How swc handle UTF-8 string

[Solo-steven]

Hi, I am new to Rust and I am reading SWC source code to learn how to build a Rust parser. I encounter some issues with UTF-8 string. It is ok to iterate UTF-8 string with chars iterator, but how can I get a slice of UTF-8 string with some function like slice(start_index, end_index) I take look into swc_common and using StringInput, I found that it still can not index UTF-8 string correctly, how should i use it for slice a UTF-8 string ?

use swc_common::input::{StringInput, Input};
use swc_common::BytePos;
fn main() {
    let utf8_str = "中文字串";
    let mut input =  StringInput::new("中文字串", BytePos(0), BytePos(utf8_str.len().try_into().unwrap()));
    println!("{:?}", input.slice(BytePos(0), BytePos(3))); // want to slice string like input.slice(BytePos(0), BytePos(1)) to get frist utf-8 char not bytes
    println!("{:?}", &utf8_str[0..3]);
}

[kdy1]

swc/crates/testing/src/lib.rs

Lines 110 to 112 in 11efd4e

	let cm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
	let (handler, errors) = self::string_errors::new_handler(cm.clone(), treat_err_as_bug);
	let result = swc_common::GLOBALS.set(&swc_common::Globals::new(), || op(cm, handler));

swc/crates/swc_ecma_parser/examples/parse-all.rs

Line 25 in 11efd4e

let fm = cm.load_file(&path).expect("failed to load file");

[Solo-steven]

Hi @kdy1 thanks for your reply, so SWC already handle UTF8 bytes when loading file ? Because slice method of StringInput seems to slice a &str by BytePos how StringInput should use to handle UTF8 index ?

[kdy1]

You should use chars. You need to learn more about string representation of rust

[Solo-steven]

@kdy1 Ya I know that I need to use chars iter for get UTF8 char from string , I just want to ask how SWC get correct UTF-8 string by index ~ thanks