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

// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements type parameter substitution.

package types2

import (
	"bytes"
	"cmd/compile/internal/syntax"
)

type substMap map[*TypeParam]Type

// makeSubstMap creates a new substitution map mapping tpars[i] to targs[i].
// If targs[i] is nil, tpars[i] is not substituted.
func makeSubstMap(tpars []*TypeParam, targs []Type) substMap {
	assert(len(tpars) == len(targs))
	proj := make(substMap, len(tpars))
	for i, tpar := range tpars {
		proj[tpar] = targs[i]
	}
	return proj
}

func (m substMap) empty() bool {
	return len(m) == 0
}

func (m substMap) lookup(tpar *TypeParam) Type {
	if t := m[tpar]; t != nil {
		return t
	}
	return tpar
}

// subst returns the type typ with its type parameters tpars replaced by the
// corresponding type arguments targs, recursively. subst doesn't modify the
// incoming type. If a substitution took place, the result type is different
// from the incoming type.
//
// If the given typMap is non-nil, it is used in lieu of check.typMap.
func (check *Checker) subst(pos syntax.Pos, typ Type, smap substMap, typMap map[string]*Named) Type {
	if smap.empty() {
		return typ
	}

	// common cases
	switch t := typ.(type) {
	case *Basic:
		return typ // nothing to do
	case *TypeParam:
		return smap.lookup(t)
	}

	// general case
	var subst subster
	subst.pos = pos
	subst.smap = smap

	if check != nil {
		subst.check = check
		if typMap == nil {
			typMap = check.typMap
		}
	}
	if typMap == nil {
		// If we don't have a *Checker and its global type map,
		// use a local version. Besides avoiding duplicate work,
		// the type map prevents infinite recursive substitution
		// for recursive types (example: type T[P any] *T[P]).
		typMap = make(map[string]*Named)
	}
	subst.typMap = typMap

	return subst.typ(typ)
}

type subster struct {
	pos    syntax.Pos
	smap   substMap
	check  *Checker // nil if called via Instantiate
	typMap map[string]*Named
}

func (subst *subster) typ(typ Type) Type {
	switch t := typ.(type) {
	case nil:
		// Call typOrNil if it's possible that typ is nil.
		panic("nil typ")

	case *Basic, *top:
		// nothing to do

	case *Array:
		elem := subst.typOrNil(t.elem)
		if elem != t.elem {
			return &Array{len: t.len, elem: elem}
		}

	case *Slice:
		elem := subst.typOrNil(t.elem)
		if elem != t.elem {
			return &Slice{elem: elem}
		}

	case *Struct:
		if fields, copied := subst.varList(t.fields); copied {
			return &Struct{fields: fields, tags: t.tags}
		}

	case *Pointer:
		base := subst.typ(t.base)
		if base != t.base {
			return &Pointer{base: base}
		}

	case *Tuple:
		return subst.tuple(t)

	case *Signature:
		// TODO(gri) rethink the recv situation with respect to methods on parameterized types
		// recv := subst.var_(t.recv) // TODO(gri) this causes a stack overflow - explain
		recv := t.recv
		params := subst.tuple(t.params)
		results := subst.tuple(t.results)
		if recv != t.recv || params != t.params || results != t.results {
			return &Signature{
				rparams: t.rparams,
				// TODO(gri) Why can't we nil out tparams here, rather than in
				//           instantiate above?
				tparams:  t.tparams,
				scope:    t.scope,
				recv:     recv,
				params:   params,
				results:  results,
				variadic: t.variadic,
			}
		}

	case *Union:
		terms, copied := subst.termlist(t.terms)
		if copied {
			// term list substitution may introduce duplicate terms (unlikely but possible).
			// This is ok; lazy type set computation will determine the actual type set
			// in normal form.
			return &Union{terms, nil}
		}

	case *Interface:
		methods, mcopied := subst.funcList(t.methods)
		embeddeds, ecopied := subst.typeList(t.embeddeds)
		if mcopied || ecopied {
			iface := &Interface{methods: methods, embeddeds: embeddeds, complete: t.complete}
			return iface
		}

	case *Map:
		key := subst.typ(t.key)
		elem := subst.typ(t.elem)
		if key != t.key || elem != t.elem {
			return &Map{key: key, elem: elem}
		}

	case *Chan:
		elem := subst.typ(t.elem)
		if elem != t.elem {
			return &Chan{dir: t.dir, elem: elem}
		}

	case *Named:
		// dump is for debugging
		dump := func(string, ...interface{}) {}
		if subst.check != nil && subst.check.conf.Trace {
			subst.check.indent++
			defer func() {
				subst.check.indent--
			}()
			dump = func(format string, args ...interface{}) {
				subst.check.trace(subst.pos, format, args...)
			}
		}

		if t.TParams().Len() == 0 {
			dump(">>> %s is not parameterized", t)
			return t // type is not parameterized
		}

		var newTArgs []Type
		assert(t.targs.Len() == t.TParams().Len())

		// already instantiated
		dump(">>> %s already instantiated", t)
		// 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.
		for i, targ := range t.targs.list() {
			dump(">>> %d targ = %s", i, targ)
			new_targ := subst.typ(targ)
			if new_targ != targ {
				dump(">>> substituted %d targ %s => %s", i, targ, new_targ)
				if newTArgs == nil {
					newTArgs = make([]Type, t.TParams().Len())
					copy(newTArgs, t.targs.list())
				}
				newTArgs[i] = new_targ
			}
		}

		if newTArgs == nil {
			dump(">>> nothing to substitute in %s", t)
			return t // nothing to substitute
		}

		// before creating a new named type, check if we have this one already
		h := instantiatedHash(t, newTArgs)
		dump(">>> new type hash: %s", h)
		if named, found := subst.typMap[h]; found {
			dump(">>> found %s", named)
			return named
		}

		// Create a new named type and populate typMap to avoid endless recursion.
		// The position used here is irrelevant because validation only occurs on t
		// (we don't call validType on named), but we use subst.pos to help with
		// debugging.
		tname := NewTypeName(subst.pos, t.obj.pkg, t.obj.name, nil)
		t.load()
		// It's ok to provide a nil *Checker because the newly created type
		// doesn't need to be (lazily) expanded; it's expanded below.
		named := (*Checker)(nil).newNamed(tname, t.orig, nil, t.tparams, t.methods) // t is loaded, so tparams and methods are available
		named.targs = NewTypeList(newTArgs)
		subst.typMap[h] = named
		t.expand(subst.typMap) // must happen after typMap update to avoid infinite recursion

		// do the substitution
		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, newTArgs)
		named.underlying = subst.typOrNil(t.underlying)
		dump(">>> underlying: %v", named.underlying)
		assert(named.underlying != nil)
		named.fromRHS = named.underlying // for consistency, though no cycle detection is necessary

		return named

	case *TypeParam:
		return subst.smap.lookup(t)

	default:
		unimplemented()
	}

	return typ
}

var instanceHashing = 0

func instantiatedHash(typ *Named, targs []Type) string {
	assert(instanceHashing == 0)
	instanceHashing++
	var buf bytes.Buffer
	writeTypeName(&buf, typ.obj, nil)
	buf.WriteByte('[')
	writeTypeList(&buf, targs, nil, nil)
	buf.WriteByte(']')
	instanceHashing--

	// With respect to the represented type, whether a
	// type is fully expanded or stored as instance
	// does not matter - they are the same types.
	// Remove the instanceMarkers printed for instances.
	res := buf.Bytes()
	i := 0
	for _, b := range res {
		if b != instanceMarker {
			res[i] = b
			i++
		}
	}

	return string(res[:i])
}

// typOrNil is like typ but if the argument is nil it is replaced with Typ[Invalid].
// A nil type may appear in pathological cases such as type T[P any] []func(_ T([]_))
// where an array/slice element is accessed before it is set up.
func (subst *subster) typOrNil(typ Type) Type {
	if typ == nil {
		return Typ[Invalid]
	}
	return subst.typ(typ)
}

func (subst *subster) var_(v *Var) *Var {
	if v != nil {
		if typ := subst.typ(v.typ); typ != v.typ {
			copy := *v
			copy.typ = typ
			return &copy
		}
	}
	return v
}

func (subst *subster) tuple(t *Tuple) *Tuple {
	if t != nil {
		if vars, copied := subst.varList(t.vars); copied {
			return &Tuple{vars: vars}
		}
	}
	return t
}

func (subst *subster) varList(in []*Var) (out []*Var, copied bool) {
	out = in
	for i, v := range in {
		if w := subst.var_(v); w != v {
			if !copied {
				// first variable that got substituted => allocate new out slice
				// and copy all variables
				new := make([]*Var, len(in))
				copy(new, out)
				out = new
				copied = true
			}
			out[i] = w
		}
	}
	return
}

func (subst *subster) func_(f *Func) *Func {
	if f != nil {
		if typ := subst.typ(f.typ); typ != f.typ {
			copy := *f
			copy.typ = typ
			return &copy
		}
	}
	return f
}

func (subst *subster) funcList(in []*Func) (out []*Func, copied bool) {
	out = in
	for i, f := range in {
		if g := subst.func_(f); g != f {
			if !copied {
				// first function that got substituted => allocate new out slice
				// and copy all functions
				new := make([]*Func, len(in))
				copy(new, out)
				out = new
				copied = true
			}
			out[i] = g
		}
	}
	return
}

func (subst *subster) typeList(in []Type) (out []Type, copied bool) {
	out = in
	for i, t := range in {
		if u := subst.typ(t); u != t {
			if !copied {
				// first function that got substituted => allocate new out slice
				// and copy all functions
				new := make([]Type, len(in))
				copy(new, out)
				out = new
				copied = true
			}
			out[i] = u
		}
	}
	return
}

func (subst *subster) termlist(in []*Term) (out []*Term, copied bool) {
	out = in
	for i, t := range in {
		if u := subst.typ(t.typ); u != t.typ {
			if !copied {
				// first function that got substituted => allocate new out slice
				// and copy all functions
				new := make([]*Term, len(in))
				copy(new, out)
				out = new
				copied = true
			}
			out[i] = NewTerm(t.tilde, u)
		}
	}
	return
}