| // Copyright 2021 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 typeparams contains common utilities for writing tools that interact |
| // with generic Go code, as introduced with Go 1.18. |
| // |
| // Many of the types and functions in this package are proxies for the new APIs |
| // introduced in the standard library with Go 1.18. For example, the |
| // typeparams.Union type is an alias for go/types.Union, and the ForTypeSpec |
| // function returns the value of the go/ast.TypeSpec.TypeParams field. At Go |
| // versions older than 1.18 these helpers are implemented as stubs, allowing |
| // users of this package to write code that handles generic constructs inline, |
| // even if the Go version being used to compile does not support generics. |
| // |
| // Additionally, this package contains common utilities for working with the |
| // new generic constructs, to supplement the standard library APIs. Notably, |
| // the StructuralTerms API computes a minimal representation of the structural |
| // restrictions on a type parameter. |
| // |
| // An external version of these APIs is available in the |
| // golang.org/x/exp/typeparams module. |
| package typeparams |
| |
| import ( |
| "go/ast" |
| "go/token" |
| "go/types" |
| ) |
| |
| // UnpackIndexExpr extracts data from AST nodes that represent index |
| // expressions. |
| // |
| // For an ast.IndexExpr, the resulting indices slice will contain exactly one |
| // index expression. For an ast.IndexListExpr (go1.18+), it may have a variable |
| // number of index expressions. |
| // |
| // For nodes that don't represent index expressions, the first return value of |
| // UnpackIndexExpr will be nil. |
| func UnpackIndexExpr(n ast.Node) (x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) { |
| switch e := n.(type) { |
| case *ast.IndexExpr: |
| return e.X, e.Lbrack, []ast.Expr{e.Index}, e.Rbrack |
| case *IndexListExpr: |
| return e.X, e.Lbrack, e.Indices, e.Rbrack |
| } |
| return nil, token.NoPos, nil, token.NoPos |
| } |
| |
| // PackIndexExpr returns an *ast.IndexExpr or *ast.IndexListExpr, depending on |
| // the cardinality of indices. Calling PackIndexExpr with len(indices) == 0 |
| // will panic. |
| func PackIndexExpr(x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) ast.Expr { |
| switch len(indices) { |
| case 0: |
| panic("empty indices") |
| case 1: |
| return &ast.IndexExpr{ |
| X: x, |
| Lbrack: lbrack, |
| Index: indices[0], |
| Rbrack: rbrack, |
| } |
| default: |
| return &IndexListExpr{ |
| X: x, |
| Lbrack: lbrack, |
| Indices: indices, |
| Rbrack: rbrack, |
| } |
| } |
| } |
| |
| // IsTypeParam reports whether t is a type parameter. |
| func IsTypeParam(t types.Type) bool { |
| _, ok := t.(*TypeParam) |
| return ok |
| } |
| |
| // OriginMethod returns the origin method associated with the method fn. |
| // For methods on a non-generic receiver base type, this is just |
| // fn. However, for methods with a generic receiver, OriginMethod returns the |
| // corresponding method in the method set of the origin type. |
| // |
| // As a special case, if fn is not a method (has no receiver), OriginMethod |
| // returns fn. |
| func OriginMethod(fn *types.Func) *types.Func { |
| recv := fn.Type().(*types.Signature).Recv() |
| if recv == nil { |
| return fn |
| } |
| base := recv.Type() |
| p, isPtr := base.(*types.Pointer) |
| if isPtr { |
| base = p.Elem() |
| } |
| named, isNamed := base.(*types.Named) |
| if !isNamed { |
| // Receiver is a *types.Interface. |
| return fn |
| } |
| if ForNamed(named).Len() == 0 { |
| // Receiver base has no type parameters, so we can avoid the lookup below. |
| return fn |
| } |
| orig := NamedTypeOrigin(named) |
| gfn, _, _ := types.LookupFieldOrMethod(orig, true, fn.Pkg(), fn.Name()) |
| return gfn.(*types.Func) |
| } |
| |
| // GenericAssignableTo is a generalization of types.AssignableTo that |
| // implements the following rule for uninstantiated generic types: |
| // |
| // If V and T are generic named types, then V is considered assignable to T if, |
| // for every possible instantation of V[A_1, ..., A_N], the instantiation |
| // T[A_1, ..., A_N] is valid and V[A_1, ..., A_N] implements T[A_1, ..., A_N]. |
| // |
| // If T has structural constraints, they must be satisfied by V. |
| // |
| // For example, consider the following type declarations: |
| // |
| // type Interface[T any] interface { |
| // Accept(T) |
| // } |
| // |
| // type Container[T any] struct { |
| // Element T |
| // } |
| // |
| // func (c Container[T]) Accept(t T) { c.Element = t } |
| // |
| // In this case, GenericAssignableTo reports that instantiations of Container |
| // are assignable to the corresponding instantiation of Interface. |
| func GenericAssignableTo(ctxt *Context, V, T types.Type) bool { |
| // If V and T are not both named, or do not have matching non-empty type |
| // parameter lists, fall back on types.AssignableTo. |
| |
| VN, Vnamed := V.(*types.Named) |
| TN, Tnamed := T.(*types.Named) |
| if !Vnamed || !Tnamed { |
| return types.AssignableTo(V, T) |
| } |
| |
| vtparams := ForNamed(VN) |
| ttparams := ForNamed(TN) |
| if vtparams.Len() == 0 || vtparams.Len() != ttparams.Len() || NamedTypeArgs(VN).Len() != 0 || NamedTypeArgs(TN).Len() != 0 { |
| return types.AssignableTo(V, T) |
| } |
| |
| // V and T have the same (non-zero) number of type params. Instantiate both |
| // with the type parameters of V. This must always succeed for V, and will |
| // succeed for T if and only if the type set of each type parameter of V is a |
| // subset of the type set of the corresponding type parameter of T, meaning |
| // that every instantiation of V corresponds to a valid instantiation of T. |
| |
| // Minor optimization: ensure we share a context across the two |
| // instantiations below. |
| if ctxt == nil { |
| ctxt = NewContext() |
| } |
| |
| var targs []types.Type |
| for i := 0; i < vtparams.Len(); i++ { |
| targs = append(targs, vtparams.At(i)) |
| } |
| |
| vinst, err := Instantiate(ctxt, V, targs, true) |
| if err != nil { |
| panic("type parameters should satisfy their own constraints") |
| } |
| |
| tinst, err := Instantiate(ctxt, T, targs, true) |
| if err != nil { |
| return false |
| } |
| |
| return types.AssignableTo(vinst, tinst) |
| } |