Fix pretty-printing of capture-variables

Author: bracevac

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

@@ -567,6 +567,25 @@ class PlainPrinter(_ctx: Context) extends Printer {
         (tparamStr, bounds.derivedTypeBounds(loRhs, hiRhs))
   end decomposeLambdas
 
+  /** Is this a capture variable's bounds? i.e. lo and hi are both CapSet-based. */
+  private def isCaptureVarBounds(lo: Type, hi: Type): Boolean =
+    lo.derivesFrom(defn.Caps_CapSet) && (hi match
+      case CapturingType(parent, _) => parent.derivesFrom(defn.Caps_CapSet)
+      case hi => hi.derivesFrom(defn.Caps_CapSet))
+
+  /** Print capture variable bounds using `^` syntax.
+   *  Plain CapSet lower bound and universal upper bound are elided.
+   */
+  private def toTextCaptureVarBounds(lo: Type, hi: Type): Text =
+    val loText = lo match
+      case CapturingType(_, refs) => " >: " ~ toTextCaptureSet(refs)
+      case _ => Text() // plain CapSet = trivial lower bound
+    val hiText = hi match
+      case CapturingType(_, refs) if isElidableUniversal(refs) => Text() // trivial upper bound
+      case CapturingType(_, refs) => " <: " ~ toTextCaptureSet(refs)
+      case _ => Text()
+    Str("^") ~ loText ~ hiText
+
   /** String representation of a definition's type following its name */
   protected def toTextRHS(tp: Type, isParameter: Boolean = false): Text = controlled {
     homogenize(tp) match {
@@ -575,6 +594,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
         val binder = rhs match
           case tp: AliasingBounds =>
             " = " ~ toText(tp.alias)
+          case TypeBounds(lo, hi) if !printDebug && Feature.ccEnabledSomewhere && isCaptureVarBounds(lo, hi) =>
+            toTextCaptureVarBounds(lo, hi)
           case TypeBounds(lo, hi) =>
             (if lo.isExactlyNothing then Text() else " >: " ~ toText(lo))
             ~ (if hi.isExactlyAny || (!printDebug && hi.isFromJavaObject) then Text() else " <: " ~ toText(hi))

docs/_docs/reference/experimental/capture-checking/safe.md

@@ -42,7 +42,6 @@ is done, the library module can be made available for use by safe code.
 This scheme is supported by a new `@assumeSafe` annotation, available in
 module `caps`.
 Modules tagged with this annotation are assumed to be callable from  agent-generated code. `@assumeSafe` comes with none of the restrictions that `safe` implies. Instead it is the obligation of the programmer to verify that the module is indeed safe. For instance caching a function results could be implemented like this:
-<!-- FIXME: the below is rejected by the compiler! -->
 ```scala sc:nocompile
 import caps.unsafe.untrackedCaptures
 import caps.assumeSafe

repl/test/dotty/tools/repl/ReplCompilerTests.scala

@@ -500,7 +500,7 @@ class ReplCompilerTests extends ReplTest:
       // Verify capture checking syntax is accepted in subsequent inputs
       run("def foo[C^](x: AnyRef^{C}): AnyRef^{x} = x")
       assertEquals(
-        "def foo[C >: caps.CapSet <: caps.CapSet^](x: AnyRef^{C}): AnyRef^{x}",
+        "def foo[C^](x: AnyRef^{C}): AnyRef^{x}",
         storedOutput().trim)
 
   @Test def `i16250b`: Unit =
@@ -520,7 +520,7 @@ class ReplCompilerTests extends ReplTest:
       // separationChecking implies captureChecking
       run("def foo[C^](x: AnyRef^{C}): AnyRef^{x} = x")
       assertEquals(
-        "def foo[C >: caps.CapSet <: caps.CapSet^](x: AnyRef^{C}): AnyRef^{x}",
+        "def foo[C^](x: AnyRef^{C}): AnyRef^{x}",
         storedOutput().trim)
 
   @Test def `i16250d`: Unit =
@@ -531,7 +531,7 @@ class ReplCompilerTests extends ReplTest:
       // safe implies captureChecking
       run("def foo[C^](x: AnyRef^{C}): AnyRef^{x} = x")
       assertEquals(
-        "def foo[C >: caps.CapSet <: caps.CapSet^](x: AnyRef^{C}): AnyRef^{x}",
+        "def foo[C^](x: AnyRef^{C}): AnyRef^{x}",
         storedOutput().trim)
 
   @Test def `i16250 nested global language imports error`: Unit = initially:

tests/neg-custom-args/captures/unbox-overrides.check

@@ -1,30 +1,21 @@
 -- [E164] Declaration Error: tests/neg-custom-args/captures/unbox-overrides.scala:8:6 ----------------------------------
 8 |  def foo[C^]: Object^{C}  // error
   |      ^
-  |error overriding method foo in trait A of type [C >: scala.caps.CapSet <: scala.caps.CapSet^]: Object^{C};
-  |  method foo of type [C >: scala.caps.CapSet <: scala.caps.CapSet^²]: Object^{C} has a parameter C with different @reserve status than the corresponding parameter in the overridden definition
-  |
-  |where:    ^  refers to a root capability in the type of type C²
-  |          ^² refers to a root capability in the type of type C³
+  |error overriding method foo in trait A of type [C^]: Object^{C};
+  |  method foo of type [C^]: Object^{C} has a parameter C with different @reserve status than the corresponding parameter in the overridden definition
   |
   | longer explanation available when compiling with `-explain`
 -- [E164] Declaration Error: tests/neg-custom-args/captures/unbox-overrides.scala:9:6 ----------------------------------
 9 |  def bar[@reserve C^]: Object^{C} // error
   |      ^
-  |error overriding method bar in trait A of type [C >: scala.caps.CapSet <: scala.caps.CapSet^]: Object^{C};
-  |  method bar of type [C >: scala.caps.CapSet <: scala.caps.CapSet^²]: Object^{C} has a parameter C with different @reserve status than the corresponding parameter in the overridden definition
-  |
-  |where:    ^  refers to a root capability in the type of type C²
-  |          ^² refers to a root capability in the type of type C³
+  |error overriding method bar in trait A of type [C^]: Object^{C};
+  |  method bar of type [C^]: Object^{C} has a parameter C with different @reserve status than the corresponding parameter in the overridden definition
   |
   | longer explanation available when compiling with `-explain`
 -- [E164] Declaration Error: tests/neg-custom-args/captures/unbox-overrides.scala:15:15 --------------------------------
 15 |abstract class C extends A, B2 // error
    |               ^
-   |error overriding method foo in trait A of type [C >: scala.caps.CapSet <: scala.caps.CapSet^]: Object^{C};
-   |  method foo in trait B2 of type [C >: scala.caps.CapSet <: scala.caps.CapSet^²]: Object^{C} has a parameter C with different @reserve status than the corresponding parameter in the overridden definition
-   |
-   |where:    ^  refers to a root capability in the type of type C²
-   |          ^² refers to a root capability in the type of type C³
+   |error overriding method foo in trait A of type [C^]: Object^{C};
+   |  method foo in trait B2 of type [C^]: Object^{C} has a parameter C with different @reserve status than the corresponding parameter in the overridden definition
    |
    | longer explanation available when compiling with `-explain`