Merge pull request #1335 from gibbyfree/natvis-doc

Add documentation for debugger_visualizer attribute

Author: ehuss

src/SUMMARY.md

@@ -44,6 +44,7 @@
     - [Code generation](attributes/codegen.md)
     - [Limits](attributes/limits.md)
     - [Type System](attributes/type_system.md)
+    - [Debugger](attributes/debugger.md)
 
 - [Statements and expressions](statements-and-expressions.md)
     - [Statements](statements.md)

src/attributes.md

@@ -271,6 +271,8 @@ The following is an index of all built-in attributes.
 - Type System
   - [`non_exhaustive`] — Indicate that a type will have more fields/variants
     added in future.
+- Debugger
+  - [`debugger_visualizer`] — Embeds a file that specifies debugger output for a type.
 
 [Doc comments]: comments.md#doc-comments
 [ECMA-334]: https://www.ecma-international.org/publications/standards/Ecma-334.htm
@@ -291,6 +293,7 @@ The following is an index of all built-in attributes.
 [`cold`]: attributes/codegen.md#the-cold-attribute
 [`crate_name`]: crates-and-source-files.md#the-crate_name-attribute
 [`crate_type`]: linkage.md
+[`debugger_visualizer`]: attributes/debugger.md#the-debugger_visualizer-attribute
 [`deny`]: attributes/diagnostics.md#lint-check-attributes
 [`deprecated`]: attributes/diagnostics.md#the-deprecated-attribute
 [`derive`]: attributes/derive.md

src/attributes/debugger.md

@@ -0,0 +1,141 @@
+# 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:
+
+<!-- ignore: requires external files, and msvc -->
+```rust ignore
+#![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
+<?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:
+
+```text
+> 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].
+1. 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.
+
+<!-- ignore: requires external files -->
+```rust ignore
+#![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:
+
+```python
+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 GDB[^rust-gdb], `print bob` will display:
+
+```text
+"Bob" is 10 years old.
+```
+
+[^rust-gdb]: Note: This assumes you are using the `rust-gdb` script which configures pretty-printers for standard library types like `String`.
+
+[auto-loading documentation]: https://sourceware.org/gdb/onlinedocs/gdb/Auto_002dloading-safe-path.html
+[attributes]: ../attributes.md
+[Natvis documentation]: https://docs.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects
+[pretty printing documentation]: https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing.html
+[_MetaListNameValueStr_]: ../attributes.md#meta-item-attribute-syntax