Rollup merge of rust-lang#156508 - oli-obk:boom, r=petrochenkov

Infer all anonymous lifetimes in assoc consts as `'static`

fixes rust-lang#115010

This FCW has been on for almost a year, let's actually do the change that T-lang asked for now that there's no breakage to be expected anymore. I first did a crater run checking if turning it into a hard error breaks anything.

There are two cases that would be a hard error (but totally fine by defaulting to `'static`), both based on an old version of `minio`. One is a personal crate on github with no changes in the last year (https://github.com/PapePathe/daaray_kaamil) and the other is https://github.com/gear-tech/gear, which is developed on github actively and does not have this issue anymore (also old `minio` dependency that was since updated). The crates.io version is 2 years outdated.

Thus we can pretty much conclude we're not going to cause anyone any confusion or subtle bugs by changing the rules. The rules for associated constants' types are now the same as the rules for free constants' types:

We now treat *all* anonymous lifetimes as `'static`, irrespective of whether other lifetimes are in scope or not. This means the following piece of code starts compiling without warnings or errors again.

```rust
struct Foo<'a>(&'a ());

impl<'a> Foo<'a> {
    const STR: &str = "hello, world";
}
```

We keep the hard error on

```rust
trait Foo {
    const B: S = S { s: &() };
}

struct S<'a> {
    s: &'a (),
}
```

in contrast to free constant, which are orthogonal to this change and cause more breakage

Author: jhpratt

compiler/rustc_lint_defs/src/builtin.rs

@@ -44,7 +44,6 @@ pub mod hardwired {
             DEPRECATED_WHERE_CLAUSE_LOCATION,
             DUPLICATE_FEATURES,
             DUPLICATE_MACRO_ATTRIBUTES,
-            ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
             ELIDED_LIFETIMES_IN_PATHS,
             EXPLICIT_BUILTIN_CFGS_IN_FLAGS,
             EXPORTED_PRIVATE_DEPENDENCIES,
@@ -4762,48 +4761,6 @@ declare_lint! {
     "impl trait in impl method signature does not match trait method signature",
 }
 
-declare_lint! {
-    /// The `elided_lifetimes_in_associated_constant` lint detects elided lifetimes
-    /// in associated constants when there are other lifetimes in scope. This was
-    /// accidentally supported, and this lint was later relaxed to allow eliding
-    /// lifetimes to `'static` when there are no lifetimes in scope.
-    ///
-    /// ### Example
-    ///
-    /// ```rust,compile_fail
-    /// #![deny(elided_lifetimes_in_associated_constant)]
-    ///
-    /// struct Foo<'a>(&'a ());
-    ///
-    /// impl<'a> Foo<'a> {
-    ///     const STR: &str = "hello, world";
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Previous version of Rust
-    ///
-    /// Implicit static-in-const behavior was decided [against] for associated
-    /// constants because of ambiguity. This, however, regressed and the compiler
-    /// erroneously treats elided lifetimes in associated constants as lifetime
-    /// parameters on the impl.
-    ///
-    /// This is a [future-incompatible] lint to transition this to a
-    /// hard error in the future.
-    ///
-    /// [against]: https://github.com/rust-lang/rust/issues/38831
-    /// [future-incompatible]: ../index.md#future-incompatible-lints
-    pub ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
-    Deny,
-    "elided lifetimes cannot be used in associated constants in impls",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: fcw!(FutureReleaseError #115010),
-    };
-}
-
 declare_lint! {
     /// The `private_macro_use` lint detects private macros that are imported
     /// with `#[macro_use]`.

compiler/rustc_resolve/src/diagnostics.rs

@@ -1661,28 +1661,6 @@ pub(crate) struct UnusedQualifications {
     pub removal_span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(
-    "{$elided ->
-        [true] `&` without an explicit lifetime name cannot be used here
-        *[false] `'_` cannot be used here
-    }"
-)]
-pub(crate) struct AssociatedConstElidedLifetime {
-    #[suggestion(
-        "use the `'static` lifetime",
-        style = "verbose",
-        code = "{code}",
-        applicability = "machine-applicable"
-    )]
-    pub span: Span,
-
-    pub code: &'static str,
-    pub elided: bool,
-    #[note("cannot automatically infer `'static` because of other lifetimes in scope")]
-    pub lifetimes_in_scope: MultiSpan,
-}
-
 #[derive(Diagnostic)]
 #[diag("lifetime parameter `{$ident}` only used once")]
 pub(crate) struct SingleUseLifetime {

compiler/rustc_resolve/src/late.rs

@@ -1,4 +1,4 @@
-#![deny(elided_lifetimes_in_associated_constant)]
+//@ check-pass
 
 use std::marker::PhantomData;
 
@@ -8,12 +8,8 @@ struct Foo<'a> {
 
 impl<'a> Foo<'a> {
     const FOO: Foo<'_> = Foo { x: PhantomData::<&()> };
-    //~^ ERROR `'_` cannot be used here
-    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
     const BAR: &() = &();
-    //~^ ERROR `&` without an explicit lifetime name cannot be used here
-    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 }
 
 fn main() {}

tests/ui/consts/assoc-const-elided-lifetime.rs

@@ -1,11 +1,9 @@
-#![deny(elided_lifetimes_in_associated_constant)]
+//@check-pass
 
 struct Foo<'a>(&'a ());
 
 impl Foo<'_> {
     const STATIC: &str = "";
-    //~^ ERROR `&` without an explicit lifetime name cannot be used here
-    //~| WARN this was previously accepted by the compiler but is being phased out
 }
 
 trait Bar {
@@ -14,9 +12,6 @@ trait Bar {
 
 impl Bar for Foo<'_> {
     const STATIC: &str = "";
-    //~^ ERROR `&` without an explicit lifetime name cannot be used here
-    //~| WARN this was previously accepted by the compiler but is being phased out
-    //~| ERROR lifetime parameters or bounds on associated constant `STATIC` do not match the trait declaration
 }
 
 fn main() {}

tests/ui/consts/assoc-const-elided-lifetime.stderr

@@ -1,18 +1,16 @@
-#![deny(elided_lifetimes_in_associated_constant)]
+//@ check-pass
+
 #![feature(generic_const_items)]
 
 struct A;
 impl A {
     const GAC_TYPE<T>: &str = "";
     const GAC_LIFETIME<'a>: &str = "";
-    //~^ ERROR `&` without an explicit lifetime name cannot be used here
-    //~| WARN this was previously accepted by the compiler but is being phased out
 }
 
 trait Trait {
     const GAC_TYPE<T>: &str = "";
     const GAC_LIFETIME<'a>: &str = "";
-    //~^ ERROR missing lifetime specifier
 }
 
 fn main() {}

tests/ui/consts/static-default-lifetime/elided-lifetime.rs

@@ -1,4 +1,4 @@
-#![deny(elided_lifetimes_in_associated_constant)]
+//@ check-pass
 
 trait Bar<'a> {
     const STATIC: &'a str;
@@ -7,9 +7,6 @@ trait Bar<'a> {
 struct A;
 impl Bar<'_> for A {
     const STATIC: &str = "";
-    //~^ ERROR `&` without an explicit lifetime name cannot be used here
-    //~| WARN this was previously accepted by the compiler but is being phased out
-    //~| ERROR lifetime parameters or bounds on associated constant `STATIC` do not match the trait declaration
 }
 
 struct B;
@@ -19,12 +16,10 @@ impl Bar<'static> for B {
 
 struct C;
 impl Bar<'_> for C {
-    // make  ^^ not cause
     const STATIC: &'static str = {
         struct B;
         impl Bar<'static> for B {
             const STATIC: &str = "";
-            //            ^ to emit a future incompat warning
         }
         ""
     };