Rollup merge of #152381 - oli-obk:non_static_reflection, r=Mark-Simulacrum

Do not require `'static` for obtaining reflection information.

tracking issue #142577

This does not affect the stable `TypeId::of`, as that has its own `'static` bound.

But it will allow obtaining `TypeId`s for non-static types via the reflection API. To obtain such a `TypeId` for any type, just use `Type::of::<(T,)>().kind` to extract the first field of a tuple.

This effectively reintroduces #41875, which @rust-lang/lang decided against allowing back in 2018 due to lack of sound use cases. We will thus need to have a T-lang meeting specifically about `TypeId` for non-static types before *stabilizing* any part of reflection (in addition to T-lang meetings about reflection in general). I'm adding an explicit point about this to the tracking issue.

cc @scottmcm @joshtriplett @9SonSteroids @SpriteOvO @izagawd @BD103

Author: JonathanBrouwer

library/core/src/intrinsics/mod.rs

@@ -2886,7 +2886,7 @@ pub const fn type_name<T: ?Sized>() -> &'static str;
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
-pub const fn type_id<T: ?Sized + 'static>() -> crate::any::TypeId;
+pub const fn type_id<T: ?Sized>() -> crate::any::TypeId;
 
 /// Tests (at compile-time) if two [`crate::any::TypeId`] instances identify the
 /// same type. This is necessary because at const-eval time the actual discriminating

library/core/src/mem/type_info.rs

@@ -2,7 +2,7 @@
 //! runtime or const-eval processable way.
 
 use crate::any::TypeId;
-use crate::intrinsics::type_of;
+use crate::intrinsics::{type_id, type_of};
 
 /// Compile-time type information.
 #[derive(Debug)]
@@ -28,11 +28,17 @@ impl TypeId {
 
 impl Type {
     /// Returns the type information of the generic type parameter.
+    ///
+    /// Note: Unlike `TypeId`s obtained via `TypeId::of`, the `Type`
+    /// struct and its fields contain `TypeId`s that are not necessarily
+    /// derived from types that outlive `'static`. This means that using
+    /// the `TypeId`s (transitively) obtained from this function will
+    /// be able to break invariants that other `TypeId` consuming crates
+    /// may have assumed to hold.
     #[unstable(feature = "type_info", issue = "146922")]
     #[rustc_const_unstable(feature = "type_info", issue = "146922")]
-    // FIXME(reflection): don't require the 'static bound
-    pub const fn of<T: ?Sized + 'static>() -> Self {
-        const { TypeId::of::<T>().info() }
+    pub const fn of<T: ?Sized>() -> Self {
+        const { type_id::<T>().info() }
     }
 }