Requirement environment: adopt the conformance's generic signature when we can.

Noticed by Slava: when the requirement is non-generic, the requirement
signature's generic signature is always going to be the same as the
conformance declaration context's generic signature, so use that immediately.

This eliminates a large number of generic signature builders created due to
requirement signatures (e.g., from 4893 -> 569 when measured against
the standard library work for SE-0157).

Short-circuit build of requirement environment.

This is all Slava and it’s magnificent.

FIXUP for “the Slava change”

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -1068,8 +1068,6 @@ RequirementEnvironment::RequirementEnvironment(
   auto selfType = cast<GenericTypeParamType>(
                             proto->getSelfInterfaceType()->getCanonicalType());
 
-  SmallVector<GenericTypeParamType*, 4> allGenericParams;
-
   // Add the generic signature of the context of the conformance. This includes
   // the generic parameters from the conforming type as well as any additional
   // constraints that might occur on the extension that declares the
@@ -1079,9 +1077,7 @@ RequirementEnvironment::RequirementEnvironment(
   if ((conformanceSig = conformanceDC->getGenericSignatureOfContext())) {
     // Use the canonical signature here.
     conformanceSig = conformanceSig->getCanonicalSignature();
-    allGenericParams.append(conformanceSig->getGenericParams().begin(),
-                            conformanceSig->getGenericParams().end());
-    depth = allGenericParams.back()->getDepth() + 1;
+    depth = conformanceSig->getGenericParams().back()->getDepth() + 1;
   }
 
   // Add the generic signature of the requirement, substituting our concrete
@@ -1121,39 +1117,46 @@ RequirementEnvironment::RequirementEnvironment(
       return ProtocolConformanceRef(proto);
     });
 
-  // First, add the generic parameters from the requirement.
-  for (auto genericParam : reqSig->getGenericParams().slice(1)) {
-    // The only depth that makes sense is depth == 1, the generic parameters
-    // of the requirement itself. Anything else is from invalid code.
-    if (genericParam->getDepth() != 1) {
-      return;
+  // If the requirement itself is non-generic, the synthetic signature
+  // is that of the conformance context.
+  if (reqSig->getGenericParams().size() == 1 &&
+      reqSig->getRequirements().size() == 1) {
+    syntheticSignature = conformanceDC->getGenericSignatureOfContext();
+    if (syntheticSignature) {
+      syntheticSignature = syntheticSignature->getCanonicalSignature();
+      syntheticEnvironment =
+        syntheticSignature->createGenericEnvironment(
+                                     *conformanceDC->getParentModule());
     }
 
-    // Create an equivalent generic parameter at the next depth.
-    auto substGenericParam =
-      GenericTypeParamType::get(depth, genericParam->getIndex(), ctx);
-
-    allGenericParams.push_back(substGenericParam);
+    return;
   }
 
-  // If there were no generic parameters, we're done.
-  if (allGenericParams.empty()) return;
-
   // Construct a generic signature builder by collecting the constraints
   // from the requirement and the context of the conformance together,
   // because both define the capabilities of the requirement.
   GenericSignatureBuilder builder(
            ctx,
            TypeChecker::LookUpConformance(tc, conformanceDC));
 
-  unsigned firstGenericParamToAdd = 0;
   if (conformanceSig) {
     builder.addGenericSignature(conformanceSig);
-    firstGenericParamToAdd = conformanceSig->getGenericParams().size();
   }
 
-  for (unsigned gp : range(firstGenericParamToAdd, allGenericParams.size()))
-    builder.addGenericParameter(allGenericParams[gp]);
+  // First, add the generic parameters from the requirement.
+  for (auto genericParam : reqSig->getGenericParams().slice(1)) {
+    // The only depth that makes sense is depth == 1, the generic parameters
+    // of the requirement itself. Anything else is from invalid code.
+    if (genericParam->getDepth() != 1) {
+      return;
+    }
+
+    // Create an equivalent generic parameter at the next depth.
+    auto substGenericParam =
+      GenericTypeParamType::get(depth, genericParam->getIndex(), ctx);
+
+    builder.addGenericParameter(substGenericParam);
+  }
 
   ++NumRequirementEnvironments;