Auto merge of #123468 - compiler-errors:precise-capturing, r=oli-obk

Implement syntax for `impl Trait` to specify its captures explicitly (`feature(precise_capturing)`)

Implements `impl use<'a, 'b, T, U> Sized` syntax that allows users to explicitly list the captured parameters for an opaque, rather than inferring it from the opaque's bounds (or capturing *all* lifetimes under 2024-edition capture rules). This allows us to exclude some implicit captures, so this syntax may be used as a migration strategy for changes due to #117587.

We represent this list of captured params as `PreciseCapturingArg` in AST and HIR, resolving them between `rustc_resolve` and `resolve_bound_vars`. Later on, we validate that the opaques only capture the parameters in this list.

We artificially limit the feature to *require* mentioning all type and const parameters, since we don't currently have support for non-lifetime bivariant generics. This can be relaxed in the future.

We also may need to limit this to require naming *all* lifetime parameters for RPITIT, since GATs have no variance. I have to investigate this. This can also be relaxed in the future.

r? `@oli-obk`

Tracking issue:

- https://github.com/rust-lang/rust/issues/123432

1 files changed, 16 insertions, 0 deletions

diff --git a/tests/ui/impl-trait/precise-capturing/ordering.rs b/tests/ui/impl-trait/precise-capturing/ordering.rs
new file mode 100644
index 00000000000..2bace798c57
--- /dev/null
+++ b/tests/ui/impl-trait/precise-capturing/ordering.rs
@@ -0,0 +1,16 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn lt<'a>() -> impl use<'a, 'a> Sized {}
+//~^ ERROR cannot capture parameter `'a` twice
+
+fn ty<T>() -> impl use<T, T> Sized {}
+//~^ ERROR cannot capture parameter `T` twice
+
+fn ct<const N: usize>() -> impl use<N, N> Sized {}
+//~^ ERROR cannot capture parameter `N` twice
+
+fn ordering<'a, T>() -> impl use<T, 'a> Sized {}
+//~^ ERROR lifetime parameter `'a` must be listed before non-lifetime parameters
+
+fn main() {}