[CS] Remove CTP_CannotFail

This is now unused.

Author: hamishknight

include/swift/Sema/ConstraintLocator.h

@@ -83,8 +83,6 @@ enum ContextualTypePurpose : uint8_t {
 
   CTP_ExprPattern,      ///< `~=` operator application associated with expression
                         /// pattern.
-
-  CTP_CannotFail,       ///< Conversion can never fail. abort() if it does.
 };
 
 namespace constraints {

lib/Sema/CSApply.cpp

@@ -9589,7 +9589,6 @@ ExprWalker::rewriteTarget(SyntacticElementTarget target) {
     case CTP_SubscriptAssignSource:
     case CTP_Condition:
     case CTP_WrappedProperty:
-    case CTP_CannotFail:
     case CTP_SingleValueStmtBranch:
       result.setExpr(rewrittenExpr);
       break;

lib/Sema/CSDiagnostics.cpp

@@ -864,7 +864,6 @@ GenericArgumentsMismatchFailure::getDiagnosticFor(
   case CTP_ThrowStmt:
   case CTP_ForEachSequence:
   case CTP_Unused:
-  case CTP_CannotFail:
   case CTP_YieldByReference:
   case CTP_EnumCaseRawValue:
   case CTP_ExprPattern:
@@ -973,7 +972,7 @@ bool GenericArgumentsMismatchFailure::diagnoseAsError() {
 
     case ConstraintLocator::ContextualType: {
       auto purpose = getContextualTypePurpose();
-      assert(!(purpose == CTP_Unused || purpose == CTP_CannotFail));
+      assert(purpose != CTP_Unused);
 
       // If this is call to a closure e.g. `let _: A = { B() }()`
       // let's point diagnostic to its result.
@@ -2949,9 +2948,7 @@ static std::optional<Diag<Type>>
 getContextualNilDiagnostic(ContextualTypePurpose CTP) {
   switch (CTP) {
   case CTP_Unused:
-  case CTP_CannotFail:
-    llvm_unreachable("These contextual type purposes cannot fail with a "
-                     "conversion type specified!");
+    llvm_unreachable("Expected a contextual purpose");
   case CTP_Initialization:
     return diag::cannot_convert_initializer_value_nil;
 
@@ -3749,7 +3746,6 @@ ContextualFailure::getDiagnosticFor(ContextualTypePurpose context,
   case CTP_ThrowStmt:
   case CTP_ForEachSequence:
   case CTP_Unused:
-  case CTP_CannotFail:
   case CTP_YieldByReference:
   case CTP_ExprPattern:
     break;

lib/Sema/CSSimplify.cpp

@@ -16680,7 +16680,6 @@ void ConstraintSystem::addContextualConversionConstraint(
 
   case CTP_ArrayElement:
   case CTP_AssignSource:
-  case CTP_CannotFail:
   case CTP_Condition:
   case CTP_Unused:
   case CTP_YieldByValue:

lib/Sema/SyntacticElementTarget.cpp

@@ -273,7 +273,6 @@ bool SyntacticElementTarget::contextualTypeIsOnlyAHint() const {
   case CTP_SubscriptAssignSource:
   case CTP_Condition:
   case CTP_WrappedProperty:
-  case CTP_CannotFail:
   case CTP_ExprPattern:
   case CTP_SingleValueStmtBranch:
     return false;