1//===--- Selection.h - What's under the cursor? -------------------*-C++-*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8// Many features are triggered at locations/ranges and operate on AST nodes.
9// (e.g. go-to-definition or code tweaks).
10// At a high level, such features need to work out which node is the correct
11// target.
12//
13// There are a few levels of ambiguity here:
14//
15// Which tokens are included:
16// int x = one + two; // what should "go to definition" do?
17// ^^^^^^
18//
19// Same token means multiple things:
20// string("foo") // class string, or a constructor?
21// ^
22//
23// Which level of the AST is interesting?
24// if (err) { // reference to 'err', or operator bool(),
25// ^ // or the if statement itself?
26//
27// Here we build and expose a data structure that allows features to resolve
28// these ambiguities in an appropriate way:
29// - we determine which low-level nodes are partly or completely covered
30// by the selection.
31// - we expose a tree of the selected nodes and their lexical parents.
32//
33// Sadly LSP specifies locations as being between characters, and this causes
34// some ambiguities we cannot cleanly resolve:
35// lhs+rhs // targeting '+' or 'lhs'?
36// ^ // in GUI editors, double-clicking 'lhs' yields this position!
37//
38// The best we can do in these cases is try both, which leads to the awkward
39// SelectionTree::createEach() API.
40//===----------------------------------------------------------------------===//
41
42#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SELECTION_H
43#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SELECTION_H
44#include "clang/AST/ASTTypeTraits.h"
45#include "clang/AST/PrettyPrinter.h"
46#include "clang/Tooling/Syntax/Tokens.h"
47#include "llvm/ADT/SmallVector.h"
48#include <stack>
49
50namespace clang {
51namespace clangd {
52
53// A selection can partially or completely cover several AST nodes.
54// The SelectionTree contains nodes that are covered, and their parents.
55// SelectionTree does not contain all AST nodes, rather only:
56// Decl, Stmt, TypeLoc, NestedNamespaceSpecifierLoc, CXXCtorInitializer.
57// (These are the nodes with source ranges that fit in DynTypedNode).
58//
59// Usually commonAncestor() is the place to start:
60// - it's the simplest answer to "what node is under the cursor"
61// - the selected Expr (for example) can be found by walking up the parent
62// chain and checking Node->ASTNode.
63// - if you want to traverse the selected nodes, they are all under
64// commonAncestor() in the tree.
65//
66// SelectionTree tries to behave sensibly in the presence of macros, but does
67// not model any preprocessor concepts: the output is a subset of the AST.
68// When a macro argument is specifically selected, only its first expansion is
69// selected in the AST. (Returning a selection forest is unreasonably difficult
70// for callers to handle correctly.)
71//
72// Comments, directives and whitespace are completely ignored.
73// Semicolons are also ignored, as the AST generally does not model them well.
74//
75// The SelectionTree owns the Node structures, but the ASTNode attributes
76// point back into the AST it was constructed with.
77class SelectionTree {
78public:
79 // Create selection trees for the given range, and pass them to Func.
80 //
81 // There may be multiple possible selection trees:
82 // - if the range is empty and borders two tokens, a tree for the right token
83 // and a tree for the left token will be yielded.
84 // - Func should return true on success (stop) and false on failure (continue)
85 //
86 // Always yields at least one tree. If no tokens are touched, it is empty.
87 static bool createEach(ASTContext &AST, const syntax::TokenBuffer &Tokens,
88 unsigned Begin, unsigned End,
89 llvm::function_ref<bool(SelectionTree)> Func);
90
91 // Create a selection tree for the given range.
92 //
93 // Where ambiguous (range is empty and borders two tokens), prefer the token
94 // on the right.
95 static SelectionTree createRight(ASTContext &AST,
96 const syntax::TokenBuffer &Tokens,
97 unsigned Begin, unsigned End);
98
99 // Copies are no good - contain pointers to other nodes.
100 SelectionTree(const SelectionTree &) = delete;
101 SelectionTree &operator=(const SelectionTree &) = delete;
102 // Moves are OK though - internal storage is pointer-stable when moved.
103 SelectionTree(SelectionTree &&) = default;
104 SelectionTree &operator=(SelectionTree &&) = default;
105
106 // Describes to what extent an AST node is covered by the selection.
107 enum Selection : unsigned char {
108 // The AST node owns no characters covered by the selection.
109 // Note that characters owned by children don't count:
110 // if (x == 0) scream();
111 // ^^^^^^
112 // The IfStmt would be Unselected because all the selected characters are
113 // associated with its children.
114 // (Invisible nodes like ImplicitCastExpr are always unselected).
115 Unselected,
116 // The AST node owns selected characters, but is not completely covered.
117 Partial,
118 // The AST node owns characters, and is covered by the selection.
119 Complete,
120 };
121 // An AST node that is implicated in the selection.
122 // (Either selected directly, or some descendant is selected).
123 struct Node {
124 // The parent within the selection tree. nullptr for TranslationUnitDecl.
125 Node *Parent;
126 // Direct children within the selection tree.
127 llvm::SmallVector<const Node *> Children;
128 // The corresponding node from the full AST.
129 DynTypedNode ASTNode;
130 // The extent to which this node is covered by the selection.
131 Selection Selected;
132 // Walk up the AST to get the lexical DeclContext of this Node, which is not
133 // the node itself.
134 const DeclContext &getDeclContext() const;
135 // Printable node kind, like "CXXRecordDecl" or "AutoTypeLoc".
136 std::string kind() const;
137 // If this node is a wrapper with no syntax (e.g. implicit cast), return
138 // its contents. (If multiple wrappers are present, unwraps all of them).
139 const Node &ignoreImplicit() const;
140 // If this node is inside a wrapper with no syntax (e.g. implicit cast),
141 // return that wrapper. (If multiple are present, unwraps all of them).
142 const Node &outerImplicit() const;
143 };
144 // The most specific common ancestor of all the selected nodes.
145 // Returns nullptr if the common ancestor is the root.
146 // (This is to avoid accidentally traversing the TUDecl and thus preamble).
147 const Node *commonAncestor() const;
148 // The selection node corresponding to TranslationUnitDecl.
149 const Node &root() const { return *Root; }
150
151private:
152 // Creates a selection tree for the given range in the main file.
153 // The range includes bytes [Start, End).
154 SelectionTree(ASTContext &AST, const syntax::TokenBuffer &Tokens,
155 unsigned Start, unsigned End);
156
157 std::deque<Node> Nodes; // Stable-pointer storage.
158 const Node *Root;
159 clang::PrintingPolicy PrintPolicy;
160
161 void print(llvm::raw_ostream &OS, const Node &N, int Indent) const;
162 friend llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
163 const SelectionTree &T) {
164 T.print(OS, N: T.root(), Indent: 1);
165 return OS;
166 }
167};
168
169} // namespace clangd
170} // namespace clang
171#endif
172