Implicit Caller Location

Approved in RFC 2091, this feature enables the accurate reporting of caller location during panics initiated from functions like Option::unwrap, Result::expect, and Index::index. This feature adds the #[track_caller] attribute for functions, the caller_location intrinsic, and the stabilization-friendly core::panic::Location::caller wrapper.

Motivating Example

Take this example program:

fn main() {
    let foo: Option<()> = None;
    foo.unwrap(); // this should produce a useful panic message!
}

Prior to Rust 1.42, panics like this unwrap() printed a location in core:

$ rustc +1.41.0 example.rs; example.exe
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value',...core\macros\mod.rs:15:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

As of 1.42, we get a much more helpful message:

$ rustc +1.42.0 example.rs; example.exe 
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', example.rs:3:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

These error messages are achieved through a combination of changes to panic! internals to make use of core::panic::Location::caller and a number of #[track_caller] annotations in the standard library which propagate caller information.

Reading Caller Location

Previously, panic! made use of the file!(), line!(), and column!() macros to construct a Location pointing to where the panic occurred. These macros couldn't be given an overridden location, so functions which intentionally invoked panic! couldn't provide their own location, hiding the actual source of error.

Internally, panic!() now calls core::panic::Location::caller() to find out where it was expanded. This function is itself annotated with #[track_caller] and wraps the caller_location compiler intrinsic implemented by rustc. This intrinsic is easiest explained in terms of how it works in a const context.

Caller Location in const

There are two main phases to returning the caller location in a const context: walking up the stack to find the right location and allocating a const value to return.

Finding the right Location

In a const context we "walk up the stack" from where the intrinsic is invoked, stopping when we reach the first function call in the stack which does not have the attribute. This walk is in InterpCx::find_closest_untracked_caller_location().

Starting at the bottom, we iterate up over stack Frames in the InterpCx::stack, calling InstanceDef::requires_caller_location on the Instances from each Frame. We stop once we find one that returns false and return the span of the previous frame which was the "topmost" tracked function.

Allocating a static Location

Once we have a Span, we need to allocate static memory for the Location, which is performed by the TyCtxt::const_caller_location() query. Internally this calls InterpCx::alloc_caller_location() and results in a unique memory kind (MemoryKind::CallerLocation). The SSA codegen backend is able to emit code for these same values, and we use this code there as well.

Once our Location has been allocated in static memory, our intrinsic returns a reference to it.

Generating code for #[track_caller] callees

To generate efficient code for a tracked function and its callers, we need to provide the same behavior from the intrinsic's point of view without having a stack to walk up at runtime. We invert the approach: as we grow the stack down we pass an additional argument to calls of tracked functions rather than walking up the stack when the intrinsic is called. That additional argument can be returned wherever the caller location is queried.

The argument we append is of type &'static core::panic::Location<'static>. A reference was chosen to avoid unnecessary copying because a pointer is a third the size of std::mem::size_of::<core::panic::Location>() == 24 at time of writing.

When generating a call to a function which is tracked, we pass the location argument the value of FunctionCx::get_caller_location.

If the calling function is tracked, get_caller_location returns the local in FunctionCx::caller_location which was populated by the current caller's caller. In these cases the intrinsic "returns" a reference which was actually provided in an argument to its caller.

If the calling function is not tracked, get_caller_location allocates a Location static from the current Span and returns a reference to that.

We more efficiently achieve the same behavior as a loop starting from the bottom by passing a single &Location value through the caller_location fields of multiple FunctionCxs as we grow the stack downward.

Codegen examples

What does this transformation look like in practice? Take this example which uses the new feature:

#![feature(track_caller)]
use std::panic::Location;

#[track_caller]
fn print_caller() {
    println!("called from {}", Location::caller());
}

fn main() {
    print_caller();
}

Here print_caller() appears to take no arguments, but we compile it to something like this:

#![feature(panic_internals)]
use std::panic::Location;

fn print_caller(caller: &Location) {
    println!("called from {}", caller);
}

fn main() {
    print_caller(&Location::internal_constructor(file!(), line!(), column!()));
}

Dynamic Dispatch

In codegen contexts we have to modify the callee ABI to pass this information down the stack, but the attribute expressly does not modify the type of the function. The ABI change must be transparent to type checking and remain sound in all uses.

Direct calls to tracked functions will always know the full codegen flags for the callee and can generate appropriate code. Indirect callers won't have this information and it's not encoded in the type of the function pointer they call, so we generate a ReifyShim around the function whenever taking a pointer to it. This shim isn't able to report the actual location of the indirect call (the function's definition site is reported instead), but it prevents miscompilation and is probably the best we can do without modifying fully-stabilized type signatures.

Note: We always emit a ReifyShim when taking a pointer to a tracked function. While the constraint here is imposed by codegen contexts, we don't know during MIR construction of the shim whether we'll be called in a const context (safe to ignore shim) or in a codegen context (unsafe to ignore shim). Even if we did know, the results from const and codegen contexts must agree.

The Attribute

The #[track_caller] attribute is checked alongside other codegen attributes to ensure the function:

  • has the "Rust" ABI (as opposed to e.g., "C")
  • is not a closure
  • is not #[naked]

If the use is valid, we set CodegenFnAttrsFlags::TRACK_CALLER. This flag influences the return value of InstanceDef::requires_caller_location which is in turn used in both const and codegen contexts to ensure correct propagation.

Traits

When applied to trait method implementations, the attribute works as it does for regular functions.

When applied to a trait method prototype, the attribute applies to all implementations of the method. When applied to a default trait method implementation, the attribute takes effect on that implementation and any overrides.

Examples:

#![feature(track_caller)]

macro_rules! assert_tracked {
    () => {{
        let location = std::panic::Location::caller();
        assert_eq!(location.file(), file!());
        assert_ne!(location.line(), line!(), "line should be outside this fn");
        println!("called at {}", location);
    }};
}

trait TrackedFourWays {
    /// All implementations inherit `#[track_caller]`.
    #[track_caller]
    fn blanket_tracked();

    /// Implementors can annotate themselves.
    fn local_tracked();

    /// This implementation is tracked (overrides are too).
    #[track_caller]
    fn default_tracked() {
        assert_tracked!();
    }

    /// Overrides of this implementation are tracked (it is too). 
    #[track_caller]
    fn default_tracked_to_override() {
        assert_tracked!();
    }
}

/// This impl uses the default impl for `default_tracked` and provides its own for 
/// `default_tracked_to_override`.
impl TrackedFourWays for () {
    fn blanket_tracked() {
        assert_tracked!();
    }

    #[track_caller]
    fn local_tracked() {
        assert_tracked!();
    }

    fn default_tracked_to_override() {
        assert_tracked!();
    }
}

fn main() {
    <() as TrackedFourWays>::blanket_tracked();
    <() as TrackedFourWays>::default_tracked();
    <() as TrackedFourWays>::default_tracked_to_override();
    <() as TrackedFourWays>::local_tracked();
}

Background/History

Broadly speaking, this feature's goal is to improve common Rust error messages without breaking stability guarantees, requiring modifications to end-user source, relying on platform-specific debug-info, or preventing user-defined types from having the same error-reporting benefits.

Improving the output of these panics has been a goal of proposals since at least mid-2016 (see non-viable alternatives in the approved RFC for details). It took two more years until RFC 2091 was approved, much of its rationale for this feature's design having been discovered through the discussion around several earlier proposals.

The design in the original RFC limited itself to implementations that could be done inside the compiler at the time without significant refactoring. However in the year and a half between the approval of the RFC and the actual implementation work, a revised design was proposed and written up on the tracking issue. During the course of implementing that, it was also discovered that an implementation was possible without modifying the number of arguments in a function's MIR, which would simplify later stages and unlock use in traits.

Because the RFC's implementation strategy could not readily support traits, the semantics were not originally specified. They have since been implemented following the path which seemed most correct to the author and reviewers.