Using clang-query

clang-query is a tool that allows you to quickly iterate and develop the difficult part of a matcher. Once the design of the matcher is completed, it can be transferred to a C++ clang-tidy plugin, similar to the ones in mozilla-central.

clang-query Options

set traversal

Traversal mode specifies how the AST Matcher will traverse the nodes in the Abstract Syntax Tree. There are two values:

AsIs

This mode notes all the nodes in the AST, even if they are not explicitly spelled out in the source. This will include nodes you have never seen and probably don’t immediately understand, for example ExprWithCleanups and MaterializeTemporaryExpr. In this mode, it is necessary to write matchers that expliticly match or otherwise traverse these potentially unexpected nodes.

IgnoreUnlessSpelledInSource

This mode skips over ‘implicit’ nodes that are created as a result of implicit casts or other usually-low-level language details. This is typically much more user-friendly. Typically you would want to use set traversal IgnoreUnlessSpelledInSource.

More examples are available in the documentation, but here is a simple example:

B func1() {
  return 42;
}

/*
  AST Dump in 'Asis' mode for C++17/C++20 dialect:

  FunctionDecl
  `-CompoundStmt
    `-ReturnStmt
      `-ImplicitCastExpr
        `-CXXConstructExpr
          `-IntegerLiteral 'int' 42

  AST Dump in 'IgnoreUnlessSpelledInSource' mode for all dialects:

  FunctionDecl
  `-CompoundStmt
    `-ReturnStmt
      `-IntegerLiteral 'int' 42
*/

set bind-root

If you are matching objects and assigning them names for later use, this option may be relevant. If you are debugging a single matcher and not using any .bind(), it is irrelevant.

Consider the output of match functionDecl().bind("x"):

clang-query> match functionDecl().bind("x")

Match #1:

testfile.cpp:1:1: note: "root" binds here
int addTwo(int num)
^~~~~~~~~~~~~~~~~~~
testfile.cpp:1:1: note: "x" binds here
int addTwo(int num)
^~~~~~~~~~~~~~~~~~~

Match #2:

testfile.cpp:6:1: note: "root" binds here
int main(int, char**)
^~~~~~~~~~~~~~~~~~~~~
testfile.cpp:6:1: note: "x" binds here
int main(int, char**)
^~~~~~~~~~~~~~~~~~~~~
2 matches.

clang-query automatically binds root to the match, but we also bound the name x to that match. The root is redundant. If you set bind-root false then the output is less noisy:

clang-query> set bind-root false
clang-query> m functionDecl().bind("x")

Match #1:

testfile.cpp:1:1: note: "x" binds here
int addtwo(int num)
^~~~~~~~~~~~~~~~~~~

Match #2:

testfile.cpp:6:1: note: "x" binds here
int main(int, char**)
^~~~~~~~~~~~~~~~~~~~~
2 matches.

set print-matcher

set print-matcher true will print a header line of the form ‘Matcher: <foo>’ where foo is the matcher you have written. It is helpful when debugging multiple matchers at the same time, and no inconvience otherwise.

enable/disable/set output <foo>

These commands will control the type of output you get from clang-query. The options are:

print

Shows you the C++ form of the node you are matching. This is typically not useful.

diag

Shows you the individual node you are matching.

dump (alias: detailed-ast)

Shows you the node you are matching and the entire subtree for the node

By default, you get diag output. You can change the output by choosing set output. You can add output by using enable output. You can disable output using disable output but this is typically not needed.

So if you want to get all three output formats you can do:

# diag output happens automatically because you did not override with 'set'
enable output print
enable output dump

Patches

This section tracks some patches; they are currently not used, but we may want them in the future.

  • Functionality:

  • Matcher Changes:

  • Plumbing: