Auto merge of rust-lang#120639 - fee1-dead-contrib:new-effects-desugaring, r=oli-obk

Implement new effects desugaring

cc `@rust-lang/project-const-traits.` Will write down notes once I have finished.

* [x] See if we want `T: Tr` to desugar into `T: Tr, T::Effects: Compat<true>`
* [x] Fix ICEs on `type Assoc: ~const Tr` and `type Assoc<T: ~const Tr>`
* [ ] add types and traits to minicore test
* [ ] update rustc-dev-guide

Fixes rust-lang#119717
Fixes rust-lang#123664
Fixes rust-lang#124857
Fixes rust-lang#126148

Author: bors

core/src/lib.rs

@@ -231,6 +231,7 @@
 #![feature(let_chains)]
 #![feature(link_llvm_intrinsics)]
 #![feature(macro_metavar_expr)]
+#![feature(marker_trait_attr)]
 #![feature(min_exhaustive_patterns)]
 #![feature(min_specialization)]
 #![feature(multiple_supertrait_upcastable)]

core/src/marker.rs

@@ -1024,3 +1024,49 @@ pub trait FnPtr: Copy + Clone {
 pub macro SmartPointer($item:item) {
     /* compiler built-in */
 }
+
+// Support traits and types for the desugaring of const traits and
+// `~const` bounds. Not supposed to be used by anything other than
+// the compiler.
+#[doc(hidden)]
+#[unstable(
+    feature = "effect_types",
+    issue = "none",
+    reason = "internal module for implementing effects"
+)]
+#[allow(missing_debug_implementations)] // these unit structs don't need `Debug` impls.
+#[cfg(not(bootstrap))]
+pub mod effects {
+    #[lang = "EffectsNoRuntime"]
+    pub struct NoRuntime;
+    #[lang = "EffectsMaybe"]
+    pub struct Maybe;
+    #[lang = "EffectsRuntime"]
+    pub struct Runtime;
+
+    #[lang = "EffectsCompat"]
+    pub trait Compat<#[rustc_runtime] const RUNTIME: bool> {}
+
+    impl Compat<false> for NoRuntime {}
+    impl Compat<true> for Runtime {}
+    impl<#[rustc_runtime] const RUNTIME: bool> Compat<RUNTIME> for Maybe {}
+
+    #[lang = "EffectsTyCompat"]
+    #[marker]
+    pub trait TyCompat<T: ?Sized> {}
+
+    impl<T: ?Sized> TyCompat<T> for T {}
+    impl<T: ?Sized> TyCompat<T> for Maybe {}
+    impl<T: ?Sized> TyCompat<Maybe> for T {}
+
+    #[lang = "EffectsIntersection"]
+    pub trait Intersection {
+        #[lang = "EffectsIntersectionOutput"]
+        type Output: ?Sized;
+    }
+
+    // FIXME(effects): remove this after next trait solver lands
+    impl Intersection for () {
+        type Output = Maybe;
+    }
+}