Merge pull request #1468 from petrochenkov/debmac

Add docs for `#[collapse_debuginfo]` attribute

Author: ehuss

src/attributes.md

@@ -275,6 +275,7 @@ The following is an index of all built-in attributes.
     added in future.
 - Debugger
   - [`debugger_visualizer`] — Embeds a file that specifies debugger output for a type.
+  - [`collapse_debuginfo`] — Controls how macro invocations are encoded in debuginfo.
 
 [Doc comments]: comments.md#doc-comments
 [ECMA-334]: https://www.ecma-international.org/publications-and-standards/standards/ecma-334/
@@ -293,6 +294,7 @@ The following is an index of all built-in attributes.
 [`cfg_attr`]: conditional-compilation.md#the-cfg_attr-attribute
 [`cfg`]: conditional-compilation.md#the-cfg-attribute
 [`cold`]: attributes/codegen.md#the-cold-attribute
+[`collapse_debuginfo`]: attributes/debugger.md#the-collapse_debuginfo-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

src/attributes/debugger.md

@@ -139,3 +139,32 @@ 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
+
+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.
+
+```rust
+#[collapse_debuginfo(yes)]
+macro_rules! example {
+    () => {
+        println!("hello!");
+    };
+}
+```
+
+[attribute]: ../attributes.md
+[_MetaListIdents_]: ../attributes.md#meta-item-attribute-syntax