Rollup merge of rust-lang#126746 - compiler-errors:no-rpitit, r=oli-obk

Deny `use<>` for RPITITs

Precise capturing `use<>` syntax is currently a no-op on RPITITs, since GATs have no variance, so all captured lifetimes are captured invariantly.

We don't currently *need* to support `use<>` on RPITITs, since `use<>` is initially intended for migrating RPIT *overcaptures* from edition 2021->2024, but since RPITITs currently capture all in-scope lifetimes, we'll never need to write `use<>` on an RPITIT.

Eventually, though, it would be desirable to support precise capturing on RPITITs, since RPITITs overcapturing by default can be annoying to some folks. But let's separate that (which will likely require some delicate types team work for adding variances to GATs and adjusting the refinement rules) from the stabilization of the feature for edition 2024.

r? oli-obk cc ```@traviscross```

Tracking:

- rust-lang#123432

compiler/rustc_ast_lowering/messages.ftl

@@ -130,6 +130,9 @@ ast_lowering_never_pattern_with_guard =
 
 ast_lowering_no_precise_captures_on_apit = `use<...>` precise capturing syntax not allowed in argument-position `impl Trait`
 
+ast_lowering_no_precise_captures_on_rpitit = `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+    .note = currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+
 ast_lowering_previously_used_here = previously used here
 
 ast_lowering_register1 = register `{$reg1_name}`

compiler/rustc_ast_lowering/src/errors.rs

@@ -424,6 +424,14 @@ pub(crate) struct NoPreciseCapturesOnApit {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_lowering_no_precise_captures_on_rpitit)]
+#[note]
+pub(crate) struct NoPreciseCapturesOnRpitit {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_lowering_yield_in_closure)]
 pub(crate) struct YieldInClosure {

compiler/rustc_ast_lowering/src/lib.rs

@@ -1594,6 +1594,26 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         };
         debug!(?captured_lifetimes_to_duplicate);
 
+        match fn_kind {
+            // Deny `use<>` on RPITIT in trait/trait-impl for now.
+            Some(FnDeclKind::Trait | FnDeclKind::Impl) => {
+                if let Some(span) = bounds.iter().find_map(|bound| match *bound {
+                    ast::GenericBound::Use(_, span) => Some(span),
+                    _ => None,
+                }) {
+                    self.tcx.dcx().emit_err(errors::NoPreciseCapturesOnRpitit { span });
+                }
+            }
+            None
+            | Some(
+                FnDeclKind::Fn
+                | FnDeclKind::Inherent
+                | FnDeclKind::ExternFn
+                | FnDeclKind::Closure
+                | FnDeclKind::Pointer,
+            ) => {}
+        }
+
         self.lower_opaque_inner(
             opaque_ty_node_id,
             origin,

tests/ui/impl-trait/precise-capturing/forgot-to-capture-type.rs

@@ -6,6 +6,7 @@ fn type_param<T>() -> impl Sized + use<> {}
 trait Foo {
     fn bar() -> impl Sized + use<>;
     //~^ ERROR `impl Trait` must mention the `Self` type of the trait
+    //~| ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
 }
 
 fn main() {}

tests/ui/impl-trait/precise-capturing/forgot-to-capture-type.stderr

@@ -1,3 +1,11 @@
+error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+  --> $DIR/forgot-to-capture-type.rs:7:30
+   |
+LL |     fn bar() -> impl Sized + use<>;
+   |                              ^^^^^
+   |
+   = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+
 error: `impl Trait` must mention all type parameters in scope in `use<...>`
   --> $DIR/forgot-to-capture-type.rs:3:23
    |
@@ -18,5 +26,5 @@ LL |     fn bar() -> impl Sized + use<>;
    |
    = note: currently, all type parameters are required to be mentioned in the precise captures list
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 

tests/ui/impl-trait/precise-capturing/redundant.normal.stderr

@@ -0,0 +1,20 @@
+warning: all possible in-scope parameters are already captured, so `use<...>` syntax is redundant
+  --> $DIR/redundant.rs:7:19
+   |
+LL | fn hello<'a>() -> impl Sized + use<'a> {}
+   |                   ^^^^^^^^^^^^^-------
+   |                                |
+   |                                help: remove the `use<...>` syntax
+   |
+   = note: `#[warn(impl_trait_redundant_captures)]` on by default
+
+warning: all possible in-scope parameters are already captured, so `use<...>` syntax is redundant
+  --> $DIR/redundant.rs:12:27
+   |
+LL |     fn inherent(&self) -> impl Sized + use<'_> {}
+   |                           ^^^^^^^^^^^^^-------
+   |                                        |
+   |                                        help: remove the `use<...>` syntax
+
+warning: 2 warnings emitted
+

tests/ui/impl-trait/precise-capturing/redundant.rpitit.stderr

@@ -0,0 +1,18 @@
+error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+  --> $DIR/redundant.rs:18:35
+   |
+LL |     fn in_trait() -> impl Sized + use<'a, Self>;
+   |                                   ^^^^^^^^^^^^^
+   |
+   = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+
+error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+  --> $DIR/redundant.rs:23:35
+   |
+LL |     fn in_trait() -> impl Sized + use<'a> {}
+   |                                   ^^^^^^^
+   |
+   = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+
+error: aborting due to 2 previous errors
+

tests/ui/impl-trait/precise-capturing/redundant.rs

@@ -1,24 +1,27 @@
 //@ compile-flags: -Zunstable-options --edition=2024
-//@ check-pass
+//@ revisions: normal rpitit
+//@[normal] check-pass
 
 #![feature(precise_capturing)]
 
 fn hello<'a>() -> impl Sized + use<'a> {}
-//~^ WARN all possible in-scope parameters are already captured
+//[normal]~^ WARN all possible in-scope parameters are already captured
 
 struct Inherent;
 impl Inherent {
     fn inherent(&self) -> impl Sized + use<'_> {}
-    //~^ WARN all possible in-scope parameters are already captured
+    //[normal]~^ WARN all possible in-scope parameters are already captured
 }
 
+#[cfg(rpitit)]
 trait Test<'a> {
     fn in_trait() -> impl Sized + use<'a, Self>;
-    //~^ WARN all possible in-scope parameters are already captured
+    //[rpitit]~^ ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
 }
+#[cfg(rpitit)]
 impl<'a> Test<'a> for () {
     fn in_trait() -> impl Sized + use<'a> {}
-    //~^ WARN all possible in-scope parameters are already captured
+    //[rpitit]~^ ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
 }
 
 fn main() {}

tests/ui/impl-trait/precise-capturing/rpitit.rs

@@ -0,0 +1,21 @@
+//@ known-bug: unknown
+
+// RPITITs don't have variances in their GATs, so they always relate invariantly
+// and act as if they capture all their args.
+// To fix this soundly, we need to make sure that all the trait header args
+// remain captured, since they affect trait selection.
+
+#![feature(precise_capturing)]
+
+trait Foo<'a> {
+    fn hello() -> impl PartialEq + use<Self>;
+}
+
+fn test<'a, 'b, T: for<'r> Foo<'r>>() {
+    PartialEq::eq(
+        &<T as Foo<'a>>::hello(),
+        &<T as Foo<'b>>::hello(),
+    );
+}
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/rpitit.stderr

@@ -0,0 +1,50 @@
+error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+  --> $DIR/rpitit.rs:11:36
+   |
+LL |     fn hello() -> impl PartialEq + use<Self>;
+   |                                    ^^^^^^^^^
+   |
+   = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+
+error: `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
+  --> $DIR/rpitit.rs:11:19
+   |
+LL | trait Foo<'a> {
+   |           -- this lifetime parameter is captured
+LL |     fn hello() -> impl PartialEq + use<Self>;
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime captured due to being mentioned in the bounds of the `impl Trait`
+
+error: lifetime may not live long enough
+  --> $DIR/rpitit.rs:15:5
+   |
+LL |   fn test<'a, 'b, T: for<'r> Foo<'r>>() {
+   |           --  -- lifetime `'b` defined here
+   |           |
+   |           lifetime `'a` defined here
+LL | /     PartialEq::eq(
+LL | |         &<T as Foo<'a>>::hello(),
+LL | |         &<T as Foo<'b>>::hello(),
+LL | |     );
+   | |_____^ argument requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+
+error: lifetime may not live long enough
+  --> $DIR/rpitit.rs:15:5
+   |
+LL |   fn test<'a, 'b, T: for<'r> Foo<'r>>() {
+   |           --  -- lifetime `'b` defined here
+   |           |
+   |           lifetime `'a` defined here
+LL | /     PartialEq::eq(
+LL | |         &<T as Foo<'a>>::hello(),
+LL | |         &<T as Foo<'b>>::hello(),
+LL | |     );
+   | |_____^ argument requires that `'b` must outlive `'a`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+help: `'a` and `'b` must be the same: replace one with the other
+
+error: aborting due to 4 previous errors
+

tests/ui/impl-trait/precise-capturing/self-capture.rs

@@ -1,9 +1,8 @@
-//@ check-pass
-
 #![feature(precise_capturing)]
 
 trait Foo {
     fn bar<'a>() -> impl Sized + use<Self>;
+    //~^ ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
 }
 
 fn main() {}

tests/ui/impl-trait/precise-capturing/self-capture.stderr

@@ -0,0 +1,10 @@
+error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+  --> $DIR/self-capture.rs:4:34
+   |
+LL |     fn bar<'a>() -> impl Sized + use<Self>;
+   |                                  ^^^^^^^^^
+   |
+   = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
+
+error: aborting due to 1 previous error
+