[MessageHandling] Decode valid JSON surrogate pairs

[Hokila]

This PR adds support to decode valid JSON surrogate pairs in string escapes. The decoder now checks for a trail surrogate after a lead surrogate. The decoder still rejects unpaired surrogates. I verified this by running the local tests with swift test --filter JSONTests.

Example

The decoder rejected some characters, such as emojis, when JSON used surrogate pairs.

// Rocket emoji (🚀) encoded as a surrogate pair
let jsonString = #""\uD83D\uDE80""#

// Before: The decoder throws an error. It treats \uD83D as an invalid lone surrogate.
// After: The decoder successfully returns the "🚀" character.
let decoded = try jsonString.withUTF8 {
    try JSON.decode(String.self, from: $0)
}

[rintaro]

Thank you!
Although this has been unreachable as long as both the compiler and the plugin are using this JSON encoder/decoder, this change is good for spec compliance!

Just a few nit-picks, and please fix the format as https://github.com/swiftlang/swift-syntax/blob/main/CONTRIBUTING.md#formatting

[rintaro]

I don't think this should be wrapped with do {} catch {}. the test method can be throws

[rintaro]

Since JSON.encode() always emits this as a bare UTF8 sequence, this test would not hit the escaped UTF16 sequence code path. Since equivalent path is covered by testComplexStruct (🛑 emoji), I don't think this test is needed.

[Hokila]

@rintaro

Thank you for review. I am new to SwiftSyntax. I looked for contribution opportunities by checking FIXME comments in the source code and reviewing recent issues. This PR addresses the surrogate-pair decoding gap I found.

Did the following change:

• Ran swift-format on the changed files. I will include formatting in my checks for future PRs.
• Changed testStringSurrogatePairDecoding to throw errors directly and removed the do/catch block.
• Removed the redundant round-trip check for raw UTF-8. testComplexStruct already covers that path. The test still checks the escaped surrogate pair directly.

I amended the commit and rebased the branch onto latest main, also ran the focused test, and it passed.

[Hokila]

@rintaro I noticed that the existing tests use XCTest. Is there a plan to migrate swift-syntax to Swift Testing, or should new tests continue to use XCTest?

[rintaro]

@swift-ci Please test

[rintaro]

I noticed that the existing tests use XCTest. Is there a plan to migrate swift-syntax to Swift Testing, or should new tests continue to use XCTest?

This repository still supports Swift 5.9 toolchain, which I don't think includes swift-testing, not 100% sure.

swift-syntax/Package.swift

Line 1 in 0a52cc4

// swift-tools-version: 5.9

[rintaro]

Thank you Hokila!

[rintaro]

@swift-ci Please test