Rollup merge of rust-lang#126531 - slanterns:error_provider, r=workin…

…gjubilee

Add codegen test for `Request::provide_*`

Codegen before & after rust-lang#126242: https://gist.github.com/slanterns/3789ee36f59ed834e1a6bd4677b68ed4.

Also adjust an outdated comment since `tag_id` is no longer attached to `TaggedOption` via `Erased`, but stored next to it in `Tagged` under the new implementation.

My first time writing FileCheck xD. Correct me if there is anything that should be amended.

r? libs

core/src/error.rs

@@ -928,7 +928,7 @@ pub(crate) mod tags {
 /// An `Option` with a type tag `I`.
 ///
 /// Since this struct implements `Erased`, the type can be erased to make a dynamically typed
-/// option. The type can be checked dynamically using `Erased::tag_id` and since this is statically
+/// option. The type can be checked dynamically using `Tagged::tag_id` and since this is statically
 /// checked for the concrete type, there is some degree of type safety.
 #[repr(transparent)]
 pub(crate) struct TaggedOption<'a, I: tags::Type<'a>>(pub Option<I::Reified>);