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

compiler/rustc_error_codes/src/error_codes/E0755.md

@@ -5,7 +5,7 @@ Erroneous code example:
 ```compile_fail,E0755
 #![feature(ffi_pure)]
 
-#[ffi_pure] // error!
+#[unsafe(ffi_pure)] // error!
 pub fn foo() {}
 # fn main() {}
 ```
@@ -17,7 +17,7 @@ side effects or infinite loops:
 #![feature(ffi_pure)]
 
 extern "C" {
-    #[ffi_pure] // ok!
+    #[unsafe(ffi_pure)] // ok!
     pub fn strlen(s: *const i8) -> isize;
 }
 # fn main() {}

compiler/rustc_error_codes/src/error_codes/E0756.md

@@ -6,7 +6,7 @@ Erroneous code example:
 ```compile_fail,E0756
 #![feature(ffi_const)]
 
-#[ffi_const] // error!
+#[unsafe(ffi_const)] // error!
 pub fn foo() {}
 # fn main() {}
 ```
@@ -18,7 +18,7 @@ which have no side effects except for their return value:
 #![feature(ffi_const)]
 
 extern "C" {
-    #[ffi_const] // ok!
+    #[unsafe(ffi_const)] // ok!
     pub fn strlen(s: *const i8) -> i32;
 }
 # fn main() {}

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;
 }
 ```