improve code and doc

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "utf8-width"
-version = "0.1.6"
+version = "0.1.7"
 authors = ["Magic Len <len@magiclen.org>"]
 edition = "2021"
 rust-version = "1.56"

src/lib.rs

@@ -17,7 +17,6 @@ assert_eq!(3, utf8_width::get_width("中".as_bytes()[0]));
 ```bash
 cargo bench
 ```
-
 */
 
 #![no_std]
@@ -36,33 +35,33 @@ pub const MIN_4: u8 = 0xF0;
 pub const MAX_4: u8 = 0xF4;
 
 #[inline]
-pub fn is_width_1(byte: u8) -> bool {
+pub const fn is_width_1(byte: u8) -> bool {
     byte <= MAX_1 // no need to check `MIN_1 <= byte`
 }
 
 #[inline]
-pub fn is_width_2(byte: u8) -> bool {
-    (MIN_2..=MAX_2).contains(&byte)
+pub const fn is_width_2(byte: u8) -> bool {
+    byte >= MIN_2 && byte <= MAX_2
 }
 
 #[inline]
-pub fn is_width_3(byte: u8) -> bool {
-    (MIN_3..=MAX_3).contains(&byte)
+pub const fn is_width_3(byte: u8) -> bool {
+    byte >= MIN_3 && byte <= MAX_3
 }
 
 #[inline]
-pub fn is_width_4(byte: u8) -> bool {
-    (MIN_4..=MAX_4).contains(&byte)
+pub const fn is_width_4(byte: u8) -> bool {
+    byte >= MIN_4 && byte <= MAX_4
 }
 
 #[inline]
-pub fn is_width_0(byte: u8) -> bool {
-    (MIN_0_1..=MAX_0_1).contains(&byte) || MIN_0_2 <= byte // no need to check `byte <= MAX_0_2`
+pub const fn is_width_0(byte: u8) -> bool {
+    byte >= MIN_0_1 && byte <= MAX_0_1 || MIN_0_2 <= byte // no need to check `byte <= MAX_0_2`
 }
 
-/// Given a first byte, determines how many bytes are in this UTF-8 character. If the UTF-8 character is invalid, returns `0`, otherwise returns `1` ~ `4`,
+/// Given a first byte, determine how many bytes are in this UTF-8 character. If the UTF-8 character is invalid, return `0`; otherwise, return `1` to `4`.
 #[inline]
-pub fn get_width(byte: u8) -> usize {
+pub const fn get_width(byte: u8) -> usize {
     if is_width_1(byte) {
         1
     } else if is_width_2(byte) {
@@ -78,10 +77,13 @@ pub fn get_width(byte: u8) -> usize {
     }
 }
 
-#[allow(clippy::missing_safety_doc)]
-/// *Assume the input first byte is from a valid UTF-8 character.* Given a first byte, determines how many bytes are in this UTF-8 character. It returns `1` ~ `4`,
+/// *Assuming the input first byte is from a valid UTF-8 character*, determine how many bytes are in this UTF-8 character. It returns `1` to `4`.
+///
+/// # Safety
+///
+/// You must ensure that the input byte is a valid UTF-8 first byte on your own.
 #[inline]
-pub unsafe fn get_width_assume_valid(byte: u8) -> usize {
+pub const unsafe fn get_width_assume_valid(byte: u8) -> usize {
     if byte <= MAX_1 {
         1
     } else if byte <= MAX_2 {