error on unsafe attributes in pre-2024 editions

the `no_mangle`, `link_section` and `export_name` attributes are exceptions, and can still be used without an unsafe in earlier editions

Author: folkertdev

compiler/rustc_error_codes/src/error_codes/E0757.md

@@ -6,8 +6,9 @@ Erroneous code example:
 #![feature(ffi_const, ffi_pure)]
 
 extern "C" {
-    #[ffi_const]
-    #[ffi_pure] // error: `#[ffi_const]` function cannot be `#[ffi_pure]`
+    #[unsafe(ffi_const)]
+    #[unsafe(ffi_pure)]
+    //~^ ERROR `#[ffi_const]` function cannot be `#[ffi_pure]`
     pub fn square(num: i32) -> i32;
 }
 ```
@@ -19,7 +20,7 @@ As `ffi_const` provides stronger guarantees than `ffi_pure`, remove the
 #![feature(ffi_const)]
 
 extern "C" {
-    #[ffi_const]
+    #[unsafe(ffi_const)]
     pub fn square(num: i32) -> i32;
 }
 ```

compiler/rustc_parse/src/validate_attr.rs

@@ -167,7 +167,20 @@ pub fn check_attribute_safety(psess: &ParseSess, safety: AttributeSafety, attr:
             // square bracket respectively.
             let diag_span = attr_item.span();
 
-            if attr.span.at_least_rust_2024() {
+            // Attributes that were safe before edition 2024, but are unsafe afterwards.
+            const SAFE_BEFORE_RUST_2024: &[Symbol] =
+                &[sym::export_name, sym::link_section, sym::no_mangle];
+
+            // For attributes whose unsafety changed in edition 2024, only emit the error when
+            // compiling for that edition or later. Other unsafe attributes will error even in
+            // earlier editions.
+            let emit_error = if SAFE_BEFORE_RUST_2024.iter().any(|s| attr.has_name(*s)) {
+                attr.span.at_least_rust_2024()
+            } else {
+                true
+            };
+
+            if emit_error {
                 psess.dcx().emit_err(errors::UnsafeAttrOutsideUnsafe {
                     span: path_span,
                     suggestion: errors::UnsafeAttrOutsideUnsafeSuggestion {

tests/codegen/cffi/ffi-const.rs

@@ -10,6 +10,6 @@ extern "C" {
     // CHECK-LABEL: declare{{.*}}void @foo()
     // CHECK-SAME: [[ATTRS:#[0-9]+]]
     // CHECK-DAG: attributes [[ATTRS]] = { {{.*}}memory(none){{.*}} }
-    #[ffi_const]
+    #[unsafe(ffi_const)]
     pub fn foo();
 }

tests/codegen/cffi/ffi-pure.rs

@@ -10,6 +10,6 @@ extern "C" {
     // CHECK-LABEL: declare{{.*}}void @foo()
     // CHECK-SAME: [[ATTRS:#[0-9]+]]
     // CHECK-DAG: attributes [[ATTRS]] = { {{.*}}memory(read){{.*}} }
-    #[ffi_pure]
+    #[unsafe(ffi_pure)]
     pub fn foo();
 }

tests/ui/feature-gates/feature-gate-ffi_const.rs

@@ -1,6 +1,6 @@
 #![crate_type = "lib"]
 
 extern "C" {
-    #[ffi_const] //~ ERROR the `#[ffi_const]` attribute is an experimental feature
+    #[unsafe(ffi_const)] //~ ERROR the `#[ffi_const]` attribute is an experimental feature
     pub fn foo();
 }

tests/ui/feature-gates/feature-gate-ffi_const.stderr

@@ -1,8 +1,8 @@
 error[E0658]: the `#[ffi_const]` attribute is an experimental feature
   --> $DIR/feature-gate-ffi_const.rs:4:5
    |
-LL |     #[ffi_const]
-   |     ^^^^^^^^^^^^
+LL |     #[unsafe(ffi_const)]
+   |     ^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #58328 <https://github.com/rust-lang/rust/issues/58328> for more information
    = help: add `#![feature(ffi_const)]` to the crate attributes to enable

tests/ui/feature-gates/feature-gate-ffi_pure.rs

@@ -1,6 +1,6 @@
 #![crate_type = "lib"]
 
 extern "C" {
-    #[ffi_pure] //~ ERROR the `#[ffi_pure]` attribute is an experimental feature
+    #[unsafe(ffi_pure)] //~ ERROR the `#[ffi_pure]` attribute is an experimental feature
     pub fn foo();
 }

tests/ui/feature-gates/feature-gate-ffi_pure.stderr

@@ -1,8 +1,8 @@
 error[E0658]: the `#[ffi_pure]` attribute is an experimental feature
   --> $DIR/feature-gate-ffi_pure.rs:4:5
    |
-LL |     #[ffi_pure]
-   |     ^^^^^^^^^^^
+LL |     #[unsafe(ffi_pure)]
+   |     ^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #58329 <https://github.com/rust-lang/rust/issues/58329> for more information
    = help: add `#![feature(ffi_pure)]` to the crate attributes to enable

tests/ui/ffi-attrs/ffi_const.rs

@@ -1,15 +1,18 @@
 #![feature(ffi_const)]
 #![crate_type = "lib"]
 
-#[ffi_const] //~ ERROR `#[ffi_const]` may only be used on foreign functions
+#[unsafe(ffi_const)] //~ ERROR `#[ffi_const]` may only be used on foreign functions
 pub fn foo() {}
 
-#[ffi_const] //~ ERROR `#[ffi_const]` may only be used on foreign functions
+#[unsafe(ffi_const)] //~ ERROR `#[ffi_const]` may only be used on foreign functions
 macro_rules! bar {
-    () => ()
+    () => {};
 }
 
 extern "C" {
-    #[ffi_const] //~ ERROR `#[ffi_const]` may only be used on foreign functions
+    #[unsafe(ffi_const)] //~ ERROR `#[ffi_const]` may only be used on foreign functions
     static INT: i32;
+
+    #[ffi_const] //~ ERROR unsafe attribute used without unsafe
+    fn bar();
 }

tests/ui/ffi-attrs/ffi_const.stderr

@@ -1,21 +1,32 @@
+error: unsafe attribute used without unsafe
+  --> $DIR/ffi_const.rs:16:7
+   |
+LL |     #[ffi_const]
+   |       ^^^^^^^^^ usage of unsafe attribute
+   |
+help: wrap the attribute in `unsafe(...)`
+   |
+LL |     #[unsafe(ffi_const)]
+   |       +++++++         +
+
 error[E0756]: `#[ffi_const]` may only be used on foreign functions
   --> $DIR/ffi_const.rs:4:1
    |
-LL | #[ffi_const]
-   | ^^^^^^^^^^^^
+LL | #[unsafe(ffi_const)]
+   | ^^^^^^^^^^^^^^^^^^^^
 
 error[E0756]: `#[ffi_const]` may only be used on foreign functions
   --> $DIR/ffi_const.rs:7:1
    |
-LL | #[ffi_const]
-   | ^^^^^^^^^^^^
+LL | #[unsafe(ffi_const)]
+   | ^^^^^^^^^^^^^^^^^^^^
 
 error[E0756]: `#[ffi_const]` may only be used on foreign functions
   --> $DIR/ffi_const.rs:13:5
    |
-LL |     #[ffi_const]
-   |     ^^^^^^^^^^^^
+LL |     #[unsafe(ffi_const)]
+   |     ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0756`.