[ConstraintSystem] Remove constraint propagation.

The current implementation isn't really useful in the face of generic
overloads. It has never been enabled by default, and isn't useful to
keep around if it is disabled. If we ever want to bring it back,
we know where to look!

Author: rudkx

include/swift/Basic/LangOptions.h

@@ -157,10 +157,6 @@ namespace swift {
     /// solver should be debugged.
     unsigned DebugConstraintSolverAttempt = 0;
 
-    /// \brief Enable the experimental constraint propagation in the
-    /// type checker.
-    bool EnableConstraintPropagation = false;
-
     /// \brief Enable the iterative type checker.
     bool IterativeTypeChecker = false;
 

include/swift/Option/FrontendOptions.td

@@ -151,9 +151,6 @@ def debug_constraints : Flag<["-"], "debug-constraints">,
 def debug_constraints_attempt : Separate<["-"], "debug-constraints-attempt">,
   HelpText<"Debug the constraint solver at a given attempt">;
 
-def propagate_constraints : Flag<["-"], "propagate-constraints">,
-  HelpText<"Enable constraint propagation in the type checker">;
-
 def iterative_type_checker : Flag<["-"], "iterative-type-checker">,
   HelpText<"Enable the iterative type checker">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -208,7 +208,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableASTScopeLookup |= Args.hasArg(OPT_enable_astscope_lookup);
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
-  Opts.EnableConstraintPropagation |= Args.hasArg(OPT_propagate_constraints);
   Opts.IterativeTypeChecker |= Args.hasArg(OPT_iterative_type_checker);
   Opts.NamedLazyMemberLoading &= !Args.hasArg(OPT_disable_named_lazy_member_loading);
   Opts.DebugGenericSignatures |= Args.hasArg(OPT_debug_generic_signatures);

lib/Sema/CMakeLists.txt

@@ -8,7 +8,6 @@ add_swift_library(swiftSema STATIC
   CSBindings.cpp
   CSDiag.cpp
   CSGen.cpp
-  CSPropagate.cpp
   CSRanking.cpp
   CSSimplify.cpp
   CSSolver.cpp

lib/Sema/CSPropagate.cpp

@@ -984,12 +984,6 @@ void ConstraintSystem::Candidate::applySolutions(
 }
 
 void ConstraintSystem::shrink(Expr *expr) {
-  // Disable the shrink pass when constraint propagation is
-  // enabled. They achieve similar effects, and the shrink pass is
-  // known to have bad behavior in some cases.
-  if (TC.Context.LangOpts.EnableConstraintPropagation)
-    return;
-
   typedef llvm::SmallDenseMap<Expr *, ArrayRef<ValueDecl *>> DomainMap;
 
   // A collection of original domains of all of the expressions,
@@ -1421,11 +1415,6 @@ bool ConstraintSystem::solve(Expr *const expr,
   if (failedConstraint || simplify())
     return true;
 
-  // If the experimental constraint propagation pass is enabled, run it.
-  if (TC.Context.LangOpts.EnableConstraintPropagation)
-    if (propagateConstraints())
-      return true;
-
   // Solve the system.
   solveRec(solutions, allowFreeTypeVariables);