[Strings] Represent string values as WTF-16 internally (#6418)

WTF-16, i.e. arbitrary sequences of 16-bit values, is the encoding of Java and
JavaScript strings, and using the same encoding makes the interpretation of
string operations trivial, even when accounting for non-ascii characters.
Specifically, use little-endian WTF-16.

Re-encode string constants from WTF-8 to WTF-16 in the parsers, then back to
WTF-8 in the writers. Update the constructor for string `Literal`s to interpret
the string as WTF-16 and store a sequence of WTF-16 code units, i.e. 16-bit
integers. Update `Builder::makeConstantExpression` accordingly to convert from
the new `Literal` string representation back to a WTF-16 string.

Update the interpreter to remove the logic for detecting non-ascii characters
and bailing out. The naive implementations of all the string operations are
correct now that our string encoding matches the JS string encoding.

1 files changed, 17 insertions, 2 deletions

diff --git a/src/support/string.h b/src/support/string.h
index 6fb3f693b..be2c3c6a3 100644
--- a/src/support/string.h
+++ b/src/support/string.h
@@ -75,9 +75,24 @@ inline bool isNumber(const std::string& str) {
   return !str.empty() && std::all_of(str.begin(), str.end(), ::isdigit);
 }
 
-std::ostream& printEscaped(std::ostream& os, const std::string_view str);
+std::ostream& printEscaped(std::ostream& os, std::string_view str);
 
-std::ostream& printEscapedJSON(std::ostream& os, const std::string_view str);
+// `str` must be a valid WTF-16 string.
+std::ostream& printEscapedJSON(std::ostream& os, std::string_view str);
+
+std::ostream& writeWTF8CodePoint(std::ostream& os, uint32_t u);
+
+std::ostream& writeWTF16CodePoint(std::ostream& os, uint32_t u);
+
+// Writes the WTF-16LE encoding of the given WTF-8 string to `os`, inserting
+// replacement characters as necessary when encountering invalid WTF-8. Returns
+// `true` iff the input was valid WTF-8.
+bool convertWTF8ToWTF16(std::ostream& os, std::string_view str);
+
+// Writes the WTF-8 encoding of the given WTF-16LE string to `os`, inserting a
+// replacement character at the end if the string is an odd number of bytes.
+// Returns `true` iff the input was valid WTF-16.
+bool convertWTF16ToWTF8(std::ostream& os, std::string_view str);
 
 } // namespace wasm::String