[SourceKit] Fix handling of @abi in AnnotatingPrinter

Make sure we avoid adding these to the entity stack entirely, which
avoids hitting the assertion that a top-level entity isn't encountered
with a non-empty stack.

Author: hamishknight

test/SourceKit/DocSupport/doc_abi.swift

@@ -0,0 +1,44 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -emit-module -module-name Mod %t/mod.swift -o %t/Mod.swiftmodule
+// RUN: %sourcekitd-test -req=doc-info -print-raw-response -module Mod -- -I %t -target %target-triple > %t/output.response
+// RUN: %diff -u %t/expected.response %t/output.response
+
+// Make sure we don't crash here, we should exclude the @abi decl from the
+// list of entities.
+
+//--- mod.swift
+@abi(func bar())
+public func foo() {}
+
+//--- expected.response
+{
+  key.annotations: [
+    {
+      key.kind: source.lang.swift.syntaxtype.attribute.builtin,
+      key.offset: 0,
+      key.length: 16
+    },
+    {
+      key.kind: source.lang.swift.syntaxtype.keyword,
+      key.offset: 17,
+      key.length: 4
+    },
+    {
+      key.kind: source.lang.swift.syntaxtype.identifier,
+      key.offset: 22,
+      key.length: 3
+    }
+  ],
+  key.entities: [
+    {
+      key.kind: source.lang.swift.decl.function.free,
+      key.name: "foo()",
+      key.usr: "s:3Mod3baryyF",
+      key.offset: 0,
+      key.length: 27,
+      key.fully_annotated_decl: "<decl.function.free><syntaxtype.attribute.builtin>@abi(<decl.function.free><syntaxtype.keyword>func</syntaxtype.keyword> <decl.name>bar</decl.name>()</decl.function.free>)</syntaxtype.attribute.builtin> <syntaxtype.keyword>func</syntaxtype.keyword> <decl.name>foo</decl.name>()</decl.function.free>"
+    }
+  ],
+  key.sourcetext: "@abi(func bar())\nfunc foo()\n\n"
+}

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -186,10 +186,22 @@ class AnnotatingPrinter : public StreamPrinter {
     TopEntities.push_back(std::move(Entity));
   }
 
+  bool shouldIgnoreDecl(const Decl *D) {
+    // Parameters are handled specially in addParameters().
+    if (isa<ParamDecl>(D))
+      return true;
+
+    // We only care about API for documentation purposes.
+    if (!ABIRoleInfo(D).providesAPI())
+      return true;
+
+    return false;
+  }
+
   void printDeclPre(const Decl *D,
                     std::optional<BracketOptions> Bracket) override {
-    if (isa<ParamDecl>(D))
-      return; // Parameters are handled specially in addParameters().
+    if (shouldIgnoreDecl(D))
+      return;
     if (!shouldContinuePre(D, Bracket))
       return;
     unsigned StartOffset = OS.tell();
@@ -212,18 +224,14 @@ class AnnotatingPrinter : public StreamPrinter {
 
   void printDeclPost(const Decl *D,
                      std::optional<BracketOptions> Bracket) override {
-    if (isa<ParamDecl>(D))
-      return; // Parameters are handled specially in addParameters().
+    if (shouldIgnoreDecl(D))
+      return;
     if (!shouldContinuePost(D, Bracket))
       return;
     assert(!EntitiesStack.empty());
     TextEntity Entity = std::move(EntitiesStack.back());
     EntitiesStack.pop_back();
 
-    // We only care about API for documentation purposes.
-    if (!ABIRoleInfo(D).providesAPI())
-      return;
-
     unsigned EndOffset = OS.tell();
     Entity.Range.Length = EndOffset - Entity.Range.Offset;
     if (EntitiesStack.empty()) {