Merge pull request #15068 from griggt/disallow-targetname-on-toplevel-classes

Disallow `@targetName` on top-level `class`, `trait`, and `object`

Author: griggt

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -176,7 +176,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID]:
     MatchableWarningID,
     CannotExtendFunctionID,
     LossyWideningConstantConversionID,
-    ImplicitSearchTooLargeID
+    ImplicitSearchTooLargeID,
+    TargetNameOnTopLevelClassID
 
   def errorNumber = ordinal - 2
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2540,3 +2540,24 @@ import transform.SymUtils._
           |
           |${openSearchPairs.reverse.map(showQuery)}%\n%
         """
+
+  class TargetNameOnTopLevelClass(symbol: Symbol)(using Context)
+  extends SyntaxMsg(TargetNameOnTopLevelClassID):
+    def msg = em"${hl("@targetName")} annotation not allowed on top-level $symbol"
+    def explain =
+      val annot = symbol.getAnnotation(defn.TargetNameAnnot).get
+      em"""The @targetName annotation may be applied to a top-level ${hl("val")} or ${hl("def")}, but not
+          |a top-level ${hl("class")}, ${hl("trait")}, or ${hl("object")}.
+          |
+          |This restriction is due to the naming convention of Java classfiles, whose filenames
+          |are based on the name of the class defined within. If @targetName were permitted
+          |here, the name of the classfile would be based on the target name, and the compiler
+          |could not associate that classfile with the Scala-visible defined name of the class.
+          |
+          |If your use case requires @targetName, consider wrapping $symbol in an ${hl("object")}
+          |(and possibly exporting it), as in the following example:
+          |
+          |${hl("object Wrapper:")}
+          |  $annot $symbol { ... }
+          |
+          |${hl("export")} Wrapper.${symbol.name}  ${hl("// optional")}"""

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -505,6 +505,8 @@ object Checking {
         fail(TailrecNotApplicable(sym))
       else if sym.is(Inline) then
         fail("Inline methods cannot be @tailrec")
+    if sym.hasAnnotation(defn.TargetNameAnnot) && sym.isClass && sym.isTopLevelClass then
+      fail(TargetNameOnTopLevelClass(sym))
     if (sym.hasAnnotation(defn.NativeAnnot)) {
       if (!sym.is(Deferred))
         fail(NativeMembersMayNotHaveImplementation(sym))

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -171,7 +171,7 @@ class CompilationTests {
         defaultOptions),
       compileFile("tests/neg-custom-args/i6300.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/infix.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
-      compileFile("tests/neg-custom-args/missing-alpha.scala", defaultOptions.and("-Yrequire-targetName", "-Xfatal-warnings")),
+      compileFile("tests/neg-custom-args/missing-targetName.scala", defaultOptions.and("-Yrequire-targetName", "-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/wildcards.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/indentRight.scala", defaultOptions.and("-no-indent", "-Xfatal-warnings")),
       compileDir("tests/neg-custom-args/adhoc-extension", defaultOptions.and("-source", "future", "-feature", "-Xfatal-warnings")),

docs/_docs/reference/other-new-features/targetName.md

@@ -29,7 +29,7 @@ The [`@targetName`](https://scala-lang.org/api/3.x/scala/annotation/targetName.h
     of type `String`. That string is called the _external name_ of the definition
     that's annotated.
 
- 2. A `@targetName` annotation can be given for all kinds of definitions.
+ 2. A `@targetName` annotation can be given for all kinds of definitions except a top-level `class`, `trait`, or `object`.
 
  3. The name given in a [`@targetName`](https://scala-lang.org/api/3.x/scala/annotation/targetName.html) annotation must be a legal name
     for the defined entities on the host platform.