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| <h1 id="implicit-caller-location"><a class="header" href="#implicit-caller-location">Implicit caller location</a></h1> |
| <p>Approved in <a href="https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md">RFC 2091</a>, this feature enables the accurate reporting of caller location during panics |
| initiated from functions like <code>Option::unwrap</code>, <code>Result::expect</code>, and <code>Index::index</code>. This feature |
| adds the <a href="https://doc.rust-lang.org/reference/attributes/codegen.html#the-track_caller-attribute"><code>#[track_caller]</code></a> attribute for functions, the |
| <a href="https://doc.rust-lang.org/nightly/core/intrinsics/fn.caller_location.html"><code>caller_location</code></a> intrinsic, and the stabilization-friendly |
| <a href="https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller"><code>core::panic::Location::caller</code></a> wrapper.</p> |
| <h2 id="motivating-example"><a class="header" href="#motivating-example">Motivating example</a></h2> |
| <p>Take this example program:</p> |
| <pre class="playground"><code class="language-rust">fn main() { |
| let foo: Option<()> = None; |
| foo.unwrap(); // this should produce a useful panic message! |
| }</code></pre> |
| <p>Prior to Rust 1.42, panics like this <code>unwrap()</code> printed a location in core:</p> |
| <pre><code>$ 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. |
| </code></pre> |
| <p>As of 1.42, we get a much more helpful message:</p> |
| <pre><code>$ 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 |
| </code></pre> |
| <p>These error messages are achieved through a combination of changes to <code>panic!</code> internals to make use |
| of <code>core::panic::Location::caller</code> and a number of <code>#[track_caller]</code> annotations in the standard |
| library which propagate caller information.</p> |
| <h2 id="reading-caller-location"><a class="header" href="#reading-caller-location">Reading caller location</a></h2> |
| <p>Previously, <code>panic!</code> made use of the <code>file!()</code>, <code>line!()</code>, and <code>column!()</code> macros to construct a |
| <a href="https://doc.rust-lang.org/core/panic/struct.Location.html"><code>Location</code></a> pointing to where the panic occurred. These macros couldn’t be given an overridden |
| location, so functions which intentionally invoked <code>panic!</code> couldn’t provide their own location, |
| hiding the actual source of error.</p> |
| <p>Internally, <code>panic!()</code> now calls <a href="https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller"><code>core::panic::Location::caller()</code></a> to find out where it |
| was expanded. This function is itself annotated with <code>#[track_caller]</code> and wraps the |
| <a href="https://doc.rust-lang.org/nightly/core/intrinsics/fn.caller_location.html"><code>caller_location</code></a> compiler intrinsic implemented by rustc. This intrinsic is easiest |
| explained in terms of how it works in a <code>const</code> context.</p> |
| <h2 id="caller-location-in-const"><a class="header" href="#caller-location-in-const">Caller location in <code>const</code></a></h2> |
| <p>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.</p> |
| <h3 id="finding-the-right-location"><a class="header" href="#finding-the-right-location">Finding the right <code>Location</code></a></h3> |
| <p>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 <em>not</em> have the attribute. This walk is in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.find_closest_untracked_caller_location"><code>InterpCx::find_closest_untracked_caller_location()</code></a>.</p> |
| <p>Starting at the bottom, we iterate up over stack <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html"><code>Frame</code></a>s in the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#structfield.stack"><code>InterpCx::stack</code></a>, calling |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/instance/enum.InstanceKind.html#method.requires_caller_location"><code>InstanceKind::requires_caller_location</code></a> on the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html#structfield.instance"><code>Instance</code>s from each <code>Frame</code></a>. We stop once we find one that returns <code>false</code> and |
| return the span of the <em>previous</em> frame which was the “topmost” tracked function.</p> |
| <h3 id="allocating-a-static-location"><a class="header" href="#allocating-a-static-location">Allocating a static <code>Location</code></a></h3> |
| <p>Once we have a <code>Span</code>, we need to allocate static memory for the <code>Location</code>, which is performed by |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.const_caller_location"><code>TyCtxt::const_caller_location()</code></a> query. Internally this calls |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.alloc_caller_location"><code>InterpCx::alloc_caller_location()</code></a> and results in a unique |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.MemoryKind.html#variant.CallerLocation">memory kind</a> (<code>MemoryKind::CallerLocation</code>). The SSA codegen backend is able |
| to emit code for these same values, and we use this code there as well.</p> |
| <p>Once our <code>Location</code> has been allocated in static memory, our intrinsic returns a reference to it.</p> |
| <h2 id="generating-code-for-track_caller-callees"><a class="header" href="#generating-code-for-track_caller-callees">Generating code for <code>#[track_caller]</code> callees</a></h2> |
| <p>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.</p> |
| <p>The argument we append is of type <code>&'static core::panic::Location<'static></code>. A reference was chosen |
| to avoid unnecessary copying because a pointer is a third the size of |
| <code>std::mem::size_of::<core::panic::Location>() == 24</code> at time of writing.</p> |
| <p>When generating a call to a function which is tracked, we pass the location argument the value of |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#method.get_caller_location"><code>FunctionCx::get_caller_location</code></a>.</p> |
| <p>If the calling function is tracked, <code>get_caller_location</code> returns the local in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#structfield.caller_location"><code>FunctionCx::caller_location</code></a> 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.</p> |
| <p>If the calling function is not tracked, <code>get_caller_location</code> allocates a <code>Location</code> static from |
| the current <code>Span</code> and returns a reference to that.</p> |
| <p>We more efficiently achieve the same behavior as a loop starting from the bottom by passing a single |
| <code>&Location</code> value through the <code>caller_location</code> fields of multiple <code>FunctionCx</code>s as we grow the |
| stack downward.</p> |
| <h3 id="codegen-examples"><a class="header" href="#codegen-examples">Codegen examples</a></h3> |
| <p>What does this transformation look like in practice? Take this example which uses the new feature:</p> |
| <pre class="playground"><code class="language-rust">#![feature(track_caller)] |
| use std::panic::Location; |
| |
| #[track_caller] |
| fn print_caller() { |
| println!("called from {}", Location::caller()); |
| } |
| |
| fn main() { |
| print_caller(); |
| }</code></pre> |
| <p>Here <code>print_caller()</code> appears to take no arguments, but we compile it to something like this:</p> |
| <pre class="playground"><code class="language-rust">#![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!())); |
| }</code></pre> |
| <h3 id="dynamic-dispatch"><a class="header" href="#dynamic-dispatch">Dynamic dispatch</a></h3> |
| <p>In codegen contexts we have to modify the callee ABI to pass this information down the stack, but |
| the attribute expressly does <em>not</em> modify the type of the function. The ABI change must be |
| transparent to type checking and remain sound in all uses.</p> |
| <p>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 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceKind.html#variant.ReifyShim"><code>ReifyShim</code></a> 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.</p> |
| <blockquote> |
| <p><em>Note:</em> We always emit a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceKind.html#variant.ReifyShim"><code>ReifyShim</code></a> 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.</p> |
| </blockquote> |
| <h2 id="the-attribute"><a class="header" href="#the-attribute">The attribute</a></h2> |
| <p>The <code>#[track_caller]</code> attribute is checked alongside other codegen attributes to ensure the |
| function:</p> |
| <ul> |
| <li>has the <code>"Rust"</code> ABI (as opposed to e.g., <code>"C"</code>)</li> |
| <li>is not a closure</li> |
| <li>is not <code>#[naked]</code></li> |
| </ul> |
| <p>If the use is valid, we set <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/middle/codegen_fn_attrs/struct.CodegenFnAttrFlags.html#associatedconstant.TRACK_CALLER"><code>CodegenFnAttrsFlags::TRACK_CALLER</code></a>. This flag influences |
| the return value of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/instance/enum.InstanceKind.html#method.requires_caller_location"><code>InstanceKind::requires_caller_location</code></a> which is in turn |
| used in both const and codegen contexts to ensure correct propagation.</p> |
| <h3 id="traits"><a class="header" href="#traits">Traits</a></h3> |
| <p>When applied to trait method implementations, the attribute works as it does for regular functions.</p> |
| <p>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 <em>and</em> any overrides.</p> |
| <p>Examples:</p> |
| <pre class="playground"><code class="language-rust">#![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(); |
| }</code></pre> |
| <h2 id="backgroundhistory"><a class="header" href="#backgroundhistory">Background/History</a></h2> |
| <p>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.</p> |
| <p>Improving the output of these panics has been a goal of proposals since at least mid-2016 (see |
| <a href="https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#non-viable-alternatives">non-viable alternatives</a> in the approved RFC for details). It took two more years until RFC 2091 |
| was approved, much of its <a href="https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#rationale">rationale</a> for this feature’s design having been discovered through the |
| discussion around several earlier proposals.</p> |
| <p>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 <a href="https://github.com/rust-lang/rust/issues/47809#issuecomment-443538059">revised design</a> 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.</p> |
| <p>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.</p> |
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