Add docs for #[collapse_debuginfo] attribute

src/attributes/debugger.md

@@ -139,3 +139,26 @@ When the crate's debug executable is passed into GDB[^rust-gdb], `print bob` wil
 [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
+
+## The `collapse_debuginfo` attribute
+
+This attribute controls whether code locations from this 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`.
+
+Both the default collapsing behavior and `#[collapse_debuginfo]` attributes can be overridden from
+command line using the `-C collapse-macro-debuginfo` option.
+
+[_MetaListIdents_]: ../attributes.md#meta-item-attribute-syntax