Disallow omitting the ABI in extern declarations

text/0000-explicit-abi.md

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+- Start Date: 2015-01-20
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Disallow omitting the ABI in `extern` declarations, both for extern functions and for external blocks.
+
+# Motivation
+
+Some advantages of making the ABI explicit are:
+ - it would ensure a more uniform coding style (even in the main rust repo you can find both extern fn and extern "C" fn)
+ - it would make it easier to search for what ABIs are used in a codebase
+ - it would be more familiar to C++ programmers
+
+Removing the default for both functions and blocks helps with ensuring consistency.
+
+# Detailed design
+
+Removing the default is simple, just a matter of making the token non-optional in the parser.
+
+Updating exisiting code to take the change into account can be automated, as it is a straightforward search-and-replace.
+
+# Drawbacks
+
+It should be trivial to add the "C" ABI (the current default) wherever it is missing, but the change would still break existing code.
+
+# Alternatives
+
+Keeping the current default will not break code; it might lead to some inconsistent usage of the explicit "C" ABI, but this could be solved on a per-project basis using coding guidelines.
+
+It would be possible to perform the change ony on extern functions or on external blocks, but this would make the grammar rules less consistent and more surprising for users.
+
+# Unresolved questions
+
+Should no-ABI empty `extern` blocks (occasionally used to link a library without explicitly accessing its functions) be allowed?