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/target_feature-unsafe.rs: New test.
	* rust/compile/unsafe11.rs: Mark function as unsafe to satisfy new check.

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

Author: Pasta-coder

gcc/rust/util/rust-attributes.cc

@@ -866,6 +866,28 @@ AttributeChecker::visit (AST::Function &fun)
 	}
       else if (result.name == "no_mangle")
 	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::ALLOW || result.name == "deny"
+	       || result.name == "warn" || result.name == "forbid")
+	{
+	  if (!attribute.has_attr_input ())
+	    {
+	      rust_error_at (attribute.get_locus (),
+			     "malformed %qs attribute input", name);
+	      rust_inform (attribute.get_locus (),
+			   "must be of the form: %<#[%s(lint1, lint2, ...)]%>",
+			   name);
+	    }
+	}
     }
   if (fun.has_body ())
     fun.get_definition ().value ()->accept_vis (*this);

gcc/testsuite/rust/compile/target_feature-unsafe.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

@@ -1,5 +1,5 @@
 #[target_feature(sse)]
-fn foo() {
+unsafe fn foo() {
     let a: usize = 0;
 }