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A macro to get the current function name. #2818

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A macro to get the current function name. #2818

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- Feature Name: fn_name_macro
- Start Date: 2019-11-14
- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)

<!-- https://github.com/rust-lang/rfcs/issues/1743 -->

# Summary
[summary]: #summary

This RFC adds an additional macro, `function!`, to the `core` crate. When invoked, the macro expands to the name of the function that contains the call site.

# Motivation
[motivation]: #motivation
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This is a useful extension of Rust's existing debug reporting: `file!`, `line!`, `column!` and `module_path!`.

For most people the name of the function is a much more immediate way to understand the context of a message than file name and line number.
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# Guide-level explanation
[guide-level-explanation]: #guide-level-explanation

For debug information about what's happening within the program there's several useful macros that you might use. One of them is the `function!` macro, which expands to the name of the current function. If used outside of a function it causes a compilation error.
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# Reference-level explanation
[reference-level-explanation]: #reference-level-explanation
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This section needs some elaboration to consider the various cases (e.g. like the one @kennytm raised) and give examples of code and what the result is.

Also, looking at the implementation of module_path! in libsyntax_ext https://github.com/rust-lang/rust/blob/9e8c4e6fb1c952048fb823e59f4c9c6487bf9a58/src/libsyntax_ext/source_util.rs#L65-L73 it looks like the information is readily available. In this case however, the information is not available in cx or reachable fields. Some elaboration on the implementation would be good. cc @petrochenkov since they are the most likely T-compiler reviewer for this RFC. Also, this probably has little to do with "debuginfo" -- that's a different part of the compiler that comes into play much later in the compilation process.


Use of the `function!` macro expands to the compiler's internal name for the function. This will generally be the name that the user wrote into the file but in the case of a closure or similar it will be something like the function's name with a unique suffix.
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What about a closure in a static?

static F: &(dyn Fn() -> &'static str + Sync) = &|| "1";

(the fully-qualified name is currently playground::F::{{closure}})

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@eddyb know this one (i hope)

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The exact text is considered "debug" information and not subject to Rust's stability guarantee.

# Drawbacks
[drawbacks]: #drawbacks
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* Doing this adds another macro to `core`.
* This macro in particular cannot be implemented in Rust itself, it requires special support from `rustc`, as well as from all potential alternative compilers for the Rust language.

# Rationale and alternatives
[rationale-and-alternatives]: #rationale-and-alternatives

Previous discussion was within an issue of the RFCs repo: https://github.com/rust-lang/rfcs/issues/1743

In summary, this is _extremely_ useful for debug purposes, and has been highly desired for nearly two years.
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Alternative: we could call it `fn_name!` instead.

# Prior art
[prior-art]: #prior-art

* C99 has a `__func__` pre-processor macro that expands to the current funciton name. [link](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1642.html)
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(Luckily) This is not a pre-processor macro, just a predefined identifier provided by the C compiler

* C# has a `nameof` operator which can be used on functions

# Unresolved questions
[unresolved-questions]: #unresolved-questions

None, other than perhaps bikeshed on the name.

# Future possibilities
[future-possibilities]: #future-possibilities

None. This is a small change that's pretty open and shut.