SwiftPM `-warnings-as-errors` and `-suppress-warnings` to dependency targets

Invoking a build with -Xswiftc -warnings-as-errors fails under the swiftbuild build system because it passes both -warnings-as-errors and -suppress-warnings to dependency (remote) targets. The Swift driver rejects the pair:

error: conflicting options '-warnings-as-errors' and '-suppress-warnings'

The native build system does not hit this: it strips -warnings-as-errors from dependency targets and compiles them with -suppress-warnings only. Because swiftbuild is the default build system as of Swift 6.4, this is a behavioural regression that breaks any project passing -Xswiftc -warnings-as-errors.

Passing -Xswiftc -warnings-as-errors (e.g. swift test -Xswiftc -warnings-as-errors) in CI to fail the build on warnings in first-party code is a widespread pattern. Dependencies are expected to be built with warnings suppressed, so a warning in a dependency never breaks the build — which is what the native engine does. swift-nio's CI uses this flag and broke when its nightly-next CI moved onto the Swift 6.4 toolchain.

Real failure: https://github.com/apple/swift-nio/actions/runs/27132399521/job/80076358853

Repro

Attached package.zip contains two throwaway packages — upstream and downstream (the root, which consumes upstream as a remote SCM dependency) — plus reproduce.sh, which git-initialises upstream and builds downstream. No network required.

Unzip it, then from inside the package directory run both commands — same toolchain, only the build system differs:

native (works):

% docker run --rm -v "$PWD:/repro" -w /repro swiftlang/swift:nightly-6.4.x-noble bash reproduce.sh native

>>> swift build --build-system native -Xswiftc -warnings-as-errors
Building for debugging...
...
Build complete! (2.30s)

swiftbuild (fails):

% docker run --rm -v "$PWD:/repro" -w /repro swiftlang/swift:nightly-6.4.x-noble bash reproduce.sh swiftbuild

>>> swift build --build-system swiftbuild -Xswiftc -warnings-as-errors
Building for debugging...
...
error: conflicting options '-warnings-as-errors' and '-suppress-warnings'
error: SwiftDriver Upstream normal aarch64 com.apple.xcode.tools.swift.compiler failed with a nonzero exit code
error: Build failed

upstream has no warnings, yet the swiftbuild build still fails — confirming the error is purely the flag combination applied to the dependency target, independent of any actual diagnostic.

Expected behaviour

Dependency (remote) targets must not receive -warnings-as-errors; they should be compiled with warnings suppressed, as the native engine does and as SE-0480 "Remote targets behavior" specifies (warning-control flags are stripped for remote targets and replaced with -suppress-warnings / -w):

https://github.com/swiftlang/swift-evolution/blob/main/proposals/0480-swiftpm-warning-control.md#remote-targets-behavior

Actual behaviour

swiftbuild adds -suppress-warnings to the dependency target but leaves the inherited -Xswiftc -warnings-as-errors in place, so both reach the driver and it errors.

Suggested fix

In the swiftbuild build-graph construction, strip -warnings-as-errors (and the other SE-0480 warning-control flags) from remote/dependency target compilation before substituting -suppress-warnings, matching the native build system.