Debugger attributes

The following attributes are used for enhancing the debugging experience when using third-party debuggers like GDB or WinDbg.

The debugger_visualizer attribute

The debugger_visualizer attribute can be used to embed a debugger visualizer file into the debug information. This enables an improved debugger experience for displaying values in the debugger. It uses the MetaListNameValueStr syntax to specify its inputs, and must be specified as a crate attribute.

Using debugger_visualizer with Natvis

Natvis is an XML-based framework for Microsoft debuggers (such as Visual Studio and WinDbg) that uses declarative rules to customize the display of types. For detailed information on the Natvis format, refer to Microsoft’s Natvis documentation.

This attribute only supports embedding Natvis files on -windows-msvc targets.

The path to the Natvis file is specified with the natvis_file key, which is a path relative to the crate source file:

#![debugger_visualizer(natvis_file = "Rectangle.natvis")]

struct FancyRect {
    x: f32,
    y: f32,
    dx: f32,
    dy: f32,
}

fn main() {
    let fancy_rect = FancyRect { x: 10.0, y: 10.0, dx: 5.0, dy: 5.0 };
    println!("set breakpoint here");
}

and Rectangle.natvis contains:

<?xml version="1.0" encoding="utf-8"?>
<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
    <Type Name="foo::FancyRect">
      <DisplayString>({x},{y}) + ({dx}, {dy})</DisplayString>
      <Expand>
        <Synthetic Name="LowerLeft">
          <DisplayString>({x}, {y})</DisplayString>
        </Synthetic>
        <Synthetic Name="UpperLeft">
          <DisplayString>({x}, {y + dy})</DisplayString>
        </Synthetic>
        <Synthetic Name="UpperRight">
          <DisplayString>({x + dx}, {y + dy})</DisplayString>
        </Synthetic>
        <Synthetic Name="LowerRight">
          <DisplayString>({x + dx}, {y})</DisplayString>
        </Synthetic>
      </Expand>
    </Type>
</AutoVisualizer>

When viewed under WinDbg, the fancy_rect variable would be shown as follows:

> Variables:
  > fancy_rect: (10.0, 10.0) + (5.0, 5.0)
    > LowerLeft: (10.0, 10.0)
    > UpperLeft: (10.0, 15.0)
    > UpperRight: (15.0, 15.0)
    > LowerRight: (15.0, 10.0)

Using debugger_visualizer with GDB

GDB supports the use of a structured Python script, called a pretty printer, that describes how a type should be visualized in the debugger view. For detailed information on pretty printers, refer to GDB’s pretty printing documentation.

Embedded pretty printers are not automatically loaded when debugging a binary under GDB. There are two ways to enable auto-loading embedded pretty printers:

1. Launch GDB with extra arguments to explicitly add a directory or binary to the auto-load safe path: gdb -iex "add-auto-load-safe-path safe-path path/to/binary" path/to/binary For more information, see GDB’s auto-loading documentation.
2. Create a file named gdbinit under $HOME/.config/gdb (you may need to create the directory if it doesn’t already exist). Add the following line to that file: add-auto-load-safe-path path/to/binary.

These scripts are embedded using the gdb_script_file key, which is a path relative to the crate source file.

#![debugger_visualizer(gdb_script_file = "printer.py")]

struct Person {
    name: String,
    age: i32,
}

fn main() {
    let bob = Person { name: String::from("Bob"), age: 10 };
    println!("set breakpoint here");
}

and printer.py contains:

import gdb

class PersonPrinter:
    "Print a Person"

    def __init__(self, val):
        self.val = val
        self.name = val["name"]
        self.age = int(val["age"])

    def to_string(self):
        return "{} is {} years old.".format(self.name, self.age)

def lookup(val):
    lookup_tag = val.type.tag
    if lookup_tag is None:
        return None
    if "foo::Person" == lookup_tag:
        return PersonPrinter(val)

    return None

gdb.current_objfile().pretty_printers.append(lookup)

When the crate’s debug executable is passed into GDB¹, print bob will display:

"Bob" is 10 years old.

The collapse_debuginfo attribute

The collapse_debuginfo attribute controls whether code locations from a macro definition are collapsed into a single location associated with the macro’s call site, when generating debuginfo for code calling this macro.

The attribute uses the MetaListIdents syntax to specify its inputs, and can only be applied to macro definitions.

Accepted options:

• #[collapse_debuginfo(yes)] — code locations in debuginfo are collapsed.
• #[collapse_debuginfo(no)] — code locations in debuginfo are not collapsed.
• #[collapse_debuginfo(external)] — code locations in debuginfo are collapsed only if the macro comes from a different crate.

The external behavior is the default for macros that don’t have this attribute, unless they are built-in macros. For built-in macros the default is yes.

Note: rustc has a -C collapse-macro-debuginfo CLI option to override both the default collapsing behavior and #[collapse_debuginfo] attributes.

#![allow(unused)]
fn main() {
#[collapse_debuginfo(yes)]
macro_rules! example {
    () => {
        println!("hello!");
    };
}
}