Debugger Visualizers

Many languages and debuggers enable developers to control how a type is displayed in a debugger. These are called “debugger visualizations” or “debugger views”.

The Windows debuggers (WinDbg\CDB) support defining custom debugger visualizations using the Natvis framework. To use Natvis, developers write XML documents using the natvis schema that describe how debugger types should be displayed with the .natvis extension. (See: https://docs.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects?view=vs-2019) The Natvis files provide patterns which match type names a description of how to display those types.

The Natvis schema can be found either online (See: https://code.visualstudio.com/docs/cpp/natvis#_schema) or locally at <VS Installation Folder>\Xml\Schemas\1033\natvis.xsd.

The GNU debugger (GDB) supports defining custom debugger views using Pretty Printers. Pretty printers are written as python scripts that describe how a type should be displayed when loaded up in GDB/LLDB. (See: https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing.html#Pretty-Printing) The pretty printers provide patterns, which match type names, and for matching types, descibe how to display those types. (For writing a pretty printer, see: https://sourceware.org/gdb/onlinedocs/gdb/Writing-a-Pretty_002dPrinter.html#Writing-a-Pretty_002dPrinter).

Embedding Visualizers

Through the use of the currently unstable #[debugger_visualizer] attribute, the smallvec crate can embed debugger visualizers into the crate metadata.

Currently the two types of visualizers supported are Natvis and Pretty printers.

For Natvis files, when linking an executable with a crate that includes Natvis files, the MSVC linker will embed the contents of all Natvis files into the generated PDB.

For pretty printers, the compiler will encode the contents of the pretty printer in the .debug_gdb_scripts section of the ELF generated.

Testing Visualizers

The smallvec crate supports testing debugger visualizers defined for this crate. The entry point for these tests are tests/debugger_visualizer.rs. These tests are defined using the debugger_test and debugger_test_parser crates. The debugger_test crate is a proc macro crate which defines a single proc macro attribute, #[debugger_test]. For more detailed information about this crate, see https://crates.io/crates/debugger_test. The CI pipeline for the smallvec crate has been updated to run the debugger visualizer tests to ensure debugger visualizers do not become broken/stale.

The #[debugger_test] proc macro attribute may only be used on test functions and will run the function under the debugger specified by the debugger meta item.

This proc macro attribute has 3 required values:

1. The first required meta item, debugger, takes a string value which specifies the debugger to launch.
2. The second required meta item, commands, takes a string of new line (\n) separated list of debugger commands to run.
3. The third required meta item, expected_statements, takes a string of new line (\n) separated list of statements that must exist in the debugger output. Pattern matching through regular expressions is also supported by using the pattern: prefix for each expected statement.

Example:

#[debugger_test(
    debugger = "cdb",
    commands = "command1\ncommand2\ncommand3",
    expected_statements = "statement1\nstatement2\nstatement3")]
fn test() {

}

Using a multiline string is also supported, with a single debugger command/expected statement per line:

#[debugger_test(
    debugger = "cdb",
    commands = "
command1
command2
command3",
    expected_statements = "
statement1
pattern:statement[0-9]+
statement3")]
fn test() {
    
}

In the example above, the second expected statement uses pattern matching through a regular expression by using the pattern: prefix.

Testing Locally

Currently, only Natvis visualizations have been defined for the smallvec crate via debug_metadata/smallvec.natvis, which means the tests/debugger_visualizer.rs tests need to be run on Windows using the *-pc-windows-msvc targets. To run these tests locally, first ensure the debugging tools for Windows are installed or install them following the steps listed here, Debugging Tools for Windows. Once the debugging tools have been installed, the tests can be run in the same manner as they are in the CI pipeline.

Note

When running the debugger visualizer tests, tests/debugger_visualizer.rs, they need to be run consecutively and not in parallel. This can be achieved by passing the flag --test-threads=1 to rustc. This is due to how the debugger tests are run. Each test marked with the #[debugger_test] attribute launches a debugger and attaches it to the current test process. If tests are running in parallel, the test will try to attach a debugger to the current process which may already have a debugger attached causing the test to fail.

For example:

cargo test --test debugger_visualizer --features debugger_visualizer -- --test-threads=1