Safer exceptions

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1261,6 +1261,16 @@ object desugar {
       makeOp(left, right, Span(left.span.start, op.span.end, op.span.start))
   }
 
+  /** Translate throws type `A throws E1 | ... | En` to
+   *  $throws[... $throws[A, E1] ... , En].
+   */
+  def throws(tpt: Tree, op: Ident, excepts: Tree)(using Context): AppliedTypeTree = excepts match
+    case InfixOp(l, bar @ Ident(tpnme.raw.BAR), r) =>
+      throws(throws(tpt, op, l), bar, r)
+    case e =>
+      AppliedTypeTree(
+        TypeTree(defn.throwsAlias.typeRef).withSpan(op.span), tpt :: excepts :: Nil)
+
   /** Translate tuple expressions of arity <= 22
    *
    *     ()             ==>   ()

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -27,6 +27,7 @@ object Feature:
   val erasedDefinitions = experimental("erasedDefinitions")
   val symbolLiterals = deprecated("symbolLiterals")
   val fewerBraces = experimental("fewerBraces")
+  val saferExceptions = experimental("saferExceptions")
 
   /** Is `feature` enabled by by a command-line setting? The enabling setting is
    *

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -657,8 +657,11 @@ class Definitions {
 
   // in scalac modified to have Any as parent
 
-  @tu lazy val ThrowableType: TypeRef          = requiredClassRef("java.lang.Throwable")
-  def ThrowableClass(using Context): ClassSymbol = ThrowableType.symbol.asClass
+  @tu lazy val ThrowableType: TypeRef             = requiredClassRef("java.lang.Throwable")
+  def ThrowableClass(using Context): ClassSymbol  = ThrowableType.symbol.asClass
+  @tu lazy val ExceptionClass: ClassSymbol        = requiredClass("java.lang.Exception")
+  @tu lazy val RuntimeExceptionClass: ClassSymbol = requiredClass("java.lang.RuntimeException")
+
   @tu lazy val SerializableType: TypeRef       = JavaSerializableClass.typeRef
   def SerializableClass(using Context): ClassSymbol = SerializableType.symbol.asClass
 
@@ -830,6 +833,9 @@ class Definitions {
       val methodName = if CanEqualClass.name == tpnme.Eql then nme.eqlAny else nme.canEqualAny
       CanEqualClass.companionModule.requiredMethod(methodName)
 
+  @tu lazy val CanThrowClass: ClassSymbol = requiredClass("scala.CanThrow")
+  @tu lazy val throwsAlias: Symbol = ScalaRuntimePackageVal.requiredType(tpnme.THROWS)
+
   @tu lazy val TypeBoxClass: ClassSymbol = requiredClass("scala.runtime.TypeBox")
     @tu lazy val TypeBox_CAP: TypeSymbol = TypeBoxClass.requiredType(tpnme.CAP)
 

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -305,6 +305,7 @@ object StdNames {
     val SPECIALIZED_INSTANCE: N     = "specInstance$"
     val THIS: N                     = "_$this"
     val TRAIT_CONSTRUCTOR: N        = "$init$"
+    val THROWS: N                   = "$throws"
     val U2EVT: N                    = "u2evt$"
     val ALLARGS: N                  = "$allArgs"
 
@@ -602,6 +603,7 @@ object StdNames {
     val this_ : N               = "this"
     val thisPrefix : N          = "thisPrefix"
     val throw_ : N              = "throw"
+    val throws: N               = "throws"
     val toArray: N              = "toArray"
     val toList: N               = "toList"
     val toObjectArray : N       = "toObjectArray"

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

@@ -24,6 +24,16 @@ object TypeUtils {
     def isErasedClass(using Context): Boolean =
       self.underlyingClassRef(refinementOK = true).typeSymbol.is(Flags.Erased)
 
+    /** Is this type a checked exception? This is the case if the type
+     *  derives from Exception but not from RuntimeException. According to
+     *  that definition Throwable is unchecked. That makes sense since you should
+     *  neither throw nor catch `Throwable` anyway, so we should not define
+     *  a capability to do so.
+     */
+    def isCheckedException(using Context): Boolean =
+      self.derivesFrom(defn.ExceptionClass)
+      && !self.derivesFrom(defn.RuntimeExceptionClass)
+
     def isByName: Boolean =
       self.isInstanceOf[ExprType]
 

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

@@ -34,8 +34,10 @@ import NameOps._
 import SymDenotations.{NoCompleter, NoDenotation}
 import Applications.unapplyArgs
 import transform.patmat.SpaceEngine.isIrrefutable
-import config.Feature._
+import config.Feature
+import config.Feature.sourceVersion
 import config.SourceVersion._
+import transform.TypeUtils.*
 
 import collection.mutable
 import reporting._
@@ -930,7 +932,7 @@ trait Checking {
                    description: => String,
                    featureUseSite: Symbol,
                    pos: SrcPos)(using Context): Unit =
-    if !enabled(name) then
+    if !Feature.enabled(name) then
       report.featureWarning(name.toString, description, featureUseSite, required = false, pos)
 
   /** Check that `tp` is a class type and that any top-level type arguments in this type
@@ -1312,6 +1314,10 @@ trait Checking {
     if !tp.derivesFrom(defn.MatchableClass) && sourceVersion.isAtLeast(`future-migration`) then
       val kind = if pattern then "pattern selector" else "value"
       report.warning(MatchableWarning(tp, pattern), pos)
+
+  def checkCanThrow(tp: Type, span: Span)(using Context): Unit =
+    if Feature.enabled(Feature.saferExceptions) && tp.isCheckedException then
+      ctx.typer.implicitArgTree(defn.CanThrowClass.typeRef.appliedTo(tp), span)
 }
 
 trait ReChecking extends Checking {
@@ -1324,6 +1330,7 @@ trait ReChecking extends Checking {
   override def checkAnnotApplicable(annot: Tree, sym: Symbol)(using Context): Boolean = true
   override def checkMatchable(tp: Type, pos: SrcPos, pattern: Boolean)(using Context): Unit = ()
   override def checkNoModuleClash(sym: Symbol)(using Context) = ()
+  override def checkCanThrow(tp: Type, span: Span)(using Context): Unit = ()
 }
 
 trait NoChecking extends ReChecking {

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

@@ -114,6 +114,9 @@ class ReTyper extends Typer with ReChecking {
       super.handleUnexpectedFunType(tree, fun)
   }
 
+  override def addCanThrowCapabilities(expr: untpd.Tree, cases: List[CaseDef])(using Context): untpd.Tree =
+    expr
+
   override def typedUnadapted(tree: untpd.Tree, pt: Type, locked: TypeVars)(using Context): Tree =
     try super.typedUnadapted(tree, pt, locked)
     catch {

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

@@ -39,7 +39,8 @@ import annotation.tailrec
 import Implicits._
 import util.Stats.record
 import config.Printers.{gadts, typr, debug}
-import config.Feature._
+import config.Feature
+import config.Feature.{sourceVersion, migrateTo3}
 import config.SourceVersion._
 import rewrites.Rewrites.patch
 import NavigateAST._
@@ -712,7 +713,7 @@ class Typer extends Namer
           case Whole(16) => // cant parse hex literal as double
           case _         => return lit(doubleFromDigits(digits))
         }
-      else if genericNumberLiteralsEnabled
+      else if Feature.genericNumberLiteralsEnabled
           && target.isValueType && isFullyDefined(target, ForceDegree.none)
       then
         // If expected type is defined with a FromDigits instance, use that one
@@ -1712,10 +1713,30 @@ class Typer extends Namer
         .withNotNullInfo(body1.notNullInfo.retractedInfo.seq(cond1.notNullInfoIf(false)))
     }
 
+  /** Add givens reflecting `CanThrow` capabilities for all checked exceptions matched
+   *  by `cases`. The givens appear in nested blocks with earlier cases leading to
+   *  more deeply nested givens. This way, given priority will be the same as pattern priority.
+   *  The functionality is enabled if the experimental.saferExceptions language feature is enabled.
+   */
+  def addCanThrowCapabilities(expr: untpd.Tree, cases: List[CaseDef])(using Context): untpd.Tree =
+    def makeCanThrow(tp: Type): untpd.Tree =
+      untpd.ValDef(
+          EvidenceParamName.fresh(),
+          untpd.TypeTree(defn.CanThrowClass.typeRef.appliedTo(tp)),
+          untpd.ref(defn.Predef_undefined))
+        .withFlags(Given | Final | Lazy | Erased)
+        .withSpan(expr.span)
+    val caps =
+      for
+        CaseDef(pat, _, _) <- cases
+        if Feature.enabled(Feature.saferExceptions) && pat.tpe.widen.isCheckedException
+      yield makeCanThrow(pat.tpe.widen)
+    caps.foldLeft(expr)((e, g) => untpd.Block(g :: Nil, e))
+
   def typedTry(tree: untpd.Try, pt: Type)(using Context): Try = {
     val expr2 :: cases2x = harmonic(harmonize, pt) {
-      val expr1 = typed(tree.expr, pt.dropIfProto)
       val cases1 = typedCases(tree.cases, EmptyTree, defn.ThrowableType, pt.dropIfProto)
+      val expr1 = typed(addCanThrowCapabilities(tree.expr, cases1), pt.dropIfProto)
       expr1 :: cases1
     }
     val finalizer1 = typed(tree.finalizer, defn.UnitType)
@@ -1734,6 +1755,7 @@ class Typer extends Namer
 
   def typedThrow(tree: untpd.Throw)(using Context): Tree = {
     val expr1 = typed(tree.expr, defn.ThrowableType)
+    checkCanThrow(expr1.tpe.widen, tree.span)
     Throw(expr1).withSpan(tree.span)
   }
 
@@ -1832,7 +1854,7 @@ class Typer extends Namer
   def typedAppliedTypeTree(tree: untpd.AppliedTypeTree)(using Context): Tree = {
     tree.args match
       case arg :: _ if arg.isTerm =>
-        if dependentEnabled then
+        if Feature.dependentEnabled then
           return errorTree(tree, i"Not yet implemented: T(...)")
         else
           return errorTree(tree, dependentStr)
@@ -1928,7 +1950,7 @@ class Typer extends Namer
     typeIndexedLambdaTypeTree(tree, tparams, body)
 
   def typedTermLambdaTypeTree(tree: untpd.TermLambdaTypeTree)(using Context): Tree =
-    if dependentEnabled then
+    if Feature.dependentEnabled then
       errorTree(tree, i"Not yet implemented: (...) =>> ...")
     else
       errorTree(tree, dependentStr)
@@ -2399,7 +2421,7 @@ class Typer extends Namer
         ctx.phase.isTyper &&
         cdef1.symbol.ne(defn.DynamicClass) &&
         cdef1.tpe.derivesFrom(defn.DynamicClass) &&
-        !dynamicsEnabled
+        !Feature.dynamicsEnabled
       if (reportDynamicInheritance) {
         val isRequired = parents1.exists(_.tpe.isRef(defn.DynamicClass))
         report.featureWarning(nme.dynamics.toString, "extension of type scala.Dynamic", cls, isRequired, cdef.srcPos)
@@ -2614,7 +2636,10 @@ class Typer extends Namer
     val untpd.InfixOp(l, op, r) = tree
     val result =
       if (ctx.mode.is(Mode.Type))
-        typedAppliedTypeTree(cpy.AppliedTypeTree(tree)(op, l :: r :: Nil))
+        typedAppliedTypeTree(
+          if op.name == tpnme.throws && Feature.enabled(Feature.saferExceptions)
+          then desugar.throws(l, op, r)
+          else cpy.AppliedTypeTree(tree)(op, l :: r :: Nil))
       else if (ctx.mode.is(Mode.Pattern))
         typedUnApply(cpy.Apply(tree)(op, l :: r :: Nil), pt)
       else {
@@ -3468,7 +3493,7 @@ class Typer extends Namer
       def isAutoApplied(sym: Symbol): Boolean =
         sym.isConstructor
         || sym.matchNullaryLoosely
-        || warnOnMigration(MissingEmptyArgumentList(sym.show), tree.srcPos)
+        || Feature.warnOnMigration(MissingEmptyArgumentList(sym.show), tree.srcPos)
            && { patch(tree.span.endPos, "()"); true }
 
       // Reasons NOT to eta expand:
@@ -3819,7 +3844,7 @@ class Typer extends Namer
         case ref: TermRef =>
           pt match {
             case pt: FunProto
-            if needsTupledDual(ref, pt) && autoTuplingEnabled =>
+            if needsTupledDual(ref, pt) && Feature.autoTuplingEnabled =>
               adapt(tree, pt.tupledDual, locked)
             case _ =>
               adaptOverloaded(ref)