Add "function name macro"

Author: mitsuhiko

text/0000-function-name-macro.md

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+- Start Date: 2014-11-17
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Introduce a `function!()` macro that expands to an `Option<&'static str>` of the function
+name it's used in.
+
+# Motivation
+
+For error reporting cases we already have `file!`, `line!`, `col!` and `module_path!`
+but there is no way yet to figure out in which function an error was reported.  Python's
+tracebacks are well received and for many people the function name information is much
+more important than the line number.
+
+# Detailed design
+
+The behavior of the new `function!` macro should be as such:
+
+* when used within a proper function `fn` (or trait method) it expands to
+  `Some(name)` where `name` is the name of the function.  It is always the
+  local name of the function which means even if a function is contained in
+  another function, it's just the the actual name.
+* when used inside a local block in a function (proc, closure or anything else) it
+  expands to the name of the innermost proper function.
+* if used anywhere else it expands to `None`.
+
+# Drawbacks
+
+Maybe `function!` is a bit generic of a name.  Alternative names could be considered.
+
+# Alternatives
+
+As an alternative name, `function_name!` could be used.