same

Wow !
What a solution!

It's beyond 8 kyu I believe.
Also, some of the test are wrong as mentioned like:
1,3,1 the result is 5 while 3*(1+1)=6 ?

I Think this is a really good solution

I knew that there is a really easy way with idiomatic rust but I wanted to see if I can solve it without looking anywhere with basic syntax.

Agreed

It's not a best practice though, if there is a unwrap there is no best practice imo

Bad Kata and Bad Description.

I don't know if it's the best practice or not but I meet the strings.Builder concept, thanks

In Golang we don't even have char.
"char" you created in your loop, represents a rune (an int32 value) of the string, not a byte or character. A rune is a code point in the Unicode character set.

When it checks char < '5' it compares the rune values
for example:
0 represents: 48
1 represents: 49
...
It compare with them like that

So your solution is correct but your fundamentals are wrong.