Document #[track_caller].

Author: anp

src/attributes.md

@@ -221,6 +221,7 @@ The following is an index of all built-in attributes.
   - [`cold`] — Hint that a function is unlikely to be called.
   - [`no_builtins`] — Disables use of certain built-in functions.
   - [`target_feature`] — Configure platform-specific code generation.
+  - [`track_caller`] - Pass the parent call location to `std::panic::Location::caller()`.
 - Documentation
   - `doc` — Specifies documentation. See [The Rustdoc Book] for more
     information. [Doc comments] are transformed into `doc` attributes.
@@ -294,6 +295,7 @@ The following is an index of all built-in attributes.
 [`should_panic`]: attributes/testing.md#the-should_panic-attribute
 [`target_feature`]: attributes/codegen.md#the-target_feature-attribute
 [`test`]: attributes/testing.md#the-test-attribute
+[`track_caller`]: attributes/codegen.md#the-track_caller-attribute
 [`type_length_limit`]: attributes/limits.md#the-type_length_limit-attribute
 [`used`]: abi.md#the-used-attribute
 [`warn`]: attributes/diagnostics.md#lint-check-attributes

src/attributes/codegen.md

@@ -143,6 +143,96 @@ feature detection on the x86 platforms.
 > may be enabled or disabled for an entire crate with the
 > [`-C target-feature`] flag.
 
+## The `track_caller` attribute
+
+The `track_caller` attribute may be applied to any function with [`"Rust"` ABI][rust-abi]. When
+applied to functions and methods in trait declarations, the attribute applies to all
+implementations. If the trait provides a default implementation with the attribute, then the
+attribute also applies to override implementations.
+
+When applied to a function in an `extern` block the attribute must also be applied to any linked
+implementations, otherwise undefined behavior results. When applied to a function which is made
+available to an `extern` block, the declaration in the `extern` block must also have the attribute,
+otherwise undefined behavior results.
+
+### Behavior
+
+Applying the attribute to a function `f` allows code within `f` to get a hint of the [`Location`] of 
+the "topmost" tracked call that led to `f`'s invocation. At the point of observation, an 
+implementation behaves as if it walks up the stack from `f`'s frame to find the nearest frame of an 
+*unattributed* function `outer`, and it returns the [`Location`] of the tracked call in `outer`.
+
+```rust
+#[track_caller]
+fn f() {
+    println!("{}", std::panic::Location::caller());
+}
+```
+
+> Note: `core` provides [`core::panic::Location::caller`] for observing caller locations. It wraps
+> the [`core::intrinsics::caller_location`] intrinsic implemented by `rustc`.
+
+> Note: because the resulting `Location` is a hint, an implementation may halt its walk up the stack
+> early. See [Limitations](#limitations) for important caveats.
+
+#### Examples
+
+When `f` is called directly by `calls_f`, code in `f` observes its callsite within `calls_f`:
+
+```rust
+fn calls_f() {
+    f(); // <-- f() prints this location
+}
+```
+
+When `f` is called by another attributed function `g` which is in turn called by `calls_g`, code in
+both `f` and `g` observes `g`'s callsite within `calls_g`: 
+
+```rust
+#[track_caller]
+fn g() {
+    println!("{}", std::panic::Location::caller());
+    f();
+}
+
+fn calls_g() {
+    g(); // <-- g() prints this location twice, once itself and once from f()
+}
+```
+
+When `g` is called by another attributed function `h` which is in turn called by `calls_h`, all code
+in `f`, `g`, and `h` observes `h`'s callsite within `calls_h`:
+
+```rust
+#[track_caller]
+fn h() {
+    println!("{}", std::panic::Location::caller());
+    g();
+}
+
+fn calls_h() {
+    h(); // <-- prints this location three times, once itself, once from g(), once from f()
+}
+```
+
+And so on.
+
+### Limitations
+
+This information is a hint and implementations are not required to preserve it.
+
+In particular, coercing a function with `#[track_caller]` to a function pointer creates a shim which
+appears to observers to have been called at the attributed function's definition site, losing actual
+caller information across virtual calls. A common example of this coercion is the creation of a
+trait object whose methods are attributed.
+
+> Note: The aforementioned shim for function pointers is necessary because `rustc` implements
+> `track_caller` in a codegen context by appending an implicit parameter to the function ABI, but
+> this would be unsound for an indirect call because the parameter is not a part of the function's
+> type and a given function pointer type may or may not refer to a function with the attribute. The
+> creation of a shim hides the implicit parameter from callers of the function pointer, preserving
+> soundness.
+
 [_MetaListNameValueStr_]: ../attributes.md#meta-item-attribute-syntax
 [`-C target-cpu`]: ../../rustc/codegen-options/index.html#target-cpu
 [`-C target-feature`]: ../../rustc/codegen-options/index.html#target-feature
@@ -155,3 +245,7 @@ feature detection on the x86 platforms.
 [trait]: ../items/traits.md
 [undefined behavior]: ../behavior-considered-undefined.md
 [unsafe function]: ../unsafe-functions.md
+[rust-abi]: ../items/external-blocks.md#abi
+[`core::intrinsics::caller_location`]: ../../core/intrinsics/fn.caller_location.html
+[`core::panic::Location::caller`]: ../../core/panic/struct.Location.html#method.caller
+[`Location`]: ../../core/panic/struct.Location.html