util/attributes: Check that #[target_feature] is only used on unsafe functions

The #[target_feature] attribute allows code generation that may not be
supported by the runtime hardware, making it inherently unsafe. This
patch adds a check to ensure it is only applied to functions declared
as 'unsafe', matching rustc behavior (E0658).

Fixes #4234

gcc/rust/ChangeLog:

	* util/rust-attributes.cc (AttributeChecker::visit):
	Reject #[target_feature] on non-unsafe functions.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4234.rs: New test.
	* rust/compile/unsafe11.rs: Mark function as unsafe to
	to satisfy new #[target_feature] restriction.

Signed-off-by: Jayant Chauhan <0001jayant@gmail.com>

Author: Pasta-coder

gcc/rust/util/rust-attributes.cc

@@ -885,6 +885,13 @@ AttributeChecker::visit (AST::Function &fun)
 			   "must be of the form: %<#[target_feature(enable = "
 			   "\"name\")]%>");
 	    }
+	  else if (!fun.get_qualifiers ().is_unsafe ())
+	    {
+	      rust_error_at (
+		attribute.get_locus (),
+		"the %<#[target_feature]%> attribute can only be applied "
+		"to %<unsafe%> functions");
+	    }
 	}
       else if (result.name == "no_mangle")
 	{

gcc/testsuite/rust/compile/issue-4234.rs

@@ -0,0 +1,3 @@
+// { dg-options "-w" }
+#[target_feature(sse)] // { dg-error "attribute can only be applied" }
+fn foo() {}

gcc/testsuite/rust/compile/unsafe11.rs

@@ -1,8 +1,9 @@
 #[target_feature(sse)]
-fn foo() {
+unsafe fn foo() {
     let a: usize = 0;
 }
 
 fn main() {
-    foo() // { dg-error "requires unsafe function or block" }
+    foo(); // { dg-error "requires unsafe function or block" }
+    unsafe { foo(); }
 }