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+## 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, describe 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:
+
+```rust
+#[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:
+
+```rust
+#[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](https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/).
+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
+```