faster byte_serialized_unchanged

[dev-ardi]

This uses a precomputed table.
This abuses the neat trick that ascii fits inside a u128.
https://godbolt.org/z/sh5MejEo5

To check that this is right you just need to call this in a loop and check the results:

    for i in 0..=255 {
        let res = byte_serialized_unchanged(i);
        println!("{}: {res}", i as char);
    }

: false
: false
: false
: false
: false
: false
: false
: false
: false
        : false

: false

: false

: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
 false
: false
: false
: false
: false
 : false
!: false
": false
#: false
$: false
%: false
&: false
': false
(: false
): false
*: true
+: false
,: false
-: true
.: true
/: false
0: true
1: true
2: true
3: true
4: true
5: true
6: true
7: true
8: true
9: true
:: false
;: false
<: false
=: false
>: false
?: false
@: false
A: true
B: true
C: true
D: true
E: true
F: true
G: true
H: true
I: true
J: true
K: true
L: true
M: true
N: true
O: true
P: true
Q: true
R: true
S: true
T: true
U: true
V: true
W: true
X: true
Y: true
Z: true
[: false
\: false
]: false
^: false
_: true
`: false
a: true
b: true
c: true
d: true
e: true
f: true
g: true
h: true
i: true
j: true
k: true
l: true
m: true
n: true
o: true
p: true
q: true
r: true
s: true
t: true
u: true
v: true
w: true
x: true
y: true
z: true
{: false
|: false
}: false
~: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
: false
 : false
¡: false
¢: false
£: false
¤: false
¥: false
¦: false
§: false
¨: false
©: false
ª: false
«: false
¬: false
­: false
®: false
¯: false
°: false
±: false
²: false
³: false
´: false
µ: false
¶: false
·: false
¸: false
¹: false
º: false
»: false
¼: false
½: false
¾: false
¿: false
À: false
Á: false
Â: false
Ã: false
Ä: false
Å: false
Æ: false
Ç: false
È: false
É: false
Ê: false
Ë: false
Ì: false
Í: false
Î: false
Ï: false
Ð: false
Ñ: false
Ò: false
Ó: false
Ô: false
Õ: false
Ö: false
×: false
Ø: false
Ù: false
Ú: false
Û: false
Ü: false
Ý: false
Þ: false
ß: false
à: false
á: false
â: false
ã: false
ä: false
å: false
æ: false
ç: false
è: false
é: false
ê: false
ë: false
ì: false
í: false
î: false
ï: false
ð: false
ñ: false
ò: false
ó: false
ô: false
õ: false
ö: false
÷: false
ø: false
ù: false
ú: false
û: false
ü: false
ý: false
þ: false
ÿ: false

[dev-ardi]

Oh, this requires bumping the msrv

[hsivonen]

It should be easy to write this as using a const function, which should make this work at least all the way back to Rust 1.57.

See

rust-url/idna/src/uts46.rs

Line 72 in 8a683ff

const fn glyphless_mask() -> u128 {

and

rust-url/idna/src/uts46.rs

Line 85 in 8a683ff

const GLYPHLESS_MASK: u128 = glyphless_mask();