Add lifetime_capture_rules_2024

compiler/rustc_ast_lowering/src/lib.rs

@@ -1571,8 +1571,10 @@ fn lower_opaque_impl_trait(
                 Vec::new()
             }
             hir::OpaqueTyOrigin::FnReturn(..) => {
-                if let FnDeclKind::Impl | FnDeclKind::Trait =
-                    fn_kind.expect("expected RPITs to be lowered with a FnKind")
+                if matches!(
+                    fn_kind.expect("expected RPITs to be lowered with a FnKind"),
+                    FnDeclKind::Impl | FnDeclKind::Trait
+                ) || self.tcx.features().lifetime_capture_rules_2024
                 {
                     // return-position impl trait in trait was decided to capture all
                     // in-scope lifetimes, which we collect for all opaques during resolution.

compiler/rustc_feature/src/unstable.rs

@@ -195,6 +195,8 @@ pub fn internal(&self, feature: Symbol) -> bool {
     (internal, intrinsics, "1.0.0", None, None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
     (internal, lang_items, "1.0.0", None, None),
+    /// Changes `impl Trait` to capture all lifetimes in scope.
+    (unstable, lifetime_capture_rules_2024, "CURRENT_RUSTC_VERSION", None, None),
     /// Allows `#[link(..., cfg(..))]`; perma-unstable per #37406
     (unstable, link_cfg, "1.14.0", None, None),
     /// Allows the `multiple_supertrait_upcastable` lint.

compiler/rustc_span/src/symbol.rs

@@ -929,6 +929,7 @@
         lib,
         libc,
         lifetime,
+        lifetime_capture_rules_2024,
         lifetimes,
         likely,
         line,

tests/ui/feature-gates/feature-gate-lifetime-capture-rules-2024.rs

@@ -0,0 +1,6 @@
+fn foo(x: &Vec<i32>) -> impl Sized {
+    x
+    //~^ ERROR hidden type for `impl Sized` captures lifetime that does not appear in bounds
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-lifetime-capture-rules-2024.stderr

@@ -0,0 +1,18 @@
+error[E0700]: hidden type for `impl Sized` captures lifetime that does not appear in bounds
+  --> $DIR/feature-gate-lifetime-capture-rules-2024.rs:2:5
+   |
+LL | fn foo(x: &Vec<i32>) -> impl Sized {
+   |           ---------     ---------- opaque type defined here
+   |           |
+   |           hidden type `&Vec<i32>` captures the anonymous lifetime defined here
+LL |     x
+   |     ^
+   |
+help: to declare that `impl Sized` captures `'_`, you can add an explicit `'_` lifetime bound
+   |
+LL | fn foo(x: &Vec<i32>) -> impl Sized + '_ {
+   |                                    ++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/impl-trait/variance.new.stderr

@@ -0,0 +1,26 @@
+error: [*, o]
+  --> $DIR/variance.rs:12:36
+   |
+LL | fn not_captured_early<'a: 'a>() -> impl Sized {}
+   |                                    ^^^^^^^^^^
+
+error: [*, o]
+  --> $DIR/variance.rs:16:32
+   |
+LL | fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {}
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: [o]
+  --> $DIR/variance.rs:18:40
+   |
+LL | fn not_captured_late<'a>(_: &'a ()) -> impl Sized {}
+   |                                        ^^^^^^^^^^
+
+error: [o]
+  --> $DIR/variance.rs:22:36
+   |
+LL | fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+

tests/ui/impl-trait/variance.old.stderr

@@ -1,23 +1,23 @@
 error: [*]
-  --> $DIR/variance.rs:8:36
+  --> $DIR/variance.rs:12:36
    |
 LL | fn not_captured_early<'a: 'a>() -> impl Sized {}
    |                                    ^^^^^^^^^^
 
 error: [*, o]
-  --> $DIR/variance.rs:10:32
+  --> $DIR/variance.rs:16:32
    |
 LL | fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {}
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: []
-  --> $DIR/variance.rs:12:40
+  --> $DIR/variance.rs:18:40
    |
 LL | fn not_captured_late<'a>(_: &'a ()) -> impl Sized {}
    |                                        ^^^^^^^^^^
 
 error: [o]
-  --> $DIR/variance.rs:14:36
+  --> $DIR/variance.rs:22:36
    |
 LL | fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^

tests/ui/impl-trait/variance.rs

@@ -1,3 +1,7 @@
+// revisions: old new
+
+#![cfg_attr(new, feature(lifetime_capture_rules_2024))]
+
 #![feature(rustc_attrs)]
 #![allow(internal_features)]
 #![rustc_variance_of_opaques]
@@ -5,11 +9,15 @@
 trait Captures<'a> {}
 impl<T> Captures<'_> for T {}
 
-fn not_captured_early<'a: 'a>() -> impl Sized {} //~ [*]
+fn not_captured_early<'a: 'a>() -> impl Sized {}
+//[old]~^ [*]
+//[new]~^^ [*, o]
 
 fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {} //~ [*, o]
 
-fn not_captured_late<'a>(_: &'a ()) -> impl Sized {} //~ []
+fn not_captured_late<'a>(_: &'a ()) -> impl Sized {}
+//[old]~^ []
+//[new]~^^ [o]
 
 fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {} //~ [o]