Add explicit_extern_abis Feature and Enforce Explicit ABIs

compiler/rustc_ast_passes/messages.ftl

@@ -79,6 +79,8 @@ ast_passes_extern_types_cannot = `type`s inside `extern` blocks cannot have {$de
     .suggestion = remove the {$remove_descr}
     .label = `extern` block begins here
 
+ast_passes_extern_without_abi = extern declarations without an explicit ABI are disallowed
+
 ast_passes_feature_on_non_nightly = `#![feature]` may not be used on the {$channel} release channel
     .suggestion = remove the attribute
     .stable_since = the feature `{$name}` has been stable since `{$since}` and no longer requires an attribute to enable

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -677,7 +677,7 @@ impl<'a> AstValidator<'a> {
                     self.dcx().emit_err(errors::PatternFnPointer { span });
                 });
                 if let Extern::Implicit(extern_span) = bfty.ext {
-                    self.maybe_lint_missing_abi(extern_span, ty.id);
+                    self.handle_missing_abi(extern_span, ty.id);
                 }
             }
             TyKind::TraitObject(bounds, ..) => {
@@ -710,10 +710,12 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId) {
+    fn handle_missing_abi(&mut self, span: Span, id: NodeId) {
         // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
         // call site which do not have a macro backtrace. See #61963.
-        if self
+        if span.edition().at_least_edition_future() && self.features.explicit_extern_abis() {
+            self.dcx().emit_err(errors::MissingAbi { span });
+        } else if self
             .sess
             .source_map()
             .span_to_snippet(span)
@@ -976,7 +978,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     }
 
                     if abi.is_none() {
-                        this.maybe_lint_missing_abi(*extern_span, item.id);
+                        this.handle_missing_abi(*extern_span, item.id);
                     }
                     visit::walk_item(this, item);
                     this.extern_mod = old_item;
@@ -1357,7 +1359,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             },
         ) = fk
         {
-            self.maybe_lint_missing_abi(*extern_span, id);
+            self.handle_missing_abi(*extern_span, id);
         }
 
         // Functions without bodies cannot have patterns.

compiler/rustc_ast_passes/src/errors.rs

@@ -823,3 +823,10 @@ pub(crate) struct DuplicatePreciseCapturing {
     #[label]
     pub bound2: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_extern_without_abi)]
+pub(crate) struct MissingAbi {
+    #[primary_span]
+    pub span: Span,
+}

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

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,

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
+```

tests/ui/feature-gates/feature-gate-explicit-extern-abis.current.fixed

@@ -0,0 +1,30 @@
+// This test differs from other feature gate tests.
+// Instead of verifying that an error occurs when the feature gate is missing,
+// it ensures that only a warning is emitted, not an error.
+
+//@ revisions: current current_feature future future_feature
+
+//@ [current] run-rustfix
+//@ [current] check-pass
+
+//@ [current_feature] run-rustfix
+//@ [current_feature] check-pass
+
+//@ [future] edition: future
+//@ [future] compile-flags: -Z unstable-options
+//@ [future] run-rustfix
+//@ [future] check-pass
+
+//@ [future_feature] edition: future
+//@ [future_feature] compile-flags: -Z unstable-options
+
+#![cfg_attr(future_feature, feature(explicit_extern_abis))]
+#![cfg_attr(current_feature, feature(explicit_extern_abis))]
+
+extern "C" fn _foo() {}
+//[current]~^ WARN extern declarations without an explicit ABI are deprecated
+//[current_feature]~^^ WARN extern declarations without an explicit ABI are deprecated
+//[future]~^^^ WARN extern declarations without an explicit ABI are deprecated
+//[future_feature]~^^^^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}

tests/ui/feature-gates/feature-gate-explicit-extern-abis.current.stderr

@@ -0,0 +1,10 @@
+warning: extern declarations without an explicit ABI are deprecated
+  --> $DIR/feature-gate-explicit-extern-abis.rs:24:1
+   |
+LL | extern fn _foo() {}
+   | ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`
+   |
+   = note: `#[warn(missing_abi)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/feature-gates/feature-gate-explicit-extern-abis.current_feature.fixed

@@ -0,0 +1,30 @@
+// This test differs from other feature gate tests.
+// Instead of verifying that an error occurs when the feature gate is missing,
+// it ensures that only a warning is emitted, not an error.
+
+//@ revisions: current current_feature future future_feature
+
+//@ [current] run-rustfix
+//@ [current] check-pass
+
+//@ [current_feature] run-rustfix
+//@ [current_feature] check-pass
+
+//@ [future] edition: future
+//@ [future] compile-flags: -Z unstable-options
+//@ [future] run-rustfix
+//@ [future] check-pass
+
+//@ [future_feature] edition: future
+//@ [future_feature] compile-flags: -Z unstable-options
+
+#![cfg_attr(future_feature, feature(explicit_extern_abis))]
+#![cfg_attr(current_feature, feature(explicit_extern_abis))]
+
+extern "C" fn _foo() {}
+//[current]~^ WARN extern declarations without an explicit ABI are deprecated
+//[current_feature]~^^ WARN extern declarations without an explicit ABI are deprecated
+//[future]~^^^ WARN extern declarations without an explicit ABI are deprecated
+//[future_feature]~^^^^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}

tests/ui/feature-gates/feature-gate-explicit-extern-abis.current_feature.stderr

@@ -0,0 +1,10 @@
+warning: extern declarations without an explicit ABI are deprecated
+  --> $DIR/feature-gate-explicit-extern-abis.rs:24:1
+   |
+LL | extern fn _foo() {}
+   | ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`
+   |
+   = note: `#[warn(missing_abi)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/feature-gates/feature-gate-explicit-extern-abis.future.fixed

@@ -0,0 +1,30 @@
+// This test differs from other feature gate tests.
+// Instead of verifying that an error occurs when the feature gate is missing,
+// it ensures that only a warning is emitted, not an error.
+
+//@ revisions: current current_feature future future_feature
+
+//@ [current] run-rustfix
+//@ [current] check-pass
+
+//@ [current_feature] run-rustfix
+//@ [current_feature] check-pass
+
+//@ [future] edition: future
+//@ [future] compile-flags: -Z unstable-options
+//@ [future] run-rustfix
+//@ [future] check-pass
+
+//@ [future_feature] edition: future
+//@ [future_feature] compile-flags: -Z unstable-options
+
+#![cfg_attr(future_feature, feature(explicit_extern_abis))]
+#![cfg_attr(current_feature, feature(explicit_extern_abis))]
+
+extern "C" fn _foo() {}
+//[current]~^ WARN extern declarations without an explicit ABI are deprecated
+//[current_feature]~^^ WARN extern declarations without an explicit ABI are deprecated
+//[future]~^^^ WARN extern declarations without an explicit ABI are deprecated
+//[future_feature]~^^^^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}

tests/ui/feature-gates/feature-gate-explicit-extern-abis.future.stderr

@@ -0,0 +1,10 @@
+warning: extern declarations without an explicit ABI are deprecated
+  --> $DIR/feature-gate-explicit-extern-abis.rs:24:1
+   |
+LL | extern fn _foo() {}
+   | ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`
+   |
+   = note: `#[warn(missing_abi)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/feature-gates/feature-gate-explicit-extern-abis.future_feature.stderr

@@ -0,0 +1,8 @@
+error: extern declarations without an explicit ABI are disallowed
+  --> $DIR/feature-gate-explicit-extern-abis.rs:24:1
+   |
+LL | extern fn _foo() {}
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+

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

@@ -0,0 +1,30 @@
+// This test differs from other feature gate tests.
+// Instead of verifying that an error occurs when the feature gate is missing,
+// it ensures that only a warning is emitted, not an error.
+
+//@ revisions: current current_feature future future_feature
+
+//@ [current] run-rustfix
+//@ [current] check-pass
+
+//@ [current_feature] run-rustfix
+//@ [current_feature] check-pass
+
+//@ [future] edition: future
+//@ [future] compile-flags: -Z unstable-options
+//@ [future] run-rustfix
+//@ [future] check-pass
+
+//@ [future_feature] edition: future
+//@ [future_feature] compile-flags: -Z unstable-options
+
+#![cfg_attr(future_feature, feature(explicit_extern_abis))]
+#![cfg_attr(current_feature, feature(explicit_extern_abis))]
+
+extern fn _foo() {}
+//[current]~^ WARN extern declarations without an explicit ABI are deprecated
+//[current_feature]~^^ WARN extern declarations without an explicit ABI are deprecated
+//[future]~^^^ WARN extern declarations without an explicit ABI are deprecated
+//[future_feature]~^^^^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}