| 1 | // Copyright 2021 The Go Authors. All rights reserved. |
|---|---|
| 2 | // Use of this source code is governed by a BSD-style |
| 3 | // license that can be found in the LICENSE file. |
| 4 | |
| 5 | // Package typeparams contains common utilities for writing tools that interact |
| 6 | // with generic Go code, as introduced with Go 1.18. |
| 7 | // |
| 8 | // Many of the types and functions in this package are proxies for the new APIs |
| 9 | // introduced in the standard library with Go 1.18. For example, the |
| 10 | // typeparams.Union type is an alias for go/types.Union, and the ForTypeSpec |
| 11 | // function returns the value of the go/ast.TypeSpec.TypeParams field. At Go |
| 12 | // versions older than 1.18 these helpers are implemented as stubs, allowing |
| 13 | // users of this package to write code that handles generic constructs inline, |
| 14 | // even if the Go version being used to compile does not support generics. |
| 15 | // |
| 16 | // Additionally, this package contains common utilities for working with the |
| 17 | // new generic constructs, to supplement the standard library APIs. Notably, |
| 18 | // the StructuralTerms API computes a minimal representation of the structural |
| 19 | // restrictions on a type parameter. |
| 20 | // |
| 21 | // An external version of these APIs is available in the |
| 22 | // golang.org/x/exp/typeparams module. |
| 23 | package typeparams |
| 24 | |
| 25 | import ( |
| 26 | "go/ast" |
| 27 | "go/token" |
| 28 | "go/types" |
| 29 | ) |
| 30 | |
| 31 | // UnpackIndexExpr extracts data from AST nodes that represent index |
| 32 | // expressions. |
| 33 | // |
| 34 | // For an ast.IndexExpr, the resulting indices slice will contain exactly one |
| 35 | // index expression. For an ast.IndexListExpr (go1.18+), it may have a variable |
| 36 | // number of index expressions. |
| 37 | // |
| 38 | // For nodes that don't represent index expressions, the first return value of |
| 39 | // UnpackIndexExpr will be nil. |
| 40 | func UnpackIndexExpr(n ast.Node) (x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) { |
| 41 | switch e := n.(type) { |
| 42 | case *ast.IndexExpr: |
| 43 | return e.X, e.Lbrack, []ast.Expr{e.Index}, e.Rbrack |
| 44 | case *IndexListExpr: |
| 45 | return e.X, e.Lbrack, e.Indices, e.Rbrack |
| 46 | } |
| 47 | return nil, token.NoPos, nil, token.NoPos |
| 48 | } |
| 49 | |
| 50 | // PackIndexExpr returns an *ast.IndexExpr or *ast.IndexListExpr, depending on |
| 51 | // the cardinality of indices. Calling PackIndexExpr with len(indices) == 0 |
| 52 | // will panic. |
| 53 | func PackIndexExpr(x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) ast.Expr { |
| 54 | switch len(indices) { |
| 55 | case 0: |
| 56 | panic("empty indices") |
| 57 | case 1: |
| 58 | return &ast.IndexExpr{ |
| 59 | X: x, |
| 60 | Lbrack: lbrack, |
| 61 | Index: indices[0], |
| 62 | Rbrack: rbrack, |
| 63 | } |
| 64 | default: |
| 65 | return &IndexListExpr{ |
| 66 | X: x, |
| 67 | Lbrack: lbrack, |
| 68 | Indices: indices, |
| 69 | Rbrack: rbrack, |
| 70 | } |
| 71 | } |
| 72 | } |
| 73 | |
| 74 | // IsTypeParam reports whether t is a type parameter. |
| 75 | func IsTypeParam(t types.Type) bool { |
| 76 | _, ok := t.(*TypeParam) |
| 77 | return ok |
| 78 | } |
| 79 | |
| 80 | // OriginMethod returns the origin method associated with the method fn. |
| 81 | // For methods on a non-generic receiver base type, this is just |
| 82 | // fn. However, for methods with a generic receiver, OriginMethod returns the |
| 83 | // corresponding method in the method set of the origin type. |
| 84 | // |
| 85 | // As a special case, if fn is not a method (has no receiver), OriginMethod |
| 86 | // returns fn. |
| 87 | func OriginMethod(fn *types.Func) *types.Func { |
| 88 | recv := fn.Type().(*types.Signature).Recv() |
| 89 | if recv == nil { |
| 90 | |
| 91 | return fn |
| 92 | } |
| 93 | base := recv.Type() |
| 94 | p, isPtr := base.(*types.Pointer) |
| 95 | if isPtr { |
| 96 | base = p.Elem() |
| 97 | } |
| 98 | named, isNamed := base.(*types.Named) |
| 99 | if !isNamed { |
| 100 | // Receiver is a *types.Interface. |
| 101 | return fn |
| 102 | } |
| 103 | if ForNamed(named).Len() == 0 { |
| 104 | // Receiver base has no type parameters, so we can avoid the lookup below. |
| 105 | return fn |
| 106 | } |
| 107 | orig := NamedTypeOrigin(named) |
| 108 | gfn, _, _ := types.LookupFieldOrMethod(orig, true, fn.Pkg(), fn.Name()) |
| 109 | return gfn.(*types.Func) |
| 110 | } |
| 111 | |
| 112 | // GenericAssignableTo is a generalization of types.AssignableTo that |
| 113 | // implements the following rule for uninstantiated generic types: |
| 114 | // |
| 115 | // If V and T are generic named types, then V is considered assignable to T if, |
| 116 | // for every possible instantation of V[A_1, ..., A_N], the instantiation |
| 117 | // T[A_1, ..., A_N] is valid and V[A_1, ..., A_N] implements T[A_1, ..., A_N]. |
| 118 | // |
| 119 | // If T has structural constraints, they must be satisfied by V. |
| 120 | // |
| 121 | // For example, consider the following type declarations: |
| 122 | // |
| 123 | // type Interface[T any] interface { |
| 124 | // Accept(T) |
| 125 | // } |
| 126 | // |
| 127 | // type Container[T any] struct { |
| 128 | // Element T |
| 129 | // } |
| 130 | // |
| 131 | // func (c Container[T]) Accept(t T) { c.Element = t } |
| 132 | // |
| 133 | // In this case, GenericAssignableTo reports that instantiations of Container |
| 134 | // are assignable to the corresponding instantiation of Interface. |
| 135 | func GenericAssignableTo(ctxt *Context, V, T types.Type) bool { |
| 136 | // If V and T are not both named, or do not have matching non-empty type |
| 137 | // parameter lists, fall back on types.AssignableTo. |
| 138 | |
| 139 | VN, Vnamed := V.(*types.Named) |
| 140 | TN, Tnamed := T.(*types.Named) |
| 141 | if !Vnamed || !Tnamed { |
| 142 | return types.AssignableTo(V, T) |
| 143 | } |
| 144 | |
| 145 | vtparams := ForNamed(VN) |
| 146 | ttparams := ForNamed(TN) |
| 147 | if vtparams.Len() == 0 || vtparams.Len() != ttparams.Len() || NamedTypeArgs(VN).Len() != 0 || NamedTypeArgs(TN).Len() != 0 { |
| 148 | return types.AssignableTo(V, T) |
| 149 | } |
| 150 | |
| 151 | // V and T have the same (non-zero) number of type params. Instantiate both |
| 152 | // with the type parameters of V. This must always succeed for V, and will |
| 153 | // succeed for T if and only if the type set of each type parameter of V is a |
| 154 | // subset of the type set of the corresponding type parameter of T, meaning |
| 155 | // that every instantiation of V corresponds to a valid instantiation of T. |
| 156 | |
| 157 | // Minor optimization: ensure we share a context across the two |
| 158 | // instantiations below. |
| 159 | if ctxt == nil { |
| 160 | ctxt = NewContext() |
| 161 | } |
| 162 | |
| 163 | var targs []types.Type |
| 164 | for i := 0; i < vtparams.Len(); i++ { |
| 165 | targs = append(targs, vtparams.At(i)) |
| 166 | } |
| 167 | |
| 168 | vinst, err := Instantiate(ctxt, V, targs, true) |
| 169 | if err != nil { |
| 170 | panic("type parameters should satisfy their own constraints") |
| 171 | } |
| 172 | |
| 173 | tinst, err := Instantiate(ctxt, T, targs, true) |
| 174 | if err != nil { |
| 175 | return false |
| 176 | } |
| 177 | |
| 178 | return types.AssignableTo(vinst, tinst) |
| 179 | } |
| 180 |
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