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

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

This is a clean revert of rust-lang#105484.

I confirmed that reverting that PR fixes the regression reported in rust-lang#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#106247 (comment) has an explanation of why rust-lang#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`

compiler/rustc_feature/src/active.rs

@@ -160,8 +160,6 @@ declare_features! (
     (active, intrinsics, "1.0.0", None, None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
     (active, lang_items, "1.0.0", None, None),
-    /// Allows the `multiple_supertrait_upcastable` lint.
-    (active, multiple_supertrait_upcastable, "CURRENT_RUSTC_VERSION", None, None),
     /// Allows using `#[omit_gdb_pretty_printer_section]`.
     (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
     /// Allows using `#[prelude_import]` on glob `use` items.

compiler/rustc_lint/src/lib.rs

@@ -61,7 +61,6 @@ mod late;
 mod let_underscore;
 mod levels;
 mod methods;
-mod multiple_supertrait_upcastable;
 mod non_ascii_idents;
 mod non_fmt_panic;
 mod nonstandard_style;
@@ -96,7 +95,6 @@ use hidden_unicode_codepoints::*;
 use internal::*;
 use let_underscore::*;
 use methods::*;
-use multiple_supertrait_upcastable::*;
 use non_ascii_idents::*;
 use non_fmt_panic::NonPanicFmt;
 use nonstandard_style::*;
@@ -231,7 +229,6 @@ late_lint_methods!(
             InvalidAtomicOrdering: InvalidAtomicOrdering,
             NamedAsmLabels: NamedAsmLabels,
             OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
-            MultipleSupertraitUpcastable: MultipleSupertraitUpcastable,
         ]
     ]
 );

compiler/rustc_span/src/symbol.rs

@@ -944,7 +944,6 @@ symbols! {
         mul,
         mul_assign,
         mul_with_overflow,
-        multiple_supertrait_upcastable,
         must_not_suspend,
         must_use,
         naked,

library/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)]

library/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.
     ///

library/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)]

src/test/ui/object-safety/issue-106247.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+#![deny(where_clauses_object_safety)]
+
+pub trait Trait {
+    fn method(&self) where Self: Sync;
+}
+
+fn main() {}