Auto merge of #142839 - oli-obk:denullarification, r=RalfJung,celinval

Stop backends from needing to support nullary intrinsics

And then remove our infrastructure special casing them. Further improvements can now be done to them by avoiding the intermediate ConstValue step, but let's leave that to follow up work

r? `@RalfJung`

Author: bors

example/mini_core.rs

@@ -660,7 +660,7 @@ pub mod intrinsics {
     #[rustc_intrinsic]
     pub unsafe fn ctlz_nonzero<T>(x: T) -> u32;
     #[rustc_intrinsic]
-    pub fn needs_drop<T: ?::Sized>() -> bool;
+    pub const fn needs_drop<T: ?::Sized>() -> bool;
     #[rustc_intrinsic]
     pub fn bitreverse<T>(x: T) -> T;
     #[rustc_intrinsic]

example/mini_core_hello_world.rs

@@ -1,4 +1,13 @@
-#![feature(no_core, lang_items, never_type, linkage, extern_types, thread_local, repr_simd)]
+#![feature(
+    no_core,
+    lang_items,
+    never_type,
+    linkage,
+    extern_types,
+    thread_local,
+    repr_simd,
+    rustc_private
+)]
 #![no_core]
 #![allow(dead_code, non_camel_case_types, internal_features)]
 
@@ -207,10 +216,14 @@ fn main() {
         assert_eq!(intrinsics::align_of::<u16>() as u8, 2);
         assert_eq!(intrinsics::align_of_val(&a) as u8, intrinsics::align_of::<&str>() as u8);
 
-        assert!(!intrinsics::needs_drop::<u8>());
-        assert!(!intrinsics::needs_drop::<[u8]>());
-        assert!(intrinsics::needs_drop::<NoisyDrop>());
-        assert!(intrinsics::needs_drop::<NoisyDropUnsized>());
+        let u8_needs_drop = const { intrinsics::needs_drop::<u8>() };
+        assert!(!u8_needs_drop);
+        let slice_needs_drop = const { intrinsics::needs_drop::<[u8]>() };
+        assert!(!slice_needs_drop);
+        let noisy_drop = const { intrinsics::needs_drop::<NoisyDrop>() };
+        assert!(noisy_drop);
+        let noisy_unsized_drop = const { intrinsics::needs_drop::<NoisyDropUnsized>() };
+        assert!(noisy_unsized_drop);
 
         Unique { pointer: NonNull(1 as *mut &str), _marker: PhantomData } as Unique<dyn SomeTrait>;
 

src/intrinsics/mod.rs

@@ -812,21 +812,6 @@ fn codegen_regular_intrinsic_call<'tcx>(
             dest.write_cvalue(fx, val);
         }
 
-        sym::needs_drop | sym::type_id | sym::type_name | sym::variant_count => {
-            intrinsic_args!(fx, args => (); intrinsic);
-
-            let const_val = fx
-                .tcx
-                .const_eval_instance(
-                    ty::TypingEnv::fully_monomorphized(),
-                    instance,
-                    source_info.span,
-                )
-                .unwrap();
-            let val = crate::constant::codegen_const_value(fx, const_val, ret.layout().ty);
-            ret.write_cvalue(fx, val);
-        }
-
         sym::ptr_offset_from | sym::ptr_offset_from_unsigned => {
             intrinsic_args!(fx, args => (ptr, base); intrinsic);
             let ptr = ptr.load_scalar(fx);