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): Error if target_feature is on safe function.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4234.rs: New test.
	* rust/compile/unsafe11.rs: Mark function as unsafe to match new restriction.

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

Author: Pasta-coder

gcc/rust/util/rust-attributes.cc

@@ -898,6 +898,16 @@ AttributeChecker::visit (AST::Function &fun)
 	  else
 	    check_no_mangle_function (attribute, fun);
 	}
+      else if (result.name == Attrs::TARGET_FEATURE)
+	{
+	  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 == Attrs::LINK_NAME)
 	{
 	  if (!attribute.has_attr_input ())

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

@@ -0,0 +1,3 @@
+// { dg-options "-w" }
+#[target_feature(enable = "sse2")] // { dg-error "attribute can only be applied to .unsafe. functions" }
+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(); }
 }