allow+update deref_into_dyn_supertrait

this commit makes `deref_into_dyn_supertrait` lint allow-by-default,
removes future incompatibility (we finally live in a broken world), and
changes the wording in the documentation.

previously documentation erroneously said that it lints against *usage*
of the deref impl, while it actually (since 104742) lints on the impl
itself (oooops, my oversight, should have updated it 2+ years ago...)

Author: WaffleLapkin

compiler/rustc_lint/src/deref_into_dyn_supertrait.rs

@@ -1,6 +1,5 @@
 use rustc_hir::{self as hir, LangItem};
 use rustc_middle::ty;
-use rustc_session::lint::FutureIncompatibilityReason;
 use rustc_session::{declare_lint, declare_lint_pass};
 use rustc_span::sym;
 use rustc_trait_selection::traits::supertraits;
@@ -9,12 +8,12 @@ use crate::lints::{SupertraitAsDerefTarget, SupertraitAsDerefTargetLabel};
 use crate::{LateContext, LateLintPass, LintContext};
 
 declare_lint! {
-    /// The `deref_into_dyn_supertrait` lint is output whenever there is a use of the
-    /// `Deref` implementation with a `dyn SuperTrait` type as `Output`.
+    /// The `deref_into_dyn_supertrait` lint is emitted whenever there is a `Deref` implementation
+    /// for `dyn SubTrait` with a `dyn SuperTrait` type as the `Output` type.
     ///
-    /// These implementations are shadowed by the `trait_upcasting` feature (stabilized since
+    /// These implementations are "shadowed" by trait upcasting (stabilized since
     /// CURRENT_RUSTC_VERSION). The `deref` functions is no longer called implicitly, which might
-    /// be behavior change compared to previous rustc versions.
+    /// change behavior compared to previous rustc versions.
     ///
     /// ### Example
     ///
@@ -44,15 +43,14 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// The dyn upcasting coercion feature added a new coercion rules, taking priority
-    /// over certain other coercion rules, which caused some behavior change.
+    /// The trait upcasting coercion added a new coercion rule, taking priority over certain other
+    /// coercion rules, which causes some behavior change compared to older `rustc` versions.
+    ///
+    /// `deref` can be still called explicitly, it just isn't called as part of a deref coercion
+    /// (since trait upcasting coercion takes priority).
     pub DEREF_INTO_DYN_SUPERTRAIT,
-    Warn,
-    "`Deref` implementation usage with a supertrait trait object for output is shadowed by trait upcasting",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseSemanticsChange,
-        reference: "issue #89460 <https://github.com/rust-lang/rust/issues/89460>",
-    };
+    Allow,
+    "`Deref` implementation with a supertrait trait object for output is shadowed by trait upcasting",
 }
 
 declare_lint_pass!(DerefIntoDynSupertrait => [DEREF_INTO_DYN_SUPERTRAIT]);

tests/ui/traits/trait-upcasting/deref-upcast-shadowing-lint.rs

@@ -0,0 +1,17 @@
+//@ check-pass
+#![warn(deref_into_dyn_supertrait)]
+use std::ops::Deref;
+
+trait Bar<T> {}
+trait Foo: Bar<i32> {}
+
+impl<'a> Deref for dyn Foo + 'a {
+    //~^ warn: this `Deref` implementation is covered by an implicit supertrait coercion
+    type Target = dyn Bar<u32> + 'a;
+
+    fn deref(&self) -> &Self::Target {
+        todo!()
+    }
+}
+
+fn main() {}

tests/ui/traits/trait-upcasting/deref-upcast-shadowing-lint.stderr

@@ -0,0 +1,17 @@
+warning: this `Deref` implementation is covered by an implicit supertrait coercion
+  --> $DIR/deref-upcast-shadowing-lint.rs:8:1
+   |
+LL | impl<'a> Deref for dyn Foo + 'a {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Foo` implements `Deref<Target = dyn Bar<u32>>` which conflicts with supertrait `Bar<i32>`
+LL |
+LL |     type Target = dyn Bar<u32> + 'a;
+   |     -------------------------------- target type is a supertrait of `dyn Foo`
+   |
+note: the lint level is defined here
+  --> $DIR/deref-upcast-shadowing-lint.rs:2:9
+   |
+LL | #![warn(deref_into_dyn_supertrait)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: 1 warning emitted
+

tests/ui/traits/trait-upcasting/migrate-lint-deny-regions.rs

@@ -1,13 +1,13 @@
-#![deny(deref_into_dyn_supertrait)]
+//@ check-pass
+#![warn(deref_into_dyn_supertrait)]
 
 use std::ops::Deref;
 
 trait Bar<'a> {}
 trait Foo<'a>: Bar<'a> {}
 
 impl<'a> Deref for dyn Foo<'a> {
-    //~^ ERROR this `Deref` implementation is covered by an implicit supertrait coercion
-    //~| WARN this will change its meaning in a future release!
+    //~^ warn: this `Deref` implementation is covered by an implicit supertrait coercion
     type Target = dyn Bar<'a>;
 
     fn deref(&self) -> &Self::Target {

tests/ui/traits/trait-upcasting/migrate-lint-deny-regions.stderr

@@ -1,19 +1,17 @@
-error: this `Deref` implementation is covered by an implicit supertrait coercion
-  --> $DIR/migrate-lint-deny-regions.rs:8:1
+warning: this `Deref` implementation is covered by an implicit supertrait coercion
+  --> $DIR/migrate-lint-deny-regions.rs:9:1
    |
 LL | impl<'a> Deref for dyn Foo<'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Foo<'_>` implements `Deref<Target = dyn Bar<'_>>` which conflicts with supertrait `Bar<'_>`
-...
+LL |
 LL |     type Target = dyn Bar<'a>;
    |     -------------------------- target type is a supertrait of `dyn Foo<'_>`
    |
-   = warning: this will change its meaning in a future release!
-   = note: for more information, see issue #89460 <https://github.com/rust-lang/rust/issues/89460>
 note: the lint level is defined here
-  --> $DIR/migrate-lint-deny-regions.rs:1:9
+  --> $DIR/migrate-lint-deny-regions.rs:2:9
    |
-LL | #![deny(deref_into_dyn_supertrait)]
+LL | #![warn(deref_into_dyn_supertrait)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 1 previous error
+warning: 1 warning emitted
 

tests/ui/traits/trait-upcasting/migrate-lint-deny.rs

@@ -1,4 +1,5 @@
 //@ check-pass
+#![warn(deref_into_dyn_supertrait)]
 
 use std::ops::Deref;
 
@@ -9,8 +10,7 @@ trait Foo: Bar<i32> {
 }
 
 impl<'a> Deref for dyn Foo + 'a {
-    //~^ WARN this `Deref` implementation is covered by an implicit supertrait coercion
-    //~| WARN this will change its meaning in a future release!
+    //~^ warn: this `Deref` implementation is covered by an implicit supertrait coercion
     type Target = dyn Bar<u32> + 'a;
 
     fn deref(&self) -> &Self::Target {

tests/ui/traits/trait-upcasting/migrate-lint-deny.stderr

@@ -1,15 +1,17 @@
 warning: this `Deref` implementation is covered by an implicit supertrait coercion
-  --> $DIR/migrate-lint-different-substs.rs:11:1
+  --> $DIR/migrate-lint-different-substs.rs:12:1
    |
 LL | impl<'a> Deref for dyn Foo + 'a {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Foo` implements `Deref<Target = dyn Bar<u32>>` which conflicts with supertrait `Bar<i32>`
-...
+LL |
 LL |     type Target = dyn Bar<u32> + 'a;
    |     -------------------------------- target type is a supertrait of `dyn Foo`
    |
-   = warning: this will change its meaning in a future release!
-   = note: for more information, see issue #89460 <https://github.com/rust-lang/rust/issues/89460>
-   = note: `#[warn(deref_into_dyn_supertrait)]` on by default
+note: the lint level is defined here
+  --> $DIR/migrate-lint-different-substs.rs:2:9
+   |
+LL | #![warn(deref_into_dyn_supertrait)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: 1 warning emitted