src/backend/implicit-caller-location.md - rust-lang/rustc-dev-guide - Git at Google


            
            # 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]`][attr-reference] attribute for functions, the

            
            [`caller_location`][intrinsic] intrinsic, and the stabilization-friendly

            
            [`core::panic::Location::caller`][wrapper] wrapper.


            ## Motivating example


            Take this example program:


            ```rust

            
            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()`][wrapper] to find out where it

            
            was expanded.

            
            This function is itself annotated with `#[track_caller]` and wraps the

            
            [`caller_location`][intrinsic] 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()`][const-find-closest].


            Starting at the bottom, we iterate up over stack [`Frame`][const-frame]s in the

            
            [`InterpCx::stack`][const-stack], calling

            
            [`InstanceKind::requires_caller_location`][requires-location] on the

            
            [`Instance`s from each `Frame`][frame-instance].

            
            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()`][const-location-query] query.

            
            Internally this calls [`InterpCx::alloc_caller_location()`][alloc-location] and results in a unique

            
            [memory kind][location-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`][fcx-get].


            If the calling function is tracked, `get_caller_location` returns the local in

            
            [`FunctionCx::caller_location`][fcx-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 `FunctionCx`s as we grow the

            
            stack downward.


            ### Codegen examples


            What does this transformation look like in practice?

            
            Take this example which uses the new feature:


            ```rust

            
            #![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:


            ```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!()));

            
            }

            
            ```


            ### 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`][attrs-flags].

            
            This flag influences the return value of

            
            [`InstanceKind::requires_caller_location`][requires-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:


            ```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();

            
            }

            
            ```


            ## 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.


            [RFC 2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md

            
            [attr-reference]: https://doc.rust-lang.org/reference/attributes/codegen.html#the-track_caller-attribute

            
            [intrinsic]: https://doc.rust-lang.org/nightly/core/intrinsics/fn.caller_location.html

            
            [wrapper]: https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller

            
            [non-viable alternatives]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#non-viable-alternatives

            
            [rationale]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#rationale

            
            [revised design]: https://github.com/rust-lang/rust/issues/47809#issuecomment-443538059

            
            [attrs-flags]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/middle/codegen_fn_attrs/struct.CodegenFnAttrFlags.html#associatedconstant.TRACK_CALLER

            
            [`ReifyShim`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceKind.html#variant.ReifyShim

            
            [`Location`]: https://doc.rust-lang.org/core/panic/struct.Location.html

            
            [const-find-closest]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.find_closest_untracked_caller_location

            
            [requires-location]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/instance/enum.InstanceKind.html#method.requires_caller_location

            
            [alloc-location]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.alloc_caller_location

            
            [fcx-location]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#structfield.caller_location

            
            [const-location-query]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.const_caller_location

            
            [location-memory-kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.MemoryKind.html#variant.CallerLocation

            
            [const-frame]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html

            
            [const-stack]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#structfield.stack

            
            [fcx-get]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#method.get_caller_location

            
            [frame-instance]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html#structfield.instance
	# 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]`][attr-reference] attribute for functions, the
	[`caller_location`][intrinsic] intrinsic, and the stabilization-friendly
	[`core::panic::Location::caller`][wrapper] wrapper.

	## Motivating example

	Take this example program:

	```rust
	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()`][wrapper] to find out where it
	was expanded.
	This function is itself annotated with `#[track_caller]` and wraps the
	[`caller_location`][intrinsic] 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()`][const-find-closest].

	Starting at the bottom, we iterate up over stack [`Frame`][const-frame]s in the
	[`InterpCx::stack`][const-stack], calling
	[`InstanceKind::requires_caller_location`][requires-location] on the
	[`Instance`s from each `Frame`][frame-instance].
	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()`][const-location-query] query.
	Internally this calls [`InterpCx::alloc_caller_location()`][alloc-location] and results in a unique
	[memory kind][location-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`][fcx-get].

	If the calling function is tracked, `get_caller_location` returns the local in
	[`FunctionCx::caller_location`][fcx-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 `FunctionCx`s as we grow the
	stack downward.

	### Codegen examples

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

	```rust
	#![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:

	```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!()));
	}
	```

	### 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`][attrs-flags].
	This flag influences the return value of
	[`InstanceKind::requires_caller_location`][requires-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:

	```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();
	}
	```

	## 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.

	[RFC 2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md
	[attr-reference]: https://doc.rust-lang.org/reference/attributes/codegen.html#the-track_caller-attribute
	[intrinsic]: https://doc.rust-lang.org/nightly/core/intrinsics/fn.caller_location.html
	[wrapper]: https://doc.rust-lang.org/nightly/core/panic/struct.Location.html#method.caller
	[non-viable alternatives]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#non-viable-alternatives
	[rationale]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#rationale
	[revised design]: https://github.com/rust-lang/rust/issues/47809#issuecomment-443538059
	[attrs-flags]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/middle/codegen_fn_attrs/struct.CodegenFnAttrFlags.html#associatedconstant.TRACK_CALLER
	[`ReifyShim`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.InstanceKind.html#variant.ReifyShim
	[`Location`]: https://doc.rust-lang.org/core/panic/struct.Location.html
	[const-find-closest]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.find_closest_untracked_caller_location
	[requires-location]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/instance/enum.InstanceKind.html#method.requires_caller_location
	[alloc-location]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#method.alloc_caller_location
	[fcx-location]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#structfield.caller_location
	[const-location-query]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.const_caller_location
	[location-memory-kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.MemoryKind.html#variant.CallerLocation
	[const-frame]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html
	[const-stack]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.InterpCx.html#structfield.stack
	[fcx-get]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#method.get_caller_location
	[frame-instance]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Frame.html#structfield.instance