[dotnet] Report a clear error for PublishReadyToRunComposite=false. Fixes #26456.

[rolfbjarne]

Setting PublishReadyToRunComposite=false made the build fail with one
MSB3030: Could not copy the file ".../R2R/<Assembly>.dylib" because it was not found. error per assembly, which gives no clue about what's actually wrong.

Non-composite ReadyToRun compilation just isn't supported with the Mach-O
container format we use for iOS, tvOS and Mac Catalyst:

• crossgen2 emits one Mach-O object file per assembly, but no component
  assemblies for the runtime to load, and the SDK replaces the IL assemblies in
  the publish output with the (never created) per-assembly dylibs.
• The runtime only knows how to locate platform-native ReadyToRun code by
  looking up the owner composite image, see
  https://github.com/dotnet/runtime/blob/main/docs/design/coreclr/botr/readytorun-platform-native-envelope.md

So report a single, actionable error instead.

Fixes #26456

🤖 Pull request created by Copilot

[Copilot]

Pull request overview

This PR improves the developer experience when PublishReadyToRunComposite=false is set for Apple-platform app bundles using the Mach-O ReadyToRun container format, by failing fast with a single actionable MSBuild error instead of many per-assembly MSB3030 copy failures. It also adds a regression test and documents the property behavior.

Changes:

• Add an MSBuild validation target that emits one clear error when non-composite R2R is requested with PublishReadyToRunContainerFormat=macho for app bundles.
• Add a unit test asserting the build fails with exactly one, actionable error for iOS/tvOS/Mac Catalyst.
• Document PublishReadyToRunComposite behavior and the platform limitation in build properties docs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/dotnet/UnitTests/ProjectTest.cs	Adds a regression test ensuring PublishReadyToRunComposite=false yields a single clear failure.
dotnet/targets/Microsoft.Sdk.R2R.targets	Adds _ValidateReadyToRunComposite to fail fast with an actionable MSBuild error before R2R compilation prep.
docs/building-apps/build-properties.md	Documents PublishReadyToRunComposite and calls out unsupported non-composite R2R for relevant Apple app bundles.

---

💡 Add a code-review agent skill for context-aware, tailored reviews. Learn more in the docs.

The base branch was changed.

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

Pipeline on Agent
Hash: 56d8ad2aa9785bf30a1522852177506bbd03240c [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build #56d8ad2] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 264 tests passed 🎉

Tests counts

✅ assembly-processing: All 1 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 7 tests passed. Html Report (VSDrops) Download
✅ linker (iOS): All 31 tests passed. Html Report (VSDrops) Download
✅ linker (MacCatalyst): All 31 tests passed. Html Report (VSDrops) Download
✅ linker (macOS): All 21 tests passed. Html Report (VSDrops) Download
✅ linker (tvOS): All 31 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 25 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 25 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 20 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 25 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ sharpie: All 1 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

macOS tests

✅ Tests on macOS Sonoma (14): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sequoia (15): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Tahoe (26): All 5 tests passed. Html Report (VSDrops) Download

Linux Build Verification

✅ Linux build succeeded

Pipeline on Agent
Hash: 56d8ad2aa9785bf30a1522852177506bbd03240c [PR build]