Fix #10198: Be robust against missing annotations

In 6f32b62, I made sure the compiler could survive parsing a Java
classfile referring to annotations missing from the classpath, but we
could still run into trouble when a Scala unpickled class does the same.

Tweaking the Tasty unpickler to drop missing annotations would require
always forcing the symbols of all annotations which I would rather avoid
doing. Instead, we adopt the same solution as Scala 2 did in
sbt/zinc#701 and special-case the single place
where we force annotations of inherited members to ignore missing
annotations.

Author: smarter

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -604,10 +604,12 @@ private class ExtractAPICollector(using Context) extends ThunkHolder {
 
     // In the Scala2 ExtractAPI phase we only extract annotations that extend
     // StaticAnnotation, but in Dotty we currently pickle all annotations so we
-    // extract everything (except inline body annotations which are handled
-    // above).
-    s.annotations.filter(_.symbol != defn.BodyAnnot) foreach { annot =>
-      annots += apiAnnotation(annot)
+    // extract everything (except annotations missing from the classpath which
+    // we simply skip over, and inline body annotations which are handled above).
+    s.annotations.foreach { annot =>
+      val sym = annot.symbol
+      if sym.exists && sym != defn.BodyAnnot then
+        annots += apiAnnotation(annot)
     }
 
     annots.toList

sbt-dotty/sbt-test/source-dependencies/missing-annot/a/A.scala

@@ -0,0 +1,4 @@
+class A {
+  @JavaAnnot def foo: Int = 1
+  @ScalaAnnot def bar: Int = 1
+}

sbt-dotty/sbt-test/source-dependencies/missing-annot/a/JavaAnnot.java

@@ -0,0 +1,3 @@
+import java.lang.annotation.*;
+
+public @interface JavaAnnot {}

sbt-dotty/sbt-test/source-dependencies/missing-annot/a/ScalaAnnot.scala

@@ -0,0 +1 @@
+class ScalaAnnot extends scala.annotation.StaticAnnotation

sbt-dotty/sbt-test/source-dependencies/missing-annot/b/B.scala

@@ -0,0 +1 @@
+class B extends A

sbt-dotty/sbt-test/source-dependencies/missing-annot/build.sbt

@@ -0,0 +1,9 @@
+lazy val a = project.in(file("a"))
+  .settings(
+    Compile / classDirectory := (ThisBuild / baseDirectory).value / "a-output"
+  )
+
+lazy val b = project.in(file("b"))
+  .settings(
+    Compile / unmanagedClasspath += (ThisBuild / baseDirectory).value / "b-input"
+  )

sbt-dotty/sbt-test/source-dependencies/missing-annot/project/DottyInjectedPlugin.scala

@@ -0,0 +1,11 @@
+import sbt._
+import Keys._
+
+object DottyInjectedPlugin extends AutoPlugin {
+  override def requires = plugins.JvmPlugin
+  override def trigger = allRequirements
+
+  override val projectSettings = Seq(
+    scalaVersion := sys.props("plugin.scalaVersion")
+  )
+}

sbt-dotty/sbt-test/source-dependencies/missing-annot/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/source-dependencies/missing-annot/test

@@ -0,0 +1,8 @@
+> a/compile
+$ mkdir b-input
+# Add A to the classpath of b but not the annotations referenced by the members of A
+$ copy-file a-output/A.class b-input/A.class
+$ copy-file a-output/A.tasty b-input/A.tasty
+# B should still be able to compile even though ExtractAPI forces the
+# annotations of all inherited members.
+> b/compile