move 'no explicit return allowed from inline' to error class

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -97,6 +97,7 @@ public enum ErrorMessageID {
     DuplicatePrivateProtectedQualifierID,
     ExpectedStartOfTopLevelDefinitionID,
     MissingReturnTypeWithReturnStatementID,
+    NoReturnFromInlineID,
     ;
 
     public int errorNumber() {

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

@@ -1737,4 +1737,14 @@ object messages {
       hl"you have to provide either ${"class"}, ${"trait"}, ${"object"}, or ${"enum"} definitions after qualifiers"
   }
 
+  case class NoReturnFromInline(owner: Symbol)(implicit ctx: Context)
+    extends Message(NoReturnFromInlineID) {
+    val kind = "Syntax"
+    val msg = hl"no explicit ${"return"} allowed from inline $owner"
+    val explanation =
+      hl"""Methods marked with ${"@inline"} may not use ${"return"} statements.
+          |Instead, you should rely on the last expression's value being
+          |returned from a method.
+          |"""
+  }
 }

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

@@ -985,7 +985,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       }
       else if (owner != cx.outer.owner && owner.isRealMethod) {
         if (owner.isInlineMethod)
-          (EmptyTree, errorType(em"no explicit return allowed from inline $owner", tree.pos))
+          (EmptyTree, errorType(NoReturnFromInline(owner), tree.pos))
         else if (!owner.isCompleted)
           (EmptyTree, errorType(MissingReturnTypeWithReturnStatement(owner), tree.pos))
         else {

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -970,4 +970,19 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val MissingReturnTypeWithReturnStatement(method) :: Nil = messages
       assertEquals(method.name.show, "bad")
     }
+
+  @Test def noReturnInInline =
+    checkMessagesAfter("frontend") {
+      """class BadFunction {
+        |  @inline def usesReturn: Int = { return 42 }
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+
+      assertMessageCount(1, messages)
+
+      val NoReturnFromInline(method) :: Nil = messages
+      assertEquals("method usesReturn", method.show)
+    }
 }