swift-syntax spans the floor to the newest release

Status: accepted, 2026-10-07. Answers #39. Serves Floors at the 26 releases. Reopen when a swift-syntax release breaks the macros in a way one source cannot serve on both ends of the range, or when the floor moves.

Context#

The macro target, BatonMacros, depends on swift-syntax, whose major version follows the Swift release: 602 for Swift 6.2, 603 for 6.3, 604 for 6.4. Package.swift asked for from: "602.0.0", which SwiftPM reads as 602 alone. An app that pins swift-syntax to its compiler's release, as swift-syntax advises a macro package's consumers to, and as an app with a second swift-syntax consumer must, could not resolve Baton at all on Swift 6.3: SwiftPM refused the graph before any of Baton was built, and its message did not say which side to change.

The issue asked for "602.0.0"..<"604.0.0". swift-syntax 604.0.0 had been released three weeks before it was filed.

Decision#

• The macros accept every swift-syntax release from the one matching the floor's toolchain (602, for Swift 6.2 tools) to the newest one released, as a closed range: "602.0.0"..<"605.0.0" today.
• The upper bound moves when a swift-syntax major ships: the range is raised to admit it once the macros build and their tests pass on it. The lower bound moves with the floor.
• CI builds and tests the package at both ends: the swift and floor jobs on the committed Package.resolved, which stays at the floor's release, and the newest-syntax job on the newest release the range allows, resolved by swift package update swift-syntax, so the day a major ships is a red job here rather than an adopter's failed resolve.

Evidence#

• The macros build with warnings as errors against 602.0.0, 603.0.2 and 604.0.0 on Swift 6.3.3 (Xcode 26.6), and the whole test suite passes against 604.0.0 (2026-10-07). The target uses SwiftSyntax, SwiftSyntaxBuilder, SwiftSyntaxMacros and SwiftCompilerPlugin in ways unchanged across those releases.
• swift-syntax's releases: 602.0.0 on 2025-09-15, 603.0.0 on 2026-03-24, 604.0.0 on 2026-09-15, none of them a prerelease.
• Resolved at the toolchain's own release (603.0.2 on Swift 6.3.3), SwiftPM downloads swift-syntax's prebuilt macro support and the macro target builds in seconds; resolved at 602 on the same toolchain, it builds swift-syntax from source, about a minute. A range lets an app have the prebuilt.

Not chosen#

• "602.0.0"..<"604.0.0", as asked: 604 was out, and an app on its toolchain would fail as the issue's did on 603.
• An open upper bound: SwiftPM picks the newest release the range allows, so an app that does not pin swift-syntax would take an untested major the day it ships, and a breaking one would fail its build with nothing Baton could release to help.
• Moving the lower bound to 603: the floor is Swift 6.2 tools, and Xcode 26.0 builds against 602.