Rollup merge of #106248 - dtolnay:revertupcastlint, r=jackh726

Revert "Implement allow-by-default `multiple_supertrait_upcastable` lint"

This is a clean revert of #105484.

I confirmed that reverting that PR fixes the regression reported in #106247. ~~I can't say I understand what this code is doing, but maybe it can be re-landed with a different implementation.~~ **Edit:** rust-lang/rust#106247 (comment) has an explanation of why #105484 ends up surfacing spurious `where_clause_object_safety` errors. The implementation of `where_clause_object_safety` assumes we only check whether a trait is object safe when somebody actually uses that trait with `dyn`. However the implementation of `multiple_supertrait_upcastable` added in the problematic PR involves checking *every* trait for whether it is object-safe.

FYI `@nbdd0121` `@compiler-errors`

alloc/src/lib.rs

@@ -87,7 +87,6 @@
 #![warn(missing_debug_implementations)]
 #![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
-#![cfg_attr(not(bootstrap), warn(multiple_supertrait_upcastable))]
 //
 // Library features:
 #![feature(alloc_layout_extra)]
@@ -192,7 +191,6 @@
 #![feature(unsized_fn_params)]
 #![feature(c_unwind)]
 #![feature(with_negative_coherence)]
-#![cfg_attr(not(bootstrap), feature(multiple_supertrait_upcastable))]
 //
 // Rustdoc features:
 #![feature(doc_cfg)]

core/src/error.rs

@@ -28,7 +28,6 @@ use crate::fmt::{Debug, Display};
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
 #[rustc_has_incoherent_inherent_impls]
-#[cfg_attr(not(bootstrap), allow(multiple_supertrait_upcastable))]
 pub trait Error: Debug + Display {
     /// The lower-level source of this error, if any.
     ///

core/src/lib.rs

@@ -95,7 +95,6 @@
 #![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
 #![allow(incomplete_features)]
-#![cfg_attr(not(bootstrap), warn(multiple_supertrait_upcastable))]
 //
 // Library features:
 #![feature(const_align_offset)]
@@ -232,7 +231,6 @@
 #![feature(unsized_fn_params)]
 #![feature(asm_const)]
 #![feature(const_transmute_copy)]
-#![cfg_attr(not(bootstrap), feature(multiple_supertrait_upcastable))]
 //
 // Target features:
 #![feature(arm_target_feature)]