mono.go

Documentation: go/types

     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 types
     6  
     7  import (
     8  	"go/ast"
     9  	"go/token"
    10  )
    11  
    12  // This file implements a check to validate that a Go package doesn't
    13  // have unbounded recursive instantiation, which is not compatible
    14  // with compilers using static instantiation (such as
    15  // monomorphization).
    16  //
    17  // It implements a sort of "type flow" analysis by detecting which
    18  // type parameters are instantiated with other type parameters (or
    19  // types derived thereof). A package cannot be statically instantiated
    20  // if the graph has any cycles involving at least one derived type.
    21  //
    22  // Concretely, we construct a directed, weighted graph. Vertices are
    23  // used to represent type parameters as well as some defined
    24  // types. Edges are used to represent how types depend on each other:
    25  //
    26  // * Everywhere a type-parameterized function or type is instantiated,
    27  //   we add edges to each type parameter from the vertices (if any)
    28  //   representing each type parameter or defined type referenced by
    29  //   the type argument. If the type argument is just the referenced
    30  //   type itself, then the edge has weight 0, otherwise 1.
    31  //
    32  // * For every defined type declared within a type-parameterized
    33  //   function or method, we add an edge of weight 1 to the defined
    34  //   type from each ambient type parameter.
    35  //
    36  // For example, given:
    37  //
    38  //	func f[A, B any]() {
    39  //		type T int
    40  //		f[T, map[A]B]()
    41  //	}
    42  //
    43  // we construct vertices representing types A, B, and T. Because of
    44  // declaration "type T int", we construct edges T<-A and T<-B with
    45  // weight 1; and because of instantiation "f[T, map[A]B]" we construct
    46  // edges A<-T with weight 0, and B<-A and B<-B with weight 1.
    47  //
    48  // Finally, we look for any positive-weight cycles. Zero-weight cycles
    49  // are allowed because static instantiation will reach a fixed point.
    50  
    51  type monoGraph struct {
    52  	vertices []monoVertex
    53  	edges    []monoEdge
    54  
    55  	// canon maps method receiver type parameters to their respective
    56  	// receiver type's type parameters.
    57  	canon map[*TypeParam]*TypeParam
    58  
    59  	// nameIdx maps a defined type or (canonical) type parameter to its
    60  	// vertex index.
    61  	nameIdx map[*TypeName]int
    62  }
    63  
    64  type monoVertex struct {
    65  	weight int // weight of heaviest known path to this vertex
    66  	pre    int // previous edge (if any) in the above path
    67  	len    int // length of the above path
    68  
    69  	// obj is the defined type or type parameter represented by this
    70  	// vertex.
    71  	obj *TypeName
    72  }
    73  
    74  type monoEdge struct {
    75  	dst, src int
    76  	weight   int
    77  
    78  	pos token.Pos
    79  	typ Type
    80  }
    81  
    82  func (check *Checker) monomorph() {
    83  	// We detect unbounded instantiation cycles using a variant of
    84  	// Bellman-Ford's algorithm. Namely, instead of always running |V|
    85  	// iterations, we run until we either reach a fixed point or we've
    86  	// found a path of length |V|. This allows us to terminate earlier
    87  	// when there are no cycles, which should be the common case.
    88  
    89  	again := true
    90  	for again {
    91  		again = false
    92  
    93  		for i, edge := range check.mono.edges {
    94  			src := &check.mono.vertices[edge.src]
    95  			dst := &check.mono.vertices[edge.dst]
    96  
    97  			// N.B., we're looking for the greatest weight paths, unlike
    98  			// typical Bellman-Ford.
    99  			w := src.weight + edge.weight
   100  			if w <= dst.weight {
   101  				continue
   102  			}
   103  
   104  			dst.pre = i
   105  			dst.len = src.len + 1
   106  			if dst.len == len(check.mono.vertices) {
   107  				check.reportInstanceLoop(edge.dst)
   108  				return
   109  			}
   110  
   111  			dst.weight = w
   112  			again = true
   113  		}
   114  	}
   115  }
   116  
   117  func (check *Checker) reportInstanceLoop(v int) {
   118  	var stack []int
   119  	seen := make([]bool, len(check.mono.vertices))
   120  
   121  	// We have a path that contains a cycle and ends at v, but v may
   122  	// only be reachable from the cycle, not on the cycle itself. We
   123  	// start by walking backwards along the path until we find a vertex
   124  	// that appears twice.
   125  	for !seen[v] {
   126  		stack = append(stack, v)
   127  		seen[v] = true
   128  		v = check.mono.edges[check.mono.vertices[v].pre].src
   129  	}
   130  
   131  	// Trim any vertices we visited before visiting v the first
   132  	// time. Since v is the first vertex we found within the cycle, any
   133  	// vertices we visited earlier cannot be part of the cycle.
   134  	for stack[0] != v {
   135  		stack = stack[1:]
   136  	}
   137  
   138  	// TODO(mdempsky): Pivot stack so we report the cycle from the top?
   139  
   140  	obj0 := check.mono.vertices[v].obj
   141  	check.errorf(obj0, _InvalidInstanceCycle, "instantiation cycle:")
   142  
   143  	qf := RelativeTo(check.pkg)
   144  	for _, v := range stack {
   145  		edge := check.mono.edges[check.mono.vertices[v].pre]
   146  		obj := check.mono.vertices[edge.dst].obj
   147  
   148  		switch obj.Type().(type) {
   149  		default:
   150  			panic("unexpected type")
   151  		case *Named:
   152  			check.errorf(atPos(edge.pos), _InvalidInstanceCycle, "\t%s implicitly parameterized by %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
   153  		case *TypeParam:
   154  			check.errorf(atPos(edge.pos), _InvalidInstanceCycle, "\t%s instantiated as %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
   155  		}
   156  	}
   157  }
   158  
   159  // recordCanon records that tpar is the canonical type parameter
   160  // corresponding to method type parameter mpar.
   161  func (w *monoGraph) recordCanon(mpar, tpar *TypeParam) {
   162  	if w.canon == nil {
   163  		w.canon = make(map[*TypeParam]*TypeParam)
   164  	}
   165  	w.canon[mpar] = tpar
   166  }
   167  
   168  // recordInstance records that the given type parameters were
   169  // instantiated with the corresponding type arguments.
   170  func (w *monoGraph) recordInstance(pkg *Package, pos token.Pos, tparams []*TypeParam, targs []Type, xlist []ast.Expr) {
   171  	for i, tpar := range tparams {
   172  		pos := pos
   173  		if i < len(xlist) {
   174  			pos = xlist[i].Pos()
   175  		}
   176  		w.assign(pkg, pos, tpar, targs[i])
   177  	}
   178  }
   179  
   180  // assign records that tpar was instantiated as targ at pos.
   181  func (w *monoGraph) assign(pkg *Package, pos token.Pos, tpar *TypeParam, targ Type) {
   182  	// Go generics do not have an analog to C++`s template-templates,
   183  	// where a template parameter can itself be an instantiable
   184  	// template. So any instantiation cycles must occur within a single
   185  	// package. Accordingly, we can ignore instantiations of imported
   186  	// type parameters.
   187  	//
   188  	// TODO(mdempsky): Push this check up into recordInstance? All type
   189  	// parameters in a list will appear in the same package.
   190  	if tpar.Obj().Pkg() != pkg {
   191  		return
   192  	}
   193  
   194  	// flow adds an edge from vertex src representing that typ flows to tpar.
   195  	flow := func(src int, typ Type) {
   196  		weight := 1
   197  		if typ == targ {
   198  			weight = 0
   199  		}
   200  
   201  		w.addEdge(w.typeParamVertex(tpar), src, weight, pos, targ)
   202  	}
   203  
   204  	// Recursively walk the type argument to find any defined types or
   205  	// type parameters.
   206  	var do func(typ Type)
   207  	do = func(typ Type) {
   208  		switch typ := typ.(type) {
   209  		default:
   210  			panic("unexpected type")
   211  
   212  		case *TypeParam:
   213  			assert(typ.Obj().Pkg() == pkg)
   214  			flow(w.typeParamVertex(typ), typ)
   215  
   216  		case *Named:
   217  			if src := w.localNamedVertex(pkg, typ.Origin()); src >= 0 {
   218  				flow(src, typ)
   219  			}
   220  
   221  			targs := typ.TypeArgs()
   222  			for i := 0; i < targs.Len(); i++ {
   223  				do(targs.At(i))
   224  			}
   225  
   226  		case *Array:
   227  			do(typ.Elem())
   228  		case *Basic:
   229  			// ok
   230  		case *Chan:
   231  			do(typ.Elem())
   232  		case *Map:
   233  			do(typ.Key())
   234  			do(typ.Elem())
   235  		case *Pointer:
   236  			do(typ.Elem())
   237  		case *Slice:
   238  			do(typ.Elem())
   239  
   240  		case *Interface:
   241  			for i := 0; i < typ.NumMethods(); i++ {
   242  				do(typ.Method(i).Type())
   243  			}
   244  		case *Signature:
   245  			tuple := func(tup *Tuple) {
   246  				for i := 0; i < tup.Len(); i++ {
   247  					do(tup.At(i).Type())
   248  				}
   249  			}
   250  			tuple(typ.Params())
   251  			tuple(typ.Results())
   252  		case *Struct:
   253  			for i := 0; i < typ.NumFields(); i++ {
   254  				do(typ.Field(i).Type())
   255  			}
   256  		}
   257  	}
   258  	do(targ)
   259  }
   260  
   261  // localNamedVertex returns the index of the vertex representing
   262  // named, or -1 if named doesn't need representation.
   263  func (w *monoGraph) localNamedVertex(pkg *Package, named *Named) int {
   264  	obj := named.Obj()
   265  	if obj.Pkg() != pkg {
   266  		return -1 // imported type
   267  	}
   268  
   269  	root := pkg.Scope()
   270  	if obj.Parent() == root {
   271  		return -1 // package scope, no ambient type parameters
   272  	}
   273  
   274  	if idx, ok := w.nameIdx[obj]; ok {
   275  		return idx
   276  	}
   277  
   278  	idx := -1
   279  
   280  	// Walk the type definition's scope to find any ambient type
   281  	// parameters that it's implicitly parameterized by.
   282  	for scope := obj.Parent(); scope != root; scope = scope.Parent() {
   283  		for _, elem := range scope.elems {
   284  			if elem, ok := elem.(*TypeName); ok && !elem.IsAlias() && elem.Pos() < obj.Pos() {
   285  				if tpar, ok := elem.Type().(*TypeParam); ok {
   286  					if idx < 0 {
   287  						idx = len(w.vertices)
   288  						w.vertices = append(w.vertices, monoVertex{obj: obj})
   289  					}
   290  
   291  					w.addEdge(idx, w.typeParamVertex(tpar), 1, obj.Pos(), tpar)
   292  				}
   293  			}
   294  		}
   295  	}
   296  
   297  	if w.nameIdx == nil {
   298  		w.nameIdx = make(map[*TypeName]int)
   299  	}
   300  	w.nameIdx[obj] = idx
   301  	return idx
   302  }
   303  
   304  // typeParamVertex returns the index of the vertex representing tpar.
   305  func (w *monoGraph) typeParamVertex(tpar *TypeParam) int {
   306  	if x, ok := w.canon[tpar]; ok {
   307  		tpar = x
   308  	}
   309  
   310  	obj := tpar.Obj()
   311  
   312  	if idx, ok := w.nameIdx[obj]; ok {
   313  		return idx
   314  	}
   315  
   316  	if w.nameIdx == nil {
   317  		w.nameIdx = make(map[*TypeName]int)
   318  	}
   319  
   320  	idx := len(w.vertices)
   321  	w.vertices = append(w.vertices, monoVertex{obj: obj})
   322  	w.nameIdx[obj] = idx
   323  	return idx
   324  }
   325  
   326  func (w *monoGraph) addEdge(dst, src, weight int, pos token.Pos, typ Type) {
   327  	// TODO(mdempsky): Deduplicate redundant edges?
   328  	w.edges = append(w.edges, monoEdge{
   329  		dst:    dst,
   330  		src:    src,
   331  		weight: weight,
   332  
   333  		pos: pos,
   334  		typ: typ,
   335  	})
   336  }
   337
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