[2.x] fix: Undefined per-project settings order (#8953)

Closes #7173

Co-authored-by: Daniil Sivak <seroperson@gmail.com>

main/src/main/scala/sbt/internal/Load.scala

@@ -427,7 +427,14 @@ private[sbt] object Load {
       inScope(GlobalScope)(loaded.autos.globalSettings) ++
       loaded.units.toSeq.flatMap { (uri, build) =>
         val pluginBuildSettings = loaded.autos.buildSettings(uri)
-        val projectSettings = build.defined flatMap { (id, project) =>
+        // Collect transformed settings per project and split them into "own-scope" and "cross-project":
+        // "own-scope" - key's project axis is `ref` itself (or global)
+        // "cross-project" - key explicitly targets a different project (`otherProj / key += ...`)
+        //
+        // It's necessary so that global defaults always precedes any other project's append
+        // See https://github.com/sbt/sbt/issues/7173
+        val (ownSettingsSeqs, crossSettingsSeqs) =
+          build.defined.toSeq.map { (id, project) =>
             val ref = ProjectRef(uri, id)
             val defineConfig: Seq[Setting[?]] =
               for (c <- project.configurations)
@@ -436,8 +443,15 @@ private[sbt] object Load {
               (thisProject :== project) +: (thisProjectRef :== ref) +: defineConfig
             val settings = builtin ++ injectSettings.project ++ project.settings
             // map This to thisScope, Select(p) to mapRef(uri, rootProject, p)
-          transformSettings(projectScope(ref), uri, rootProject, settings)
+            val transformed = transformSettings(projectScope(ref), uri, rootProject, settings)
+            transformed.partition { s =>
+              s.key.scope.project match {
+                case Select(r) => r == ref
+                case _         => true // global scope
               }
+            }
+          }.unzip
+        val projectSettings = (ownSettingsSeqs ++ crossSettingsSeqs).flatten
         val buildScope = Scope(Select(BuildRef(uri)), Zero, Zero, Zero)
         val buildBase = baseDirectory :== build.localBase
         val settings3 = pluginBuildSettings ++ (buildBase +: build.buildSettings)

sbt-app/src/sbt-test/project/i7173-cross-project-setting/build.sbt

@@ -0,0 +1,27 @@
+val check = taskKey[Unit]("Verify cross-project source generator is applied")
+
+lazy val foo = project
+  .in(file("foo"))
+  .settings(
+    check := Def.uncached {
+      val gens = (Compile / sourceGenerators).value
+      assert(
+        gens.nonEmpty,
+        s"#7173: `foo / Compile / sourceGenerators` should contain the generator registered by fooAux"
+      )
+    }
+  )
+
+lazy val fooAux = project
+  .in(file("foo-aux"))
+  .settings(
+    foo / Compile / sourceGenerators += Def.task(Seq.empty[File])
+  )
+
+lazy val bar = project
+
+// Fails:
+lazy val baz = project
+
+// Succeeds without i7173 edits:
+// lazy val ewikqelkqweqopweo = project

sbt-app/src/sbt-test/project/i7173-cross-project-setting/test

@@ -0,0 +1,3 @@
+# Adding an unrelated project (baz) should not prevent foo / Compile / sourceGenerators
+# from including the generator registered by fooAux.
+> foo/check