Rollup merge of #105484 - nbdd0121:upcast, r=compiler-errors

Implement allow-by-default `multiple_supertrait_upcastable` lint

The lint detects when an object-safe trait has multiple supertraits.

Enabled in libcore and liballoc as they are low-level enough that many embedded programs will use them.

r? `@nikomatsakis`

src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![deny(multiple_supertrait_upcastable)]
+//~^ WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+#![warn(multiple_supertrait_upcastable)]
+//~^ WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.stderr

@@ -0,0 +1,57 @@
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:3:1
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+   = note: `#[warn(unknown_lints)]` on by default
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:7:1
+   |
+LL | #![warn(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:3:1
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:7:1
+   |
+LL | #![warn(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:3:1
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:7:1
+   |
+LL | #![warn(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: 6 warnings emitted
+

src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.rs

@@ -0,0 +1,10 @@
+#![feature(multiple_supertrait_upcastable)]
+#![deny(multiple_supertrait_upcastable)]
+
+trait A {}
+trait B {}
+
+trait C: A + B {}
+//~^ ERROR `C` is object-safe and has multiple supertraits
+
+fn main() {}

src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.stderr

@@ -0,0 +1,14 @@
+error: `C` is object-safe and has multiple supertraits
+  --> $DIR/multiple_supertrait_upcastable.rs:7:1
+   |
+LL | trait C: A + B {}
+   | ^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/multiple_supertrait_upcastable.rs:2:9
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+