SILGen: Support mutable consuming parameters.

Emit a box like we would for a local variable, and move the parameter value into the box, as
part of the prolog.

lib/AST/Decl.cpp

@@ -6387,9 +6387,9 @@ Type VarDecl::getType() const {
 /// is a let member in an initializer.
 bool VarDecl::isSettable(const DeclContext *UseDC,
                          const DeclRefExpr *base) const {
-  // Only inout parameters are settable.
+  // Parameters are settable or not depending on their ownership convention.
   if (auto *PD = dyn_cast<ParamDecl>(this))
-    return PD->isInOut();
+    return !PD->isImmutableInFunctionBody();
 
   // If this is a 'var' decl, then we're settable if we have storage or a
   // setter.

lib/SILGen/SILGenProlog.cpp

@@ -429,18 +429,20 @@ struct ArgumentInitHelper {
     ManagedValue argrv = makeArgument(ty, pd->isInOut(), isNoImplicitCopy,
                                       lifetimeAnnotation, parent, loc);
 
+    SILValue value = argrv.getValue();
     if (pd->isInOut()) {
       assert(argrv.getType().isAddress() && "expected inout to be address");
-    } else {
-    #warning "todo"
-      assert(pd->isImmutableInFunctionBody()
-             && "consuming mutable params not implemented yet");
+    } else if (!pd->isImmutableInFunctionBody()) {
+      // If it's a locally mutable parameter, then we need to move the argument
+      // value into a local box to hold the mutated value.
+      auto mutableBox = SGF.emitLocalVariableWithCleanup(pd,
+                                                    MarkUninitializedInst::Var);
+      argrv.ensurePlusOne(SGF, loc).forwardInto(SGF, loc, mutableBox.get());
+      return;
+    }
     // If the variable is immutable, we can bind the value as is.
     // Leave the cleanup on the argument, if any, in place to consume the
     // argument if we're responsible for it.
-    }
-    SILValue value = argrv.getValue();
-    #warning "todo"
     SILDebugVariable varinfo(pd->isImmutableInFunctionBody(), ArgNo);
     if (!argrv.getType().isAddress()) {
       // NOTE: We setup SGF.VarLocs[pd] in updateArgumentValueForBinding.

lib/Sema/TypeCheckStorage.cpp

@@ -3358,9 +3358,9 @@ StorageImplInfoRequest::evaluate(Evaluator &evaluator,
                                  AbstractStorageDecl *storage) const {
   if (auto *param = dyn_cast<ParamDecl>(storage)) {
     return StorageImplInfo::getSimpleStored(
-      param->isInOut()
-      ? StorageIsMutable
-      : StorageIsNotMutable);
+      param->isImmutableInFunctionBody()
+        ? StorageIsNotMutable
+        : StorageIsMutable);
   }
 
   if (auto *var = dyn_cast<VarDecl>(storage)) {

test/SILGen/consuming_parameter.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+func bar(_: String) {}
+
+// CHECK-LABEL: sil {{.*}} @${{.*}}3foo
+func foo(y: consuming String, z: String) -> () -> String {
+    // CHECK: bb0(%0 : @owned $String, %1 : @guaranteed $String):
+    // CHECK:   [[BOX:%.*]] = alloc_box ${ var String }
+    // CHECK:   [[BOX0:%.*]] = mark_uninitialized [var] [[BOX]]
+    // CHECK:   [[BOX1:%.*]] = begin_borrow [lexical] [[BOX0]]
+    // CHECK:   [[Y:%.*]] = project_box [[BOX1]]
+    // CHECK:   store %0 to [init] [[Y]]
+
+    // CHECK:   [[YCAPTURE:%.*]] = copy_value [[BOX1]]
+    // CHECK:   partial_apply {{.*}} {{%.*}}([[YCAPTURE]])
+    let r = { y }
+
+    // CHECK:   [[ZCOPY:%.*]] = copy_value %1
+    // CHECK:   [[YACCESS:%.*]] = begin_access [modify] [unknown] [[Y]]
+    // CHECK:   assign [[ZCOPY]] to [[YACCESS]]
+    y = z
+
+    // CHECK:   [[YACCESS:%.*]] = begin_access [read] [unknown] [[Y]]
+    // CHECK:   [[YVAL:%.*]] = load [copy] [[YACCESS]]
+    // CHECK:   apply {{%.*}}([[YVAL]]
+    bar(y)
+
+    return r
+}