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Copy pathsubst.go
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577 lines (515 loc) · 17.6 KB
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// Copyright 2022 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.
package ir
import (
"fmt"
"go/types"
"honnef.co/go/tools/go/types/typeutil"
"honnef.co/go/tools/internal/xtools-internal/aliases"
)
// subster defines a type substitution operation of a set of type parameters
// to type parameter free replacement types. Substitution is done within
// the context of a package-level function instantiation. *Named types
// declared in the function are unique to the instantiation.
//
// For example, given a parameterized function F
//
// func F[S, T any]() any {
// type X struct{ s S; next *X }
// var p *X
// return p
// }
//
// calling the instantiation F[string, int]() returns an interface
// value (*X[string,int], nil) where the underlying value of
// X[string,int] is a struct{s string; next *X[string,int]}.
//
// A nil *subster is a valid, empty substitution map. It always acts as
// the identity function. This allows for treating parameterized and
// non-parameterized functions identically while compiling to ssa.
//
// Not concurrency-safe.
//
// Note: Some may find it helpful to think through some of the most
// complex substitution cases using lambda calculus inspired notation.
// subst.typ() solves evaluating a type expression E
// within the body of a function Fn[m] with the type parameters m
// once we have applied the type arguments N.
// We can succinctly write this as a function application:
//
// ((λm. E) N)
//
// go/types does not provide this interface directly.
// So what subster provides is a type substitution operation
//
// E[m:=N]
type subster struct {
replacements map[*types.TypeParam]types.Type // values should contain no type params
cache map[types.Type]types.Type // cache of subst results
origin *types.Func // types.Objects declared within this origin function are unique within this context
ctxt *types.Context // speeds up repeated instantiations
uniqueness typeutil.Map[types.Type] // determines the uniqueness of the instantiations within the function
// TODO(taking): consider adding Pos
}
// Returns a subster that replaces rtparams[i] with rtargs[i] and tparams[i] with targs[i].
// Uses ctxt as a cache. rtargs and targs should not contain any types in rtparams or tparams.
// fn is the generic function for which we are substituting.
func makeSubster(ctxt *types.Context, fn *types.Func, rtparams *types.TypeParamList, rtargs []types.Type, tparams *types.TypeParamList, targs []types.Type) *subster {
got := len(rtargs) + len(targs)
want := rtparams.Len() + tparams.Len()
if got != want {
panic(fmt.Sprintf("makeSubster argument count must match: got %d; want %d", got, want))
}
subst := &subster{
replacements: make(map[*types.TypeParam]types.Type, want),
cache: make(map[types.Type]types.Type),
origin: fn.Origin(),
ctxt: ctxt,
}
for i := 0; i < rtparams.Len(); i++ {
subst.replacements[rtparams.At(i)] = rtargs[i]
}
for i := 0; i < tparams.Len(); i++ {
subst.replacements[tparams.At(i)] = targs[i]
}
return subst
}
// typ returns the type of t with the type parameter tparams[i] substituted
// for the type targs[i] where subst was created using tparams and targs.
func (subst *subster) typ(t types.Type) (res types.Type) {
if subst == nil {
return t // A nil subst is type preserving.
}
if r, ok := subst.cache[t]; ok {
return r
}
defer func() {
subst.cache[t] = res
}()
switch t := t.(type) {
case *types.TypeParam:
if r := subst.replacements[t]; r != nil {
return r
}
return t
case *types.Basic:
return t
case *types.Array:
if r := subst.typ(t.Elem()); r != t.Elem() {
return types.NewArray(r, t.Len())
}
return t
case *types.Slice:
if r := subst.typ(t.Elem()); r != t.Elem() {
return types.NewSlice(r)
}
return t
case *types.Pointer:
if r := subst.typ(t.Elem()); r != t.Elem() {
return types.NewPointer(r)
}
return t
case *types.Tuple:
return subst.tuple(t)
case *types.Struct:
return subst.struct_(t)
case *types.Map:
key := subst.typ(t.Key())
elem := subst.typ(t.Elem())
if key != t.Key() || elem != t.Elem() {
return types.NewMap(key, elem)
}
return t
case *types.Chan:
if elem := subst.typ(t.Elem()); elem != t.Elem() {
return types.NewChan(t.Dir(), elem)
}
return t
case *types.Signature:
return subst.signature(t)
case *types.Union:
return subst.union(t)
case *types.Interface:
return subst.interface_(t)
case *types.Alias:
return subst.alias(t)
case *types.Named:
return subst.named(t)
case *typeutil.Iterator:
elem := subst.typ(t.Elem())
if elem != t.Elem() {
return typeutil.NewIterator(elem)
}
return t
case *typeutil.DeferStack:
return t
default:
panic(fmt.Sprintf("internal error: unhandled type %T", t))
}
}
// types returns the result of {subst.typ(ts[i])}.
func (subst *subster) types(ts []types.Type) []types.Type {
res := make([]types.Type, len(ts))
for i := range ts {
res[i] = subst.typ(ts[i])
}
return res
}
func (subst *subster) tuple(t *types.Tuple) *types.Tuple {
if t != nil {
if vars := subst.varlist(t); vars != nil {
return types.NewTuple(vars...)
}
}
return t
}
type varlist interface {
At(i int) *types.Var
Len() int
}
// fieldlist is an adapter for structs for the varlist interface.
type fieldlist struct {
str *types.Struct
}
func (fl fieldlist) At(i int) *types.Var { return fl.str.Field(i) }
func (fl fieldlist) Len() int { return fl.str.NumFields() }
func (subst *subster) struct_(t *types.Struct) *types.Struct {
if t != nil {
if fields := subst.varlist(fieldlist{t}); fields != nil {
tags := make([]string, t.NumFields())
for i, n := 0, t.NumFields(); i < n; i++ {
tags[i] = t.Tag(i)
}
return types.NewStruct(fields, tags)
}
}
return t
}
// varlist returns subst(in[i]) or return nils if subst(v[i]) == v[i] for all i.
func (subst *subster) varlist(in varlist) []*types.Var {
var out []*types.Var // nil => no updates
for i, n := 0, in.Len(); i < n; i++ {
v := in.At(i)
w := subst.var_(v)
if v != w && out == nil {
out = make([]*types.Var, n)
for j := 0; j < i; j++ {
out[j] = in.At(j)
}
}
if out != nil {
out[i] = w
}
}
return out
}
func (subst *subster) var_(v *types.Var) *types.Var {
if v != nil {
if typ := subst.typ(v.Type()); typ != v.Type() {
if v.IsField() {
return types.NewField(v.Pos(), v.Pkg(), v.Name(), typ, v.Embedded())
}
return types.NewParam(v.Pos(), v.Pkg(), v.Name(), typ)
}
}
return v
}
func (subst *subster) union(u *types.Union) *types.Union {
var out []*types.Term // nil => no updates
for i, n := 0, u.Len(); i < n; i++ {
t := u.Term(i)
r := subst.typ(t.Type())
if r != t.Type() && out == nil {
out = make([]*types.Term, n)
for j := 0; j < i; j++ {
out[j] = u.Term(j)
}
}
if out != nil {
out[i] = types.NewTerm(t.Tilde(), r)
}
}
if out != nil {
return types.NewUnion(out)
}
return u
}
func (subst *subster) interface_(iface *types.Interface) *types.Interface {
if iface == nil {
return nil
}
// methods for the interface. Initially nil if there is no known change needed.
// Signatures for the method where recv is nil. NewInterfaceType fills in the receivers.
var methods []*types.Func
initMethods := func(n int) { // copy first n explicit methods
methods = make([]*types.Func, iface.NumExplicitMethods())
for i := range n {
f := iface.ExplicitMethod(i)
norecv := changeRecv(f.Type().(*types.Signature), nil)
methods[i] = types.NewFunc(f.Pos(), f.Pkg(), f.Name(), norecv)
}
}
for i := 0; i < iface.NumExplicitMethods(); i++ {
f := iface.ExplicitMethod(i)
// On interfaces, we need to cycle break on anonymous interface types
// being in a cycle with their signatures being in cycles with their receivers
// that do not go through a Named.
norecv := changeRecv(f.Type().(*types.Signature), nil)
sig := subst.typ(norecv)
if sig != norecv && methods == nil {
initMethods(i)
}
if methods != nil {
methods[i] = types.NewFunc(f.Pos(), f.Pkg(), f.Name(), sig.(*types.Signature))
}
}
var embeds []types.Type
initEmbeds := func(n int) { // copy first n embedded types
embeds = make([]types.Type, iface.NumEmbeddeds())
for i := range n {
embeds[i] = iface.EmbeddedType(i)
}
}
for i := 0; i < iface.NumEmbeddeds(); i++ {
e := iface.EmbeddedType(i)
r := subst.typ(e)
if e != r && embeds == nil {
initEmbeds(i)
}
if embeds != nil {
embeds[i] = r
}
}
if methods == nil && embeds == nil {
return iface
}
if methods == nil {
initMethods(iface.NumExplicitMethods())
}
if embeds == nil {
initEmbeds(iface.NumEmbeddeds())
}
return types.NewInterfaceType(methods, embeds).Complete()
}
func (subst *subster) alias(t *types.Alias) types.Type {
// See subster.named. This follows the same strategy.
tparams := t.TypeParams()
targs := t.TypeArgs()
tname := t.Obj()
torigin := t.Origin()
if !declaredWithin(tname, subst.origin) {
// t is declared outside of the function origin. So t is a package level type alias.
if targs.Len() == 0 {
// No type arguments so no instantiation needed.
return t
}
// Instantiate with the substituted type arguments.
newTArgs := subst.typelist(targs)
return subst.instantiate(torigin, newTArgs)
}
if targs.Len() == 0 {
// t is declared within the function origin and has no type arguments.
//
// Example: This corresponds to A or B in F, but not A[int]:
//
// func F[T any]() {
// type A[S any] = struct{t T, s S}
// type B = T
// var x A[int]
// ...
// }
//
// This is somewhat different than *Named as *Alias cannot be created recursively.
// Copy and substitute type params.
var newTParams []*types.TypeParam
for cur := range tparams.TypeParams() {
cobj := cur.Obj()
cname := types.NewTypeName(cobj.Pos(), cobj.Pkg(), cobj.Name(), nil)
ntp := types.NewTypeParam(cname, nil)
subst.cache[cur] = ntp // See the comment "Note: Subtle" in subster.named.
newTParams = append(newTParams, ntp)
}
// Substitute rhs.
rhs := subst.typ(t.Rhs())
// Create the fresh alias.
obj := aliases.New(tname.Pos(), tname.Pkg(), tname.Name(), rhs, newTParams)
// Substitute into all of the constraints after they are created.
for i, ntp := range newTParams {
bound := tparams.At(i).Constraint()
ntp.SetConstraint(subst.typ(bound))
}
return obj.Type()
}
// t is declared within the function origin and has type arguments.
//
// Example: This corresponds to A[int] in F. Cases A and B are handled above.
// func F[T any]() {
// type A[S any] = struct{t T, s S}
// type B = T
// var x A[int]
// ...
// }
subOrigin := subst.typ(torigin)
subTArgs := subst.typelist(targs)
return subst.instantiate(subOrigin, subTArgs)
}
func (subst *subster) named(t *types.Named) types.Type {
// A Named type is a user defined type.
// Ignoring generics, Named types are canonical: they are identical if
// and only if they have the same defining symbol.
// Generics complicate things, both if the type definition itself is
// parameterized, and if the type is defined within the scope of a
// parameterized function. In this case, two named types are identical if
// and only if their identifying symbols are identical, and all type
// arguments bindings in scope of the named type definition (including the
// type parameters of the definition itself) are equivalent.
//
// Notably:
// 1. For type definition type T[P1 any] struct{}, T[A] and T[B] are identical
// only if A and B are identical.
// 2. Inside the generic func Fn[m any]() any { type T struct{}; return T{} },
// the result of Fn[A] and Fn[B] have identical type if and only if A and
// B are identical.
// 3. Both 1 and 2 could apply, such as in
// func F[m any]() any { type T[x any] struct{}; return T{} }
//
// A subster replaces type parameters within a function scope, and therefore must
// also replace free type parameters in the definitions of local types.
//
// Note: There are some detailed notes sprinkled throughout that borrow from
// lambda calculus notation. These contain some over simplifying math.
//
// LC: One way to think about subster is that it is a way of evaluating
// ((λm. E) N) as E[m:=N].
// Each Named type t has an object *TypeName within a scope S that binds an
// underlying type expression U. U can refer to symbols within S (+ S's ancestors).
// Let x = t.TypeParams() and A = t.TypeArgs().
// Each Named type t is then either:
// U where len(x) == 0 && len(A) == 0
// λx. U where len(x) != 0 && len(A) == 0
// ((λx. U) A) where len(x) == len(A)
// In each case, we will evaluate t[m:=N].
tparams := t.TypeParams() // x
targs := t.TypeArgs() // A
if !declaredWithin(t.Obj(), subst.origin) {
// t is declared outside of Fn[m].
//
// In this case, we can skip substituting t.Underlying().
// The underlying type cannot refer to the type parameters.
//
// LC: Let free(E) be the set of free type parameters in an expression E.
// Then whenever m ∉ free(E), then E = E[m:=N].
// t ∉ Scope(fn) so therefore m ∉ free(U) and m ∩ x = ∅.
if targs.Len() == 0 {
// t has no type arguments. So it does not need to be instantiated.
//
// This is the normal case in real Go code, where t is not parameterized,
// declared at some package scope, and m is a TypeParam from a parameterized
// function F[m] or method.
//
// LC: m ∉ free(A) lets us conclude m ∉ free(t). So t=t[m:=N].
return t
}
// t is declared outside of Fn[m] and has type arguments.
// The type arguments may contain type parameters m so
// substitute the type arguments, and instantiate the substituted
// type arguments.
//
// LC: Evaluate this as ((λx. U) A') where A' = A[m := N].
newTArgs := subst.typelist(targs)
return subst.instantiate(t.Origin(), newTArgs)
}
// t is declared within Fn[m].
if targs.Len() == 0 { // no type arguments?
assert(t == t.Origin(), "local parameterized type abstraction must be an origin type")
// t has no type arguments.
// The underlying type of t may contain the function's type parameters,
// replace these, and create a new type.
//
// Subtle: We short circuit substitution and use a newly created type in
// subst, i.e. cache[t]=fresh, to preemptively replace t with fresh
// in recursive types during traversal. This both breaks infinite cycles
// and allows for constructing types with the replacement applied in
// subst.typ(U).
//
// A new copy of the Named and Typename (and constraints) per function
// instantiation matches the semantics of Go, which treats all function
// instantiations F[N] as having distinct local types.
//
// LC: x.Len()=0 can be thought of as a special case of λx. U.
// LC: Evaluate (λx. U)[m:=N] as (λx'. U') where U'=U[x:=x',m:=N].
tname := t.Obj()
obj := types.NewTypeName(tname.Pos(), tname.Pkg(), tname.Name(), nil)
fresh := types.NewNamed(obj, nil, nil)
var newTParams []*types.TypeParam
for cur := range tparams.TypeParams() {
cobj := cur.Obj()
cname := types.NewTypeName(cobj.Pos(), cobj.Pkg(), cobj.Name(), nil)
ntp := types.NewTypeParam(cname, nil)
subst.cache[cur] = ntp
newTParams = append(newTParams, ntp)
}
fresh.SetTypeParams(newTParams)
subst.cache[t] = fresh
subst.cache[fresh] = fresh
fresh.SetUnderlying(subst.typ(t.Underlying()))
// Substitute into all of the constraints after they are created.
for i, ntp := range newTParams {
bound := tparams.At(i).Constraint()
ntp.SetConstraint(subst.typ(bound))
}
return fresh
}
// t is defined within Fn[m] and t has type arguments (an instantiation).
// We reduce this to the two cases above:
// (1) substitute the function's type parameters into t.Origin().
// (2) substitute t's type arguments A and instantiate the updated t.Origin() with these.
//
// LC: Evaluate ((λx. U) A)[m:=N] as (t' A') where t' = (λx. U)[m:=N] and A'=A [m:=N]
subOrigin := subst.typ(t.Origin())
subTArgs := subst.typelist(targs)
return subst.instantiate(subOrigin, subTArgs)
}
func (subst *subster) instantiate(orig types.Type, targs []types.Type) types.Type {
i, err := types.Instantiate(subst.ctxt, orig, targs, false)
assert(err == nil, "failed to Instantiate named (Named or Alias) type")
if c, _ := subst.uniqueness.At(i); c != nil {
return c
}
subst.uniqueness.Set(i, i)
return i
}
func (subst *subster) typelist(l *types.TypeList) []types.Type {
res := make([]types.Type, l.Len())
for i := 0; i < l.Len(); i++ {
res[i] = subst.typ(l.At(i))
}
return res
}
func (subst *subster) signature(t *types.Signature) types.Type {
tparams := t.TypeParams()
// We are choosing not to support tparams.Len() > 0 until a need has been observed in practice.
//
// There are some known usages for types.Types coming from types.{Eval,CheckExpr}.
// To support tparams.Len() > 0, we just need to do the following [pseudocode]:
// targs := {subst.replacements[tparams[i]]]}; Instantiate(ctxt, t, targs, false)
assert(tparams.Len() == 0, "Substituting types.Signatures with generic functions are currently unsupported.")
// Either:
// (1)non-generic function.
// no type params to substitute
// (2)generic method and recv needs to be substituted.
// Receivers can be either:
// named
// pointer to named
// interface
// nil
// interface is the problematic case. We need to cycle break there!
recv := subst.var_(t.Recv())
params := subst.tuple(t.Params())
results := subst.tuple(t.Results())
if recv != t.Recv() || params != t.Params() || results != t.Results() {
return types.NewSignatureType(recv, nil, nil, params, results, t.Variadic())
}
return t
}