error: unused attribute const_fn_union with incr-comp

[spastorino]

Every time I try to recompile the compiler using incremental compilation with ./x.py build -i --stage 1 --keep-stage 1 src/libstd I get the following error ...

error: unused attribute                                                                                                
  --> src/libcore/slice/mod.rs:66:5                                                                                    
   |                                                                                                                   
66 |     #[allow_internal_unstable(const_fn_union)]                                                                    
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                                                 
   = note: `-D unused-attributes` implied by `-D warnings`
                                                                                                                       
error: unused attribute                                                                                                
    --> src/libcore/str/mod.rs:2170:5                      
     |                                                                                                                 
2170 |     #[allow_internal_unstable(const_fn_union)]                                                                  
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                  
                                                                                                                       
error: aborting due to 2 previous errors                                                                               
                                                                                                                                                                                                                                              
error: could not compile `core`.                   

[Centril]

@oli-obk, J'Accuse…! :)

[spastorino]

The following change could be used as a workaround ...

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index b5462d98837..e55baded913 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -64,6 +64,7 @@ impl<T> [T] {
     #[inline]
     // SAFETY: const sound because we transmute out the length field as a usize (which it must be)
     #[allow_internal_unstable(const_fn_union)]
+    #[allow(unused_attributes)]
     pub const fn len(&self) -> usize {
         unsafe {
             crate::ptr::Repr { rust: self }.raw.len
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index ece61dde490..d02d232a20e 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -2168,6 +2168,7 @@ impl str {
     #[inline(always)]
     // SAFETY: const sound because we transmute two types with the same layout
     #[allow_internal_unstable(const_fn_union)]
+    #[allow(unused_attributes)]
     pub const fn as_bytes(&self) -> &[u8] {
         #[repr(C)]
         union Slices<'a> {