cmd/compile/internal/types2: don't set a Config.Context if none is provided

This CL is a clean port of CL 363175 from go/types to types2.

Change-Id: I149789be07c0ca7ddef7bfaa4ea9507778a63775
Reviewed-on: https://go-review.googlesource.com/c/go/+/364454
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/call.go

@@ -78,7 +78,7 @@ func (check *Checker) instantiateSignature(pos syntax.Pos, typ *Signature, targs
 		}()
 	}
 
-	inst := check.instance(pos, typ, targs, check.conf.Context).(*Signature)
+	inst := check.instance(pos, typ, targs, check.bestContext(nil)).(*Signature)
 	assert(len(posList) <= len(targs))
 	tparams := typ.TypeParams().list()
 	if i, err := check.verify(pos, tparams, targs); err != nil {

src/cmd/compile/internal/types2/check.go

@@ -103,6 +103,7 @@ type Checker struct {
 	// package information
 	// (initialized by NewChecker, valid for the life-time of checker)
 	conf *Config
+	ctxt *Context // context for de-duplicating instances
 	pkg  *Package
 	*Info
 	version version                // accepted language version
@@ -200,11 +201,6 @@ func NewChecker(conf *Config, pkg *Package, info *Info) *Checker {
 		conf = new(Config)
 	}
 
-	// make sure we have a context
-	if conf.Context == nil {
-		conf.Context = NewContext()
-	}
-
 	// make sure we have an info struct
 	if info == nil {
 		info = new(Info)
@@ -217,6 +213,7 @@ func NewChecker(conf *Config, pkg *Package, info *Info) *Checker {
 
 	return &Checker{
 		conf:    conf,
+		ctxt:    conf.Context,
 		pkg:     pkg,
 		Info:    info,
 		version: version,
@@ -333,6 +330,7 @@ func (check *Checker) checkFiles(files []*syntax.File) (err error) {
 	check.seenPkgMap = nil
 	check.recvTParamMap = nil
 	check.defTypes = nil
+	check.ctxt = nil
 
 	// TODO(gri) There's more memory we should release at this point.
 

src/cmd/compile/internal/types2/decl.go

@@ -69,7 +69,7 @@ func (check *Checker) objDecl(obj Object, def *Named) {
 	// Funcs with m.instRecv set have not yet be completed. Complete them now
 	// so that they have a type when objDecl exits.
 	if m, _ := obj.(*Func); m != nil && m.instRecv != nil {
-		check.completeMethod(check.conf.Context, m)
+		check.completeMethod(nil, m)
 	}
 
 	// Checking the declaration of obj means inferring its type

src/cmd/compile/internal/types2/named.go

@@ -220,15 +220,17 @@ func (n *Named) setUnderlying(typ Type) {
 
 // bestContext returns the best available context. In order of preference:
 // - the given ctxt, if non-nil
-// - check.Config.Context, if check is non-nil
+// - check.ctxt, if check is non-nil
 // - a new Context
 func (check *Checker) bestContext(ctxt *Context) *Context {
 	if ctxt != nil {
 		return ctxt
 	}
 	if check != nil {
-		assert(check.conf.Context != nil)
-		return check.conf.Context
+		if check.ctxt == nil {
+			check.ctxt = NewContext()
+		}
+		return check.ctxt
 	}
 	return NewContext()
 }

src/cmd/compile/internal/types2/signature.go

@@ -216,7 +216,7 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 			var err string
 			switch T := rtyp.(type) {
 			case *Named:
-				T.resolve(check.conf.Context)
+				T.resolve(check.bestContext(nil))
 				// The receiver type may be an instantiated type referred to
 				// by an alias (which cannot have receiver parameters for now).
 				if T.TypeArgs() != nil && sig.RecvTypeParams() == nil {

src/cmd/compile/internal/types2/typexpr.go

@@ -437,9 +437,10 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 	}
 
 	// create the instance
-	h := check.conf.Context.instanceHash(orig, targs)
+	ctxt := check.bestContext(nil)
+	h := ctxt.instanceHash(orig, targs)
 	// targs may be incomplete, and require inference. In any case we should de-duplicate.
-	inst, _ := check.conf.Context.lookup(h, orig, targs).(*Named)
+	inst, _ := ctxt.lookup(h, orig, targs).(*Named)
 	// If inst is non-nil, we can't just return here. Inst may have been
 	// constructed via recursive substitution, in which case we wouldn't do the
 	// validation below. Ensure that the validation (and resulting errors) runs
@@ -448,7 +449,7 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 		tname := NewTypeName(x.Pos(), orig.obj.pkg, orig.obj.name, nil)
 		inst = check.newNamed(tname, orig, nil, nil, nil) // underlying, methods and tparams are set when named is resolved
 		inst.targs = NewTypeList(targs)
-		inst = check.conf.Context.update(h, orig, targs, inst).(*Named)
+		inst = ctxt.update(h, orig, targs, inst).(*Named)
 	}
 	def.setUnderlying(inst)
 
@@ -474,7 +475,7 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 		// This is an instance from the source, not from recursive substitution,
 		// and so it must be resolved during type-checking so that we can report
 		// errors.
-		inst.resolve(check.conf.Context)
+		inst.resolve(ctxt)
 		// Since check is non-nil, we can still mutate inst. Unpinning the resolver
 		// frees some memory.
 		inst.resolver = nil