diff --git a/src/attributes/codegen.md b/src/attributes/codegen.md index cf9837d9e..eeb341b81 100644 --- a/src/attributes/codegen.md +++ b/src/attributes/codegen.md @@ -502,111 +502,119 @@ in the standard library for runtime feature detection on these platforms. r[attributes.codegen.track_caller] ## The `track_caller` attribute -r[attributes.codegen.track_caller.allowed-positions] -The `track_caller` attribute may be applied to any function with [`"Rust"` ABI][rust-abi] -with the exception of the entry point `fn main`. - -r[attributes.codegen.track_caller.traits] -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. - -r[attributes.codegen.track_caller.extern] -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. +r[attributes.codegen.track_caller.intro] +The *`track_caller` [attribute][attributes]* is used on functions to indicate that the caller should be tracked for the purpose of using [`Location`] to determine the caller. -r[attributes.codegen.track_caller.behavior] -### 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`. +> [!EXAMPLE] +> ```rust +> #[track_caller] +> fn f() { +> println!("{}", std::panic::Location::caller()); +> } +> ``` -```rust -#[track_caller] -fn f() { - println!("{}", std::panic::Location::caller()); -} -``` +r[attributes.codegen.track_caller.syntax] +The `track_caller` attribute uses the [MetaWord] syntax and thus does not take any inputs. -> [!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. +r[attributes.codegen.track_caller.allowed-positions] +The `track_caller` attribute may only be applied to: -#### Examples +- [Free functions][items.fn] +- [Inherent associated functions][items.associated.fn] +- [Trait impl functions][items.impl.trait] +- [Trait definition functions][items.traits] +- [External block functions][items.extern.fn] +- [Closures][expr.closure] -When `f` is called directly by `calls_f`, code in `f` observes its callsite within `calls_f`: +All functions must have the [`"Rust"` ABI][rust-abi]. -```rust -# #[track_caller] -# fn f() { -# println!("{}", std::panic::Location::caller()); -# } -fn calls_f() { - f(); // <-- f() prints this location -} -``` +It may not be applied to the [the `main` function][crate.main]. -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`: +r[attributes.codegen.track_caller.duplicates] +Duplicate instances of the `track_caller` attribute are ignored. -```rust -# #[track_caller] -# fn f() { -# println!("{}", std::panic::Location::caller()); -# } -#[track_caller] -fn g() { - println!("{}", std::panic::Location::caller()); - f(); -} +> [!NOTE] +> `rustc` warns on on duplicate `track_caller` attributes. -fn calls_g() { - g(); // <-- g() prints this location twice, once itself and once from f() -} -``` +r[attributes.codegen.track_caller.traits] +When applied to functions and methods in trait declarations, the `track_caller` attribute applies to all implementations. If the trait provides a default implementation with the attribute, then the attribute also applies to override implementations. -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`: +r[attributes.codegen.track_caller.extern] +When applied to a function in an `extern` block, the `track_caller` 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. -```rust -# #[track_caller] -# fn f() { -# println!("{}", std::panic::Location::caller()); -# } -# #[track_caller] -# fn g() { -# println!("{}", std::panic::Location::caller()); -# f(); -# } -#[track_caller] -fn h() { - println!("{}", std::panic::Location::caller()); - g(); -} +r[attributes.codegen.track_caller.behavior] +Applying the `track_caller` 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`. -fn calls_h() { - h(); // <-- prints this location three times, once itself, once from g(), once from f() -} -``` +> [!NOTE] +> `core` provides [`core::panic::Location::caller`] for observing caller locations. It wraps the [`core::intrinsics::caller_location`] intrinsic implemented by `rustc`. -And so on. +> [!NOTE] +> Because the resulting `Location` is a hint, an implementation may halt its walk up the stack early. See [Limitations](#track_caller-limitations) for important caveats. + +> [!EXAMPLE] +> When `f` is called directly by `calls_f`, code in `f` observes its callsite within `calls_f`: +> +> ```rust +> # #[track_caller] +> # fn f() { +> # println!("{}", std::panic::Location::caller()); +> # } +> 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 f() { +> # println!("{}", std::panic::Location::caller()); +> # } +> #[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 f() { +> # println!("{}", std::panic::Location::caller()); +> # } +> # #[track_caller] +> # fn g() { +> # println!("{}", std::panic::Location::caller()); +> # f(); +> # } +> #[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. r[attributes.codegen.track_caller.limits] -### Limitations +### `track_caller` limitations r[attributes.codegen.track_caller.hint] This information is a hint and implementations are not required to preserve it. r[attributes.codegen.track_caller.decay] -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. +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.