refactor(es/parser): Remove needless unsafe (#7818)

**Related issue:**

 - Closes #7797

Author: kdy1

crates/swc_ecma_parser/src/lexer/mod.rs

@@ -55,6 +55,7 @@ impl From<u32> for Char {
 
 pub(crate) struct CharIter(SmallVec<[char; 7]>);
 
+/// Ported from https://github.com/web-infra-dev/oxc/blob/99a4816ce7b6132b2667257984f9d92ae3768f03/crates/oxc_parser/src/lexer/mod.rs#L1349-L1374
 impl IntoIterator for Char {
     type IntoIter = CharIter;
     type Item = char;
@@ -69,20 +70,28 @@ impl IntoIterator for Char {
         CharIter(match char::from_u32(self.0) {
             Some(c) => smallvec![c],
             None => {
-                let c = unsafe { char::from_u32_unchecked(self.0) };
-                let escaped = c.escape_unicode().to_string();
+                let mut buf = smallvec![];
 
-                debug_assert!(escaped.starts_with('\\'));
+                let high = self.0 & 0xffff0000 >> 16;
 
-                let mut buf = smallvec![];
-                buf.push('\\');
-                buf.push('\0');
-                buf.push('u');
+                let low = self.0 & 0x0000ffff;
 
-                if escaped.len() == 8 {
-                    buf.extend(escaped[3..=6].chars());
+                // The second code unit of a surrogate pair is always in the range from 0xDC00
+                // to 0xDFFF, and is called a low surrogate or a trail surrogate.
+                if !(0xdc00..=0xdfff).contains(&low) {
+                    buf.push('\\');
+                    buf.push('u');
+                    buf.extend(format!("{high:x}").chars());
+                    buf.push('\\');
+                    buf.push('u');
+                    buf.extend(format!("{low:x}").chars());
                 } else {
-                    buf.extend(escaped[2..].chars());
+                    // `https://tc39.es/ecma262/#sec-utf16decodesurrogatepair`
+                    let astral_code_point = (high - 0xd800) * 0x400 + low - 0xdc00 + 0x10000;
+
+                    buf.push('\\');
+                    buf.push('u');
+                    buf.extend(format!("{astral_code_point:x}").chars());
                 }
 
                 buf