[dev.typeparams] cmd/compile: lazy import resolution for types2

This CL adds three new functions to the types2 API to support lazy
import resolution:

1. A new Scope.InsertLazy method to allow recording that Objects exist
in a particular Scope (in particular, package scopes) without having
to yet fully construct those objects. Instead, types2 will call the
provided `resolve` function if/when the object is actually needed.

2. Similarly, a new NewTypeNameLazy function to create TypeName
objects without yet instantiating their underlying Named
instance.

3. Finally, an InstantiateLazy method, that allows creating type
instances without requiring any of the types to be expanded right
away. Importantly, this requires providing a types2.Checker argument
to handle recursive types correctly.

The APIs as-is are a bit clumsy (esp. NewTypeNameLazy), but seem to
work well for cmd/compile's needs. In particular, they simplify some
of the complexities of handling recursive type definitions within the
importer.

Also, the current prototype is a bit fragile. It uses sync.Once to
manage concurrent lazy resolution, which is frustrating to debug in
the presence of reentrancy issues. It also means the importer needs to
deal with concurrency as well. These aren't issues for types2 though
as cmd/compile only walks the type-checked AST sequentially.

Finally, it looks like some of the details of lazy type names are
similar to the lazy "instance" stuff used for generics, so maybe
there's opportunity for unifying them under a more general (but still
internal) lazy type mechanism.

I had originally intended for this CL to also update the types2
importer, but (1) it doesn't have access to the types2.Checker
instance needed to call InstantiateLazy, and (2) it creates a new
TypeName/TypeParam at each use rather than reusing them, which
evidently works with types2.Instantiate but not
types2.(*Checker).instantiate (i.e., InstantiateLazy). I spent a while
trying to fix these issues, but kept running into more subtle
issues. Instead, I've included my WIP "unified IR" CL as a followup CL
that demonstrates these Lazy methods (see noder/reader2.go).

Updates #46449.

Change-Id: I4d1e8e649f6325a11790d25fd90c39fa07c8d41d
Reviewed-on: https://go-review.googlesource.com/c/go/+/323569
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 6 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index 35ca197d64..dd8dd74161 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -76,7 +76,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 	var tparams []*TypeName
 	switch t := typ.(type) {
 	case *Named:
-		tparams = t.tparams
+		tparams = t.TParams()
 	case *Signature:
 		tparams = t.tparams
 		defer func() {
@@ -347,7 +347,7 @@ func (subst *subster) typ(typ Type) Type {
 			}
 		}
 
-		if t.tparams == nil {
+		if t.TParams() == nil {
 			dump(">>> %s is not parameterized", t)
 			return t // type is not parameterized
 		}
@@ -357,7 +357,7 @@ func (subst *subster) typ(typ Type) Type {
 		if len(t.targs) > 0 {
 			// already instantiated
 			dump(">>> %s already instantiated", t)
-			assert(len(t.targs) == len(t.tparams))
+			assert(len(t.targs) == len(t.TParams()))
 			// For each (existing) type argument targ, determine if it needs
 			// to be substituted; i.e., if it is or contains a type parameter
 			// that has a type argument for it.
@@ -367,7 +367,7 @@ func (subst *subster) typ(typ Type) Type {
 				if new_targ != targ {
 					dump(">>> substituted %d targ %s => %s", i, targ, new_targ)
 					if new_targs == nil {
-						new_targs = make([]Type, len(t.tparams))
+						new_targs = make([]Type, len(t.TParams()))
 						copy(new_targs, t.targs)
 					}
 					new_targs[i] = new_targ
@@ -397,7 +397,7 @@ func (subst *subster) typ(typ Type) Type {
 
 		// create a new named type and populate caches to avoid endless recursion
 		tname := NewTypeName(subst.pos, t.obj.pkg, t.obj.name, nil)
-		named := subst.check.newNamed(tname, t, t.underlying, t.tparams, t.methods) // method signatures are updated lazily
+		named := subst.check.newNamed(tname, t, t.Underlying(), t.TParams(), t.methods) // method signatures are updated lazily
 		named.targs = new_targs
 		if subst.check != nil {
 			subst.check.typMap[h] = named
@@ -406,7 +406,7 @@ func (subst *subster) typ(typ Type) Type {
 
 		// do the substitution
 		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, new_targs)
-		named.underlying = subst.typOrNil(t.underlying)
+		named.underlying = subst.typOrNil(t.Underlying())
 		named.fromRHS = named.underlying // for cycle detection (Checker.validType)
 
 		return named