide_bytes := []byte{len: 64, cap: 64}
-	copy(wide_bytes, x)
+	copy(mut wide_bytes, x)
 	// for i, item in x {
 	//	wide_bytes[i] = item
 	//}
@@ -176,7 +176,7 @@ pub fn (mut s Scalar) set_bytes_with_clamping(x []byte) ?Scalar {
 // bytes returns the canonical 32-byte little-endian encoding of s.
 pub fn (mut s Scalar) bytes() []byte {
 	mut buf := []byte{len: 32}
-	copy(buf, s.s[..])
+	copy(mut buf, s.s[..])
 	return buf
 }
 
@@ -1116,7 +1116,7 @@ fn generate_scalar(size int) ?Scalar {
 			// using builtin rand.read([]buf)
 			rand.read(mut s.s[..16])
 			// buf := rand.read(s.s[..16].len) ?
-			// copy(s.s[..16], buf)
+			// copy(mut s.s[..16], buf)
 
 			/*
 			for i, item in buf {
@@ -1133,7 +1133,7 @@ fn generate_scalar(size int) ?Scalar {
 			// rand.Read(s.s[:16])
 			rand.read(mut s.s[..16])
 			// buf := rand.read(s.s[..16].len) ?
-			// copy(s.s[..16], buf)
+			// copy(mut s.s[..16], buf)
 
 			/*
 			for i, item in buf {
@@ -1149,7 +1149,7 @@ fn generate_scalar(size int) ?Scalar {
 			// rand.Read(s.s[:])
 			rand.read(mut s.s[..])
 			// buf := crand.read(s.s.len) ?
-			// copy(s.s[..], buf)
+			// copy(mut s.s[..], buf)
 
 			/*
 			for i, item in buf {
diff --git a/vlib/crypto/md5/md5.v b/vlib/crypto/md5/md5.v
index 17ca5da6f2..496e18053f 100644
--- a/vlib/crypto/md5/md5.v
+++ b/vlib/crypto/md5/md5.v
@@ -58,7 +58,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 		nn := p.len
 		d.len += u64(nn)
 		if d.nx > 0 {
-			n := copy(d.x[d.nx..], p)
+			n := copy(mut d.x[d.nx..], p)
 			d.nx += n
 			if d.nx == md5.block_size {
 				block(mut d, d.x)
@@ -80,7 +80,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 			}
 		}
 		if p.len > 0 {
-			d.nx = copy(d.x, p)
+			d.nx = copy(mut d.x, p)
 		}
 		return nn
 	}
diff --git a/vlib/crypto/sha1/sha1.v b/vlib/crypto/sha1/sha1.v
index ee606df8e5..b7a80f3af0 100644
--- a/vlib/crypto/sha1/sha1.v
+++ b/vlib/crypto/sha1/sha1.v
@@ -62,7 +62,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 		mut p := p_
 		d.len += u64(nn)
 		if d.nx > 0 {
-			n := copy(d.x[d.nx..], p)
+			n := copy(mut d.x[d.nx..], p)
 			d.nx += n
 			if d.nx == sha1.chunk {
 				block(mut d, d.x)
@@ -84,7 +84,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 			}
 		}
 		if p.len > 0 {
-			d.nx = copy(d.x, p)
+			d.nx = copy(mut d.x, p)
 		}
 	}
 	return nn
diff --git a/vlib/crypto/sha256/sha256.v b/vlib/crypto/sha256/sha256.v
index e5d391986f..76be52e314 100644
--- a/vlib/crypto/sha256/sha256.v
+++ b/vlib/crypto/sha256/sha256.v
@@ -96,7 +96,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 		nn := p.len
 		d.len += u64(nn)
 		if d.nx > 0 {
-			n := copy(d.x[d.nx..], p)
+			n := copy(mut d.x[d.nx..], p)
 			d.nx += n
 			if d.nx == sha256.chunk {
 				block(mut d, d.x)
@@ -118,7 +118,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 			}
 		}
 		if p.len > 0 {
-			d.nx = copy(d.x, p)
+			d.nx = copy(mut d.x, p)
 		}
 		return nn
 	}
@@ -191,8 +191,8 @@ pub fn sum224(data []byte) []byte {
 	mut d := new224()
 	d.write(data) or { panic(err) }
 	sum := d.checksum()
-	sum224 := []byte{len: sha256.size224}
-	copy(sum224, sum[..sha256.size224])
+	mut sum224 := []byte{len: sha256.size224}
+	copy(mut sum224, sum[..sha256.size224])
 	return sum224
 }
 
diff --git a/vlib/crypto/sha512/sha512.v b/vlib/crypto/sha512/sha512.v
index a13c7fa65a..6a22ee7d26 100644
--- a/vlib/crypto/sha512/sha512.v
+++ b/vlib/crypto/sha512/sha512.v
@@ -155,7 +155,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 		nn := p.len
 		d.len += u64(nn)
 		if d.nx > 0 {
-			n := copy(d.x[d.nx..], p)
+			n := copy(mut d.x[d.nx..], p)
 			d.nx += n
 			if d.nx == sha512.chunk {
 				block(mut d, d.x)
@@ -177,7 +177,7 @@ pub fn (mut d Digest) write(p_ []byte) ?int {
 			}
 		}
 		if p.len > 0 {
-			d.nx = copy(d.x, p)
+			d.nx = copy(mut d.x, p)
 		}
 		return nn
 	}
@@ -258,8 +258,8 @@ pub fn sum384(data []byte) []byte {
 	mut d := new_digest(.sha384)
 	d.write(data) or { panic(err) }
 	sum := d.checksum()
-	sum384 := []byte{len: sha512.size384}
-	copy(sum384, sum[..sha512.size384])
+	mut sum384 := []byte{len: sha512.size384}
+	copy(mut sum384, sum[..sha512.size384])
 	return sum384
 }
 
@@ -268,8 +268,8 @@ pub fn sum512_224(data []byte) []byte {
 	mut d := new_digest(.sha512_224)
 	d.write(data) or { panic(err) }
 	sum := d.checksum()
-	sum224 := []byte{len: sha512.size224}
-	copy(sum224, sum[..sha512.size224])
+	mut sum224 := []byte{len: sha512.size224}
+	copy(mut sum224, sum[..sha512.size224])
 	return sum224
 }
 
@@ -278,8 +278,8 @@ pub fn sum512_256(data []byte) []byte {
 	mut d := new_digest(.sha512_256)
 	d.write(data) or { panic(err) }
 	sum := d.checksum()
-	sum256 := []byte{len: sha512.size256}
-	copy(sum256, sum[..sha512.size256])
+	mut sum256 := []byte{len: sha512.size256}
+	copy(mut sum256, sum[..sha512.size256])
 	return sum256
 }
 
diff --git a/vlib/encoding/base58/alphabet.v b/vlib/encoding/base58/alphabet.v
index 44d4fc30f9..60e469e083 100644
--- a/vlib/encoding/base58/alphabet.v
+++ b/vlib/encoding/base58/alphabet.v
@@ -47,7 +47,7 @@ pub fn new_alphabet(str string) ?Alphabet {
 	}
 
 	mut ret := Alphabet{}
-	copy(ret.encode, str.bytes())
+	copy(mut ret.encode, str.bytes())
 
 	mut distinct := 0
 	for i, b in ret.encode {
diff --git a/vlib/io/buffered_reader.v b/vlib/io/buffered_reader.v
index f336e2dfed..3df4128464 100644
--- a/vlib/io/buffered_reader.v
+++ b/vlib/io/buffered_reader.v
@@ -47,7 +47,7 @@ pub fn (mut r BufferedReader) read(mut buf []byte) ?int {
 			return none
 		}
 	}
-	read := copy(buf, r.buf[r.offset..r.len])
+	read := copy(mut buf, r.buf[r.offset..r.len])
 	if read == 0 {
 		return none
 	}
diff --git a/vlib/io/custom_string_reading_test.v b/vlib/io/custom_string_reading_test.v
index 76e3307395..432fcf3b30 100644
--- a/vlib/io/custom_string_reading_test.v
+++ b/vlib/io/custom_string_reading_test.v
@@ -19,7 +19,7 @@ fn (mut s StringReader) read(mut buf []byte) ?int {
 	}
 	mut howmany := imin(buf.len, s.text.len - s.place)
 	xxx := s.text[s.place..s.place + howmany].bytes()
-	read := copy(buf, xxx)
+	read := copy(mut buf, xxx)
 	s.place += read
 	return read
 }
diff --git a/vlib/io/io_test.v b/vlib/io/io_test.v
index 20c949eca7..682a47a861 100644
--- a/vlib/io/io_test.v
+++ b/vlib/io/io_test.v
@@ -16,7 +16,7 @@ fn (mut b Buf) read(mut buf []byte) ?int {
 	if !(b.i < b.bytes.len) {
 		return none
 	}
-	n := copy(buf, b.bytes[b.i..])
+	n := copy(mut buf, b.bytes[b.i..])
 	b.i += n
 	return n
 }
diff --git a/vlib/io/reader_test.v b/vlib/io/reader_test.v
index c7a2265af9..7918744228 100644
--- a/vlib/io/reader_test.v
+++ b/vlib/io/reader_test.v
@@ -11,7 +11,7 @@ fn (mut b Buf) read(mut buf []byte) ?int {
 	if !(b.i < b.bytes.len) {
 		return none
 	}
-	n := copy(buf, b.bytes[b.i..])
+	n := copy(mut buf, b.bytes[b.i..])
 	b.i += n
 	return n
 }
@@ -48,7 +48,7 @@ fn (mut s StringReader) read(mut buf []byte) ?int {
 	if s.place >= s.text.len {
 		return none
 	}
-	read := copy(buf, s.text[s.place..].bytes())
+	read := copy(mut buf, s.text[s.place..].bytes())
 	s.place += read
 	return read
 }
diff --git a/vlib/net/http/request_test.v b/vlib/net/http/request_test.v
index 27abb35a23..ce7b92a63e 100644
--- a/vlib/net/http/request_test.v
+++ b/vlib/net/http/request_test.v
@@ -14,7 +14,7 @@ fn (mut s StringReader) read(mut buf []byte) ?int {
 	}
 	max_bytes := 100
 	end := if s.place + max_bytes >= s.text.len { s.text.len } else { s.place + max_bytes }
-	n := copy(buf, s.text[s.place..end].bytes())
+	n := copy(mut buf, s.text[s.place..end].bytes())
 	s.place += n
 	return n
 }
diff --git a/vlib/net/urllib/urllib.v b/vlib/net/urllib/urllib.v
index 5217703720..dc632f254f 100644
--- a/vlib/net/urllib/urllib.v
+++ b/vlib/net/urllib/urllib.v
@@ -274,7 +274,7 @@ fn escape(s string, mode EncodingMode) string {
 	required := s.len + 2 * hex_count
 	mut t := []byte{len: required}
 	if hex_count == 0 {
-		copy(t, s.bytes())
+		copy(mut t, s.bytes())
 		for i in 0 .. s.len {
 			if s[i] == ` ` {
 				t[i] = `+`
diff --git a/vlib/v/gen/js/sourcemap/vlq/vlq_decode_test.v b/vlib/v/gen/js/sourcemap/vlq/vlq_decode_test.v
index fdb4acfc51..6d188e1220 100644
--- a/vlib/v/gen/js/sourcemap/vlq/vlq_decode_test.v
+++ b/vlib/v/gen/js/sourcemap/vlq/vlq_decode_test.v
@@ -39,7 +39,7 @@ fn (mut b TestReader) read(mut buf []byte) ?int {
 	if !(b.i < b.bytes.len) {
 		return none
 	}
-	n := copy(buf, b.bytes[b.i..])
+	n := copy(mut buf, b.bytes[b.i..])
 	b.i += n
 	return n
 }