Rollup merge of #135340 - obeis:explicit-extern-abis, r=traviscross,nadrieril

Add `explicit_extern_abis` Feature and Enforce Explicit ABIs

The unstable `explicit_extern_abis` feature is introduced, requiring explicit ABIs in `extern` blocks. Hard errors will be enforced with this feature enabled in a future edition.

RFC rust-lang/rfcs#3722

Update #134986

src/doc/unstable-book/src/language-features/explicit-extern-abis.md

@@ -0,0 +1,23 @@
+# `explicit_extern_abis`
+
+The tracking issue for this feature is: #134986
+
+------
+
+Disallow `extern` without an explicit ABI. We should write `extern "C"`
+(or another ABI) instead of just `extern`.
+
+By making the ABI explicit, it becomes much clearer that "C" is just one of the
+possible choices, rather than the "standard" way for external functions.
+Removing the default makes it easier to add a new ABI on equal footing as "C".
+
+```rust,editionfuture,compile_fail
+#![feature(explicit_extern_abis)]
+
+extern fn function1() {}  // ERROR `extern` declarations without an explicit ABI
+                          // are disallowed
+
+extern "C" fn function2() {} // compiles
+
+extern "aapcs" fn function3() {} // compiles
+```