Add explicit_extern_abis unstable feature

also add `explicit-extern-abis` feature section to
the unstable book.

Author: obeis

Diff for: compiler/rustc_feature/src/unstable.rs

@@ -478,6 +478,8 @@ declare_features! (
     (incomplete, ergonomic_clones, "CURRENT_RUSTC_VERSION", Some(132290)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (unstable, exhaustive_patterns, "1.13.0", Some(51085)),
+    /// Disallows `extern` without an explicit ABI.
+    (unstable, explicit_extern_abis, "CURRENT_RUSTC_VERSION", Some(134986)),
     /// Allows explicit tail calls via `become` expression.
     (incomplete, explicit_tail_calls, "1.72.0", Some(112788)),
     /// Allows using `aapcs`, `efiapi`, `sysv64` and `win64` as calling conventions

Diff for: compiler/rustc_span/src/symbol.rs

@@ -881,6 +881,7 @@ symbols! {
         expf16,
         expf32,
         expf64,
+        explicit_extern_abis,
         explicit_generic_args_with_impl_trait,
         explicit_tail_calls,
         export_name,

Diff for: 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
+```

Diff for: tests/ui/feature-gates/feature-gate-explicit-extern-abis.rs

@@ -0,0 +1,6 @@
+#![feature(explicit_extern_abis)]
+
+extern fn foo() {}
+//~^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}

Diff for: tests/ui/feature-gates/feature-gate-explicit-extern-abis.stderr

@@ -0,0 +1,8 @@
+error: extern declarations without an explicit ABI are disallowed
+  --> $DIR/feature-gate-explicit-extern-abis.rs:3:1
+   |
+LL | extern fn foo() {}
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+