Merge #6307

6307: Add whitelist of safe intrinsics r=frazar a=frazar

This PR should fix #5996, where intrinsic operations where all marked as unsafe.

I'm rather new to this codebase, so I might be doing something *very* wrong. Please forgive me!

In particular, I'm not sure how to "check that we are in extern `rust-intrinsics`" as mentioned [in this comment](https://github.com/rust-analyzer/rust-analyzer/issues/5996#issuecomment-709234802). 

Co-authored-by: Francesco Zardi <frazar@users.noreply.github.com>

crates/hir_ty/src/diagnostics/unsafe_check.rs

@@ -199,6 +199,24 @@ fn main() {
         let x = STATIC_MUT.a;
     }
 }
+"#,
+        );
+    }
+
+    #[test]
+    fn no_missing_unsafe_diagnostic_with_safe_intrinsic() {
+        check_diagnostics(
+            r#"
+extern "rust-intrinsic" {
+    pub fn bitreverse(x: u32) -> u32; // Safe intrinsic
+    pub fn floorf32(x: f32) -> f32; // Unsafe intrinsic
+}
+
+fn main() {
+    let _ = bitreverse(12);
+    let _ = floorf32(12.0);
+          //^^^^^^^^^^^^^^ This operation is unsafe and requires an unsafe function or block
+}
 "#,
         );
     }