dev: Add better compile-time diagnostics

main/src/io/github/iltotore/iron/internal/DecodingFailure.scala

@@ -0,0 +1,9 @@
+package io.github.iltotore.iron.internal
+
+import scala.quoted.*
+
+enum DecodingFailure:
+  case Unknown
+  case NotInlined(term: Term)
+  case HasRuntimeBindings(bindings: List[Quotes.reflectModule.Definition])(using Quotes)
+  case HasStatements(statements: List[quotes.reflect.Statement])(using Quotes)

main/src/io/github/iltotore/iron/internal/ExprDecoder.scala

@@ -0,0 +1,32 @@
+package io.github.iltotore.iron.internal
+
+import io.github.iltotore.iron.compileTime.NumConstant
+import scala.quoted.*
+
+trait ExprDecoder[T]:
+
+  def decode(expr: Expr[T])(using Quotes): Either[DecodingFailure, T]
+
+object ExprDecoder:
+
+  given [T](using fromExpr: FromExpr[T]): ExprDecoder[T] with
+
+    override def decode(expr: Expr[T])(using Quotes): Either[DecodingFailure, T] =
+      fromExpr.unapply(expr).toRight(DecodingFailure.Unknown)
+
+  private class PrimitiveExprDecoder[T <: NumConstant | Byte | Short | Boolean | String] extends ExprDecoder[T]:
+
+    override def decode(expr: Expr[T])(using Quotes): Either[DecodingFailure, T] =
+      import quotes.reflect.*
+
+      def rec(tree: Term): Either[DecodingFailure, T] = tree match
+        case Block(stats, e)         => if stats.isEmpty then rec(e) else Left(DecodingFailure.HasStatements(stats))
+        case Inlined(_, bindings, e) => if bindings.isEmpty then rec(e) else Left(DecodingFailure.HasRuntimeBindings(bindings))
+        case Typed(e, _)             => rec(e)
+        case Ident(name)             => Left(DecodingFailure.NotInlined(Symbol()))
+        case _ =>
+          tree.tpe.widenTermRefByName match
+            case ConstantType(c) => Right(c.value.asInstanceOf[T])
+            case _               => Left(DecodingFailure.Unknown)
+
+      rec(expr.asTerm)