Rollup merge of #133237 - fee1-dead-contrib:constadd, r=compiler-errors

Minimally constify `Add`

* This PR removes the requirement for `impl const` to have a const stability attribute. cc ``@RalfJung`` I believe you mentioned that it would make much more sense to require `const_trait`s to have const stability instead. I agree with that sentiment but I don't think that is _required_ for a small scale experimentation like this PR. https://github.com/rust-lang/project-const-traits/issues/16 should definitely be prioritized in the future, but removing the impl check should be good for now as all callers need `const_trait_impl` enabled for any const impl to work.
* This PR is intentionally minimal as constifying other traits can become more complicated (`PartialEq`, for example, would run into requiring implementing it for `str` as that is used in matches, which runs into the implementation for slice equality which uses specialization)

Per the reasons above, anyone who is interested in making traits `const` in the standard library are **strongly encouraged** to reach out to us on the [Zulip channel](https://rust-lang.zulipchat.com/#narrow/channel/419616-t-compiler.2Fproject-const-traits) before proceeding with the work.

cc ``@rust-lang/project-const-traits``

I believe there is prior approval from libs that we can experiment, so

r? project-const-traits

tests/ui/stability-attribute/missing-const-stability.rs

@@ -27,7 +27,7 @@ pub trait Bar {
 }
 #[stable(feature = "stable", since = "1.0.0")]
 impl const Bar for Foo {
-    //~^ ERROR implementation has missing const stability attribute
+    // ok because all users must enable `const_trait_impl`
     fn fun() {}
 }
 

tests/ui/stability-attribute/missing-const-stability.stderr

@@ -4,15 +4,6 @@ error: function has missing const stability attribute
 LL | pub const fn foo() {}
    | ^^^^^^^^^^^^^^^^^^^^^
 
-error: implementation has missing const stability attribute
-  --> $DIR/missing-const-stability.rs:29:1
-   |
-LL | / impl const Bar for Foo {
-LL | |
-LL | |     fn fun() {}
-LL | | }
-   | |_^
-
 error: function has missing const stability attribute
   --> $DIR/missing-const-stability.rs:36:1
    |
@@ -25,5 +16,5 @@ error: associated function has missing const stability attribute
 LL |     pub const fn foo() {}
    |     ^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 

tests/ui/traits/const-traits/call-const-trait-method-pass.stderr

@@ -1,12 +1,3 @@
-error: const `impl` for trait `Add` which is not marked with `#[const_trait]`
-  --> $DIR/call-const-trait-method-pass.rs:7:12
-   |
-LL | impl const std::ops::Add for Int {
-   |            ^^^^^^^^^^^^^
-   |
-   = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
-   = note: adding a non-const method body in the future would be a breaking change
-
 error: const `impl` for trait `PartialEq` which is not marked with `#[const_trait]`
   --> $DIR/call-const-trait-method-pass.rs:15:12
    |
@@ -16,14 +7,6 @@ LL | impl const PartialEq for Int {
    = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
    = note: adding a non-const method body in the future would be a breaking change
 
-error[E0015]: cannot call non-const operator in constants
-  --> $DIR/call-const-trait-method-pass.rs:39:22
-   |
-LL | const ADD_INT: Int = Int(1i32) + Int(2i32);
-   |                      ^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
-
 error[E0015]: cannot call non-const fn `<Int as PartialEq>::eq` in constant functions
   --> $DIR/call-const-trait-method-pass.rs:20:15
    |
@@ -32,6 +15,6 @@ LL |         !self.eq(other)
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0015`.

tests/ui/traits/const-traits/const-and-non-const-impl.stderr

@@ -1,21 +1,3 @@
-error: const `impl` for trait `Add` which is not marked with `#[const_trait]`
-  --> $DIR/const-and-non-const-impl.rs:7:12
-   |
-LL | impl const std::ops::Add for i32 {
-   |            ^^^^^^^^^^^^^
-   |
-   = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
-   = note: adding a non-const method body in the future would be a breaking change
-
-error: const `impl` for trait `Add` which is not marked with `#[const_trait]`
-  --> $DIR/const-and-non-const-impl.rs:23:12
-   |
-LL | impl const std::ops::Add for Int {
-   |            ^^^^^^^^^^^^^
-   |
-   = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
-   = note: adding a non-const method body in the future would be a breaking change
-
 error[E0119]: conflicting implementations of trait `Add` for type `Int`
   --> $DIR/const-and-non-const-impl.rs:23:1
    |
@@ -38,7 +20,7 @@ LL | impl const std::ops::Add for i32 {
    = note: for more information see https://doc.rust-lang.org/reference/items/implementations.html#orphan-rules
    = note: define and implement a trait or new type instead
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0117, E0119.
 For more information about an error, try `rustc --explain E0117`.

tests/ui/traits/const-traits/generic-bound.rs

@@ -1,4 +1,4 @@
-//@ known-bug: #110395
+//@ check-pass
 
 #![feature(const_trait_impl)]
 
@@ -26,5 +26,6 @@ const fn twice<T: std::ops::Add>(arg: S<T>) -> S<T> {
 }
 
 fn main() {
+    const _: S<i32> = twice(S(PhantomData));
     let _ = twice(S(PhantomData::<i32>));
 }

tests/ui/traits/const-traits/generic-bound.stderr

@@ -1,20 +0,0 @@
-error: const `impl` for trait `Add` which is not marked with `#[const_trait]`
-  --> $DIR/generic-bound.rs:16:15
-   |
-LL | impl<T> const std::ops::Add for S<T> {
-   |               ^^^^^^^^^^^^^
-   |
-   = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
-   = note: adding a non-const method body in the future would be a breaking change
-
-error[E0015]: cannot call non-const operator in constant functions
-  --> $DIR/generic-bound.rs:25:5
-   |
-LL |     arg + arg
-   |     ^^^^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0015`.