Better printing of capabilities in error messages

 - Special case in some situations so that we only print the name,
   not the underlying type.
 - Print TermParamRefs like other singleton types

Author: odersky

compiler/src/dotty/tools/dotc/cc/Capability.scala

@@ -788,6 +788,9 @@ object Capabilities:
           case _: Maybe => MaybeCapability(c1)
           case _ => c1
 
+    def showAsCapability(using Context) =
+      i"capability ${ctx.printer.toTextCapability(this).show}"
+
     def toText(printer: Printer): Text = printer.toTextCapability(this)
   end Capability
 

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -1300,24 +1300,23 @@ object CaptureSet:
         case cs: Var =>
           if !cs.levelOK(elem) then
             val levelStr = elem match
-              case ref: TermRef => i", defined in ${ref.symbol.maybeOwner}"
-              case _ => ""
-            i"""capability ${elem}$levelStr
-                |cannot be included in outer capture set $cs"""
+              case ref: TermRef => i", defined in ${ref.symbol.maybeOwner}\n"
+              case _ => " "
+            i"""${elem.showAsCapability}${levelStr}cannot be included in outer capture set $cs"""
           else if !elem.tryClassifyAs(cs.classifier) then
-            i"""capability ${elem} is not classified as ${cs.classifier}, therefore it
+            i"""${elem.showAsCapability} is not classified as ${cs.classifier}, therefore it
                 |cannot be included in capture set $cs of ${cs.classifier} elements"""
           else if cs.isBadRoot(elem) then
             elem match
               case elem: FreshCap =>
-                i"""local capability $elem created in ${elem.ccOwner}
+                i"""local ${elem.showAsCapability} created in ${elem.ccOwner}
                   |cannot be included in outer capture set $cs"""
               case _ =>
-                i"universal capability $elem cannot be included in capture set $cs"
+                i"universal ${elem.showAsCapability} cannot be included in capture set $cs"
           else
-            i"capability $elem cannot be included in capture set $cs"
+            i"${elem.showAsCapability} cannot be included in capture set $cs"
         case _ =>
-          i"capability $elem is not included in capture set $cs$why"
+          i"${elem.showAsCapability} is not included in capture set $cs$why"
 
     override def toText(printer: Printer): Text =
       inContext(printer.printerContext):

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -242,8 +242,6 @@ class PlainPrinter(_ctx: Context) extends Printer {
           selectionString(tp)
         else
           toTextPrefixOf(tp) ~ selectionString(tp)
-      case tp: TermParamRef =>
-        ParamRefNameString(tp) ~ hashStr(tp.binder) ~ ".type"
       case tp: TypeParamRef =>
         val suffix =
           if showNestingLevel then

docs/_docs/reference/experimental/cc.md

@@ -43,10 +43,12 @@ followed by `^`. We'll see that this turns the parameter into a _capability_ who
 
 If we now try to define the problematic value `later`, we get a static error:
 ```
-   |  val later = usingLogFile { f => () => f.write(0) }
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |The expression's type () => Unit is not allowed to capture the root capability `cap`.
-   |This usually means that a capability persists longer than its allowed lifetime.
+   |val later = usingLogFile { f => () => f.write(0) } // error
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |Found:    (f: java.io.FileOutputStream^?) ->? () ->{f} Unit
+   |Required: java.io.FileOutputStream^ => () ->? Unit
+   |
+   |Note that capability f cannot be included in outer capture set ?.
 ```
 In this case, it was easy to see that the `logFile` capability escapes in the closure passed to `usingLogFile`. But capture checking also works for more complex cases.
 For instance, capture checking is able to distinguish between the following safe code:

tests/neg-custom-args/captures/box-adapt-cases.check

@@ -3,14 +3,14 @@
    |          ^^^^^^^^^^^^^^^^
    |          Found:    (cap: Cap^{io}) ->{io} Int
    |          Required: Cap^{io} -> Int
-   |          Note that capability (io : Cap^) is not included in capture set {}.
+   |          Note that capability io is not included in capture set {}.
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/box-adapt-cases.scala:29:10 ------------------------------
 29 |  x.value(cap => cap.use())  // error
    |          ^^^^^^^^^^^^^^^^
    |          Found:    (cap: Cap^?) ->{io, fs} Int
    |          Required: Cap^{io, fs} ->{io} Int
-   |          Note that capability (fs : Cap^) is not included in capture set {io}.
+   |          Note that capability fs is not included in capture set {io}.
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/byname.check

@@ -12,22 +12,22 @@
    |
    |where:    ?=> refers to a fresh root capability created in method test when checking argument to parameter ff of method h
    |
-   |Note that capability (cap1 : Cap) is not included in capture set {cap2}.
+   |Note that capability cap1 is not included in capture set {cap2}.
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:19:5 ----------------------------------------
 19 |  h(g()) // error
    |    ^^^
    |    Found:    () ?->{cap2} I^?
    |    Required: () ?->{cap1} I
-   |    Note that capability (cap2 : Cap) is not included in capture set {cap1}.
+   |    Note that capability cap2 is not included in capture set {cap1}.
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:22:5 ----------------------------------------
 22 |  h2(() => g())() // error
    |     ^^^^^^^^^
    |     Found:    () ->{cap2} I^?
    |     Required: () ->{cap1} I
-   |     Note that capability (cap2 : Cap) is not included in capture set {cap1}.
+   |     Note that capability cap2 is not included in capture set {cap1}.
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/capt-depfun.check

@@ -6,6 +6,6 @@
    |
    |                                         where:    => refers to a fresh root capability in the type of value dc
    |
-   |                                         Note that capability (y : C^) is not included in capture set {}.
+   |                                         Note that capability y is not included in capture set {}.
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/capt1.check

@@ -3,23 +3,23 @@
   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |  Found:    () ->{x} C
   |  Required: () -> C
-  |  Note that capability (x : C^) is not included in capture set {}.
+  |  Note that capability x is not included in capture set {}.
   |
   | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:8:2 ------------------------------------------
 8 |  () => if x == null then y else y  // error
   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |  Found:    () ->{x} C^?
   |  Required: Matchable
-  |  Note that capability (x : C^) is not included in capture set {}.
+  |  Note that capability x is not included in capture set {}.
   |
   | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:15:2 -----------------------------------------
 15 |  def f(y: Int) = if x == null then y else y  // error
    |  ^
    |  Found:    (y: Int) ->{x} Int
    |  Required: Matchable
-   |  Note that capability (x : C^) is not included in capture set {}.
+   |  Note that capability x is not included in capture set {}.
 16 |  f
    |
    | longer explanation available when compiling with `-explain`
@@ -28,7 +28,7 @@
    |  ^
    |  Found:    A^{x}
    |  Required: A
-   |  Note that capability (x : C^) is not included in capture set {}.
+   |  Note that capability x is not included in capture set {}.
 23 |    def m() = if x == null then y else y
 24 |  F(22)
    |
@@ -38,7 +38,7 @@
    |  ^
    |  Found:    A^{x}
    |  Required: A
-   |  Note that capability (x : C^) is not included in capture set {}.
+   |  Note that capability x is not included in capture set {}.
 28 |    def m() = if x == null then y else y
    |
    | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/cc-glb.check

@@ -3,6 +3,6 @@
   |                   ^^
   |                   Found:    (x1 : (Foo[T] & Foo[Any])^{io})
   |                   Required: Foo[T]
-  |                   Note that capability (io : Cap^) is not included in capture set {}.
+  |                   Note that capability io is not included in capture set {}.
   |
   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/cc-poly-2.check

@@ -14,6 +14,6 @@
    |                    ^
    |                    Found:    (x : Test.D^{d})
    |                    Required: Test.D^{c1}
-   |                    Note that capability (d : Test.D^) is not included in capture set {c1}.
+   |                    Note that capability d is not included in capture set {c1}.
    |
    | longer explanation available when compiling with `-explain`