Improve message when -Xmax-inlines limit reached

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -593,8 +593,14 @@ object Erasure {
       */
     private def checkNotErased(tree: Tree)(using Context): tree.type = {
       if (!ctx.mode.is(Mode.Type)) {
-        if (isErased(tree))
-          report.error(em"${tree.symbol} is declared as erased, but is in fact used", tree.srcPos)
+        if isErased(tree) then
+          val msg =
+            if tree.symbol.is(Flags.Inline) then
+              em"""${tree.symbol} is declared as `inline`, but was not inlined
+                  |
+                  |Try increasing `-Xmax-inlines` above ${ctx.settings.XmaxInlines.value}""".stripMargin
+            else em"${tree.symbol} is declared as `erased`, but is in fact used"
+          report.error(msg, tree.srcPos)
         tree.symbol.getAnnotation(defn.CompileTimeOnlyAnnot) match {
           case Some(annot) =>
             def defaultMsg =

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

@@ -63,6 +63,7 @@ class CompilationTests {
       compileFile("tests/pos-special/extend-java-enum.scala", defaultOptions.and("-source", "3.0-migration")),
       compileFile("tests/pos-custom-args/help.scala", defaultOptions.and("-help", "-V", "-W", "-X", "-Y")),
       compileFile("tests/pos-custom-args/i10383.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
+      compileFile("tests/pos-custom-args/i13044.scala", defaultOptions.and("-Xmax-inlines:33")),
     ).checkCompile()
   }
 

tests/neg/i13044.check

@@ -0,0 +1,44 @@
+-- Error: tests/neg/i13044.scala:50:40 ---------------------------------------------------------------------------------
+50 |   implicit def typeSchema: Schema[A] = Schema.gen // error // error
+   |                                        ^^^^^^^^^^
+   |                                        given instance gen is declared as `inline`, but was not inlined
+   |
+   |                                        Try increasing `-Xmax-inlines` above 32
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:18
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+-- Error: tests/neg/i13044.scala:50:40 ---------------------------------------------------------------------------------
+50 |   implicit def typeSchema: Schema[A] = Schema.gen // error // error
+   |                                        ^^^^^^^^^^
+   |                                        method recurse is declared as `inline`, but was not inlined
+   |
+   |                                        Try increasing `-Xmax-inlines` above 32
+   | This location contains code that was inlined from i13044.scala:18
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37
+   | This location contains code that was inlined from i13044.scala:17
+   | This location contains code that was inlined from i13044.scala:18
+   | This location contains code that was inlined from i13044.scala:31
+   | This location contains code that was inlined from i13044.scala:37

tests/neg/i13044.scala

@@ -0,0 +1,51 @@
+import scala.deriving.Mirror
+import scala.compiletime._
+
+trait Schema[T] {
+  def build: T
+}
+
+object Schema extends SchemaDerivation {
+  implicit lazy val int: Schema[Int]   = ???
+  implicit def option[A](implicit ev: Schema[A]): Schema[Option[A]] = ???
+}
+
+trait SchemaDerivation {
+  inline def recurse[A <: Tuple]: List[Schema[Any]] =
+    inline erasedValue[A] match {
+      case _: (t *: ts) =>
+        val builder = summonInline[Schema[t]].asInstanceOf[Schema[Any]]
+        builder :: recurse[ts]
+      case _: EmptyTuple => Nil
+    }
+
+  inline def derived[A]: Schema[A] =
+    inline summonInline[Mirror.Of[A]] match {
+      case m: Mirror.SumOf[A] =>
+        lazy val subTypes = recurse[m.MirroredElemTypes]
+        new Schema[A] {
+          def build: A = ???
+        }
+
+      case m: Mirror.ProductOf[A] =>
+        lazy val fields = recurse[m.MirroredElemTypes]
+        new Schema[A] {
+          def build: A = ???
+        }
+    }
+
+  inline given gen[A]: Schema[A] = derived
+}
+
+case class H(i: Int)
+case class G(h: H)
+case class F(g: G)
+case class E(f: Option[F])
+case class D(e: E)
+case class C(d: D)
+case class B(c: C)
+case class A(a: A, b: B)
+
+object TestApp {
+   implicit def typeSchema: Schema[A] = Schema.gen // error // error
+}

tests/pos-custom-args/i13044.scala

@@ -0,0 +1,51 @@
+import scala.deriving.Mirror
+import scala.compiletime._
+
+trait Schema[T] {
+  def build: T
+}
+
+object Schema extends SchemaDerivation {
+  implicit lazy val int: Schema[Int]   = ???
+  implicit def option[A](implicit ev: Schema[A]): Schema[Option[A]] = ???
+}
+
+trait SchemaDerivation {
+  inline def recurse[A <: Tuple]: List[Schema[Any]] =
+    inline erasedValue[A] match {
+      case _: (t *: ts) =>
+        val builder = summonInline[Schema[t]].asInstanceOf[Schema[Any]]
+        builder :: recurse[ts]
+      case _: EmptyTuple => Nil
+    }
+
+  inline def derived[A]: Schema[A] =
+    inline summonInline[Mirror.Of[A]] match {
+      case m: Mirror.SumOf[A] =>
+        lazy val subTypes = recurse[m.MirroredElemTypes]
+        new Schema[A] {
+          def build: A = ???
+        }
+
+      case m: Mirror.ProductOf[A] =>
+        lazy val fields = recurse[m.MirroredElemTypes]
+        new Schema[A] {
+          def build: A = ???
+        }
+    }
+
+  inline given gen[A]: Schema[A] = derived
+}
+
+case class H(i: Int)
+case class G(h: H)
+case class F(g: G)
+case class E(f: Option[F])
+case class D(e: E)
+case class C(d: D)
+case class B(c: C)
+case class A(a: A, b: B)
+
+object TestApp {
+   implicit def typeSchema: Schema[A] = Schema.gen
+}