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typeck: silence lint for "collision safe" items #99898

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typeck: silence lint for "collision safe" items #99898

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Commits on Nov 21, 2022

  1. typeck: silence lint for collision safe items 

    When an unstable method exists on a type and the user is invoking a
method that would no longer have priority when the unstable method is
stabilized, an "unstable name collision" lint is emitted. In the vast
majority of circumstances, this is desirable.

However, when adding a new inherent method to the standard library,
which deliberately shadows the name of a method in the `Deref` target,
then this lint can be triggered, affecting users on stable who don't
even know about the new unstable inherent method. As the new method is
being added to the standard library, by the library team, it can be
known that the lint isn't necessary, as the library team can ensure that
the new inherent method has the same behaviour and won't cause any
problems for users when it is stabilized.

```rust
pub struct Foo;
pub struct Bar;

impl std::ops::Deref for Foo {
    type Target = Bar;
    fn deref(&self) -> &Self::Target { &Bar }
}

impl Foo {
    #[unstable(feature = "new_feature", issue = "none", collision_safe)]
    pub fn example(&self) -> u32 { 4 }
}

impl Bar {
    #[stable(feature = "old_feature", since = "1.0.0")]
    pub fn example(&self) -> u32 { 3 }
}

// ..in another crate..
fn main() {
    let foo = Foo;
    assert_eq!(foo.example(), 3);
    // still invokes `Bar`'s `example`, as the `new_feature` isn't enabled, but doesn't
    // trigger a name collision lint (in practice, both `example` functions should
    // have identical behaviour)
}
```

Without this addition, the new inherent method would need to be
insta-stable in order to avoid breaking stable users.

Signed-off-by: David Wood <david.wood@huawei.com>
    @davidtwco
    davidtwco committed Nov 21, 2022
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  2. typeck: always lint for user's shadows 

    `collision_safe` works when the new-unstable item is only shadowing a
from-std stable item, but it could also shadow a user-defined item from
a extension trait, in which case the lint should still fire. Use the
presence of a stability attribute on a chosen item as a heuristic for a
std item and continue to lint if the chosen item isn't from std even if
the unstable items are collision-safe.

Signed-off-by: David Wood <david.wood@huawei.com>
    @davidtwco
    davidtwco committed Nov 21, 2022
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