Add unsafe_extern_blocks feature flag

Author: spastorino

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -440,16 +440,14 @@ impl<'a> AstValidator<'a> {
     }
 
     fn check_foreign_item_safety(&self, item_span: Span, safety: Safety) {
-        match safety {
-            Safety::Unsafe(_) | Safety::Safe(_)
-                if self.extern_mod_safety == Some(Safety::Default) =>
-            {
-                self.dcx().emit_err(errors::InvalidSafetyOnExtern {
-                    item_span,
-                    block: self.current_extern_span(),
-                });
-            }
-            _ => {}
+        if matches!(safety, Safety::Unsafe(_) | Safety::Safe(_))
+            && (self.extern_mod_safety == Some(Safety::Default)
+                || !self.features.unsafe_extern_blocks)
+        {
+            self.dcx().emit_err(errors::InvalidSafetyOnExtern {
+                item_span,
+                block: self.current_extern_span(),
+            });
         }
     }
 
@@ -1044,13 +1042,17 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         errors::VisibilityNotPermittedNote::IndividualForeignItems,
                     );
 
-                    if &Safety::Default == safety {
-                        this.lint_buffer.buffer_lint(
-                            MISSING_UNSAFE_ON_EXTERN,
-                            item.id,
-                            item.span,
-                            BuiltinLintDiag::MissingUnsafeOnExtern,
-                        );
+                    if this.features.unsafe_extern_blocks {
+                        if &Safety::Default == safety {
+                            this.lint_buffer.buffer_lint(
+                                MISSING_UNSAFE_ON_EXTERN,
+                                item.id,
+                                item.span,
+                                BuiltinLintDiag::MissingUnsafeOnExtern,
+                            );
+                        }
+                    } else if let &Safety::Unsafe(span) = safety {
+                        this.dcx().emit_err(errors::UnsafeItem { span, kind: "extern block" });
                     }
 
                     if abi.is_none() {

compiler/rustc_feature/src/unstable.rs

@@ -624,6 +624,8 @@ declare_features! (
     (unstable, type_changing_struct_update, "1.58.0", Some(86555)),
     /// Allows unnamed fields of struct and union type
     (incomplete, unnamed_fields, "1.74.0", Some(49804)),
+    /// Allows unsafe on extern declarations and safety qualifiers over internal items.
+    (unstable, unsafe_extern_blocks, "CURRENT_RUSTC_VERSION", Some(123743)),
     /// Allows unsized fn parameters.
     (unstable, unsized_fn_params, "1.49.0", Some(48055)),
     /// Allows unsized rvalues at arguments and parameters.

compiler/rustc_lint_defs/src/builtin.rs

@@ -4858,7 +4858,10 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,edition2024
+    /// ```rust,edition2024,ignore
+    /// #![feature(unsafe_extern_blocks)]
+    /// #![allow(dead_code)]
+    ///
     /// extern "C" {
     ///     fn foo(_: i32);
     /// }
@@ -4880,5 +4883,5 @@ declare_lint! {
     pub MISSING_UNSAFE_ON_EXTERN,
     Allow,
     "detects missing unsafe keyword on extern declarations",
-    @edition Edition2024 => Warn;
+    @edition Edition2024 => Deny;
 }

compiler/rustc_span/src/symbol.rs

@@ -1965,6 +1965,7 @@ symbols! {
         unsafe_block_in_unsafe_fn,
         unsafe_cell,
         unsafe_cell_raw_get,
+        unsafe_extern_blocks,
         unsafe_no_drop_flag,
         unsafe_pin_internals,
         unsize,

tests/ui/feature-gates/feature-gate-unsafe-extern-blocks.rs

@@ -0,0 +1,5 @@
+unsafe extern "C" {
+    //~^ ERROR extern block cannot be declared unsafe
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-unsafe-extern-blocks.stderr

@@ -0,0 +1,8 @@
+error: extern block cannot be declared unsafe
+  --> $DIR/feature-gate-unsafe-extern-blocks.rs:1:1
+   |
+LL | unsafe extern "C" {
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/parser/unsafe-foreign-mod-2.rs

@@ -1,6 +1,8 @@
 extern "C" unsafe {
     //~^ ERROR expected `{`, found keyword `unsafe`
+    //~| ERROR extern block cannot be declared unsafe
     unsafe fn foo();
+    //~^ ERROR items in unadorned `extern` blocks cannot have safety qualifiers
 }
 
 fn main() {}

tests/ui/parser/unsafe-foreign-mod-2.stderr

@@ -4,5 +4,20 @@ error: expected `{`, found keyword `unsafe`
 LL | extern "C" unsafe {
    |            ^^^^^^ expected `{`
 
-error: aborting due to 1 previous error
+error: extern block cannot be declared unsafe
+  --> $DIR/unsafe-foreign-mod-2.rs:1:12
+   |
+LL | extern "C" unsafe {
+   |            ^^^^^^
+
+error: items in unadorned `extern` blocks cannot have safety qualifiers
+  --> $DIR/unsafe-foreign-mod-2.rs:4:5
+   |
+LL | extern "C" unsafe {
+   | ----------------- help: add unsafe to this `extern` block
+...
+LL |     unsafe fn foo();
+   |     ^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
 

tests/ui/parser/unsafe-foreign-mod.rs

@@ -1,5 +1,5 @@
-//@ build-pass
-
-unsafe extern "C" {}
+unsafe extern "C" {
+    //~^ ERROR extern block cannot be declared unsafe
+}
 
 fn main() {}

tests/ui/parser/unsafe-foreign-mod.stderr

@@ -0,0 +1,8 @@
+error: extern block cannot be declared unsafe
+  --> $DIR/unsafe-foreign-mod.rs:1:1
+   |
+LL | unsafe extern "C" {
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+