`rustc_on_unimplemented` is not std-agnostic

[WaffleLapkin]

If rustc_on_unimplemented contains a path mentioning std::, it won't work in #![no_std] crates, even if the type exists in the core. Similarly if you use core:: path instead, the diagnostic will only work in #![no_std] crates.

As an example take this annotation for ? desugaring:

rust/library/core/src/ops/try_trait.rs

Lines 250 to 260 in fa06a37

	on(
	all(
	from_desugaring = "QuestionMark",
	_Self = "std::option::Option<T>",
	R = "std::result::Result<T, E>",
	),
	message = "the `?` operator can only be used on `Option`s, not `Result`s, \
	in {ItemContext} that returns `Option`",
	label = "use `.ok()?` if you want to discard the `{R}` error information",
	parent_label = "this function returns an `Option`"
	),

It should (and does) trigger for the following code:

fn f() -> Option<()> {
    Err::<(), _>(())?;
    None
}

error[E0277]: the `?` operator can only be used on `Option`s, not `Result`s, in a function that returns `Option`
 --> src/lib.rs:2:21
  |
1 | fn f() -> Option<()> {
  | -------------------- this function returns an `Option`
2 |     Err::<(), _>(())?;
  |                     ^ use `.ok()?` if you want to discard the `Result<Infallible, ()>` error information
  |
  = help: the trait `FromResidual<Result<Infallible, ()>>` is not implemented for `Option<()>`
  = help: the following other types implement trait `FromResidual<R>`:
            <Option<T> as FromResidual<Yeet<()>>>
            <Option<T> as FromResidual>

(play;note the use .ok()? label)

However, if you add #![no_std] the label disappears:

#![no_std]

fn f() -> Option<()> {
    Err::<(), _>(())?;
    None
}

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
 --> src/lib.rs:4:21
  |
3 | fn f() -> Option<()> {
  | -------------------- this function should return `Result` or `Option` to accept `?`
4 |     Err::<(), _>(())?;
  |                     ^ cannot use the `?` operator in a function that returns `Option<()>`
  |
  = help: the trait `FromResidual<Result<Infallible, ()>>` is not implemented for `Option<()>`
  = help: the following other types implement trait `FromResidual<R>`:
            <Option<T> as FromResidual<Yeet<()>>>
            <Option<T> as FromResidual>

(play; the error message is also misleading, the function does return option)

---

rustc in general is not great at being std-agnostic in diagnostics, and this is just another annoying example of that.

Meta

rustc --version --verbose:

rustc 1.70.0 (90c541806 2023-05-31)
binary: rustc
commit-hash: 90c541806f23a127002de5b4038be731ba1458ca
commit-date: 2023-05-31
host: aarch64-unknown-linux-gnu
release: 1.70.0
LLVM version: 16.0.2

[estebank]

The filters can be extended to also account for core:: paths, by mixing any and all in the condition. Sadly that solution has to be peppered throughout all the annotations in std, but is a solution we can implement today.

[WaffleLapkin]

@estebank I think we should still (eventually) investigate how we can make rustc_on_unimplemented more reliable in general, but for the time being your solution is a step forward.

I'll mark this as E-easy Call for participation: Easy difficulty. Experience needed to fix: Not much. Good first issue. + E-mentor Call for participation: This issue has a mentor. Use #t-compiler/help on Zulip for discussion. , so someone can work on this.

Instructions

If you want to work on this, use @rustbot claim to assign this issue to yourself, so that everyone knows someone is already working on it and won't duplicate the work.

Then find all mentions of rustc_on_unimplemented in the standard libraries (./library) and for each path that mentions std::, replace it with any(... = "core::...", ... = "std::...") if the type is present in the core or with any(... = "alloc::...", ... = "std::...") if the type is present in alloc (or with any(... = "core::...", ... = "alloc::...", ... = "std::...") in the rare case that the type is present in both core and alloc).

For example:

--- a/library/core/src/convert/mod.rs
+++ b/library/core/src/convert/mod.rs
@@ -534,7 +534,10 @@ pub trait Into<T>: Sized {
 #[rustc_on_unimplemented(on(
     all(
         _Self = "&str",
-        T = "std::string::String"
+        any(
+            T = "alloc::string::String",
+            T = "std::string::String",
+        )
     ),
     note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
 ))]

[Rageking8]

@rustbot claim

[estebank]

investigate how we can make rustc_on_unimplemented more reliable in general, but for the time being your solution is a step forward.

I believe that the way forward is with annotations in the types themselves that influence the E0277 errors. This design will have to be explored as part of the #[diagnostic::*] namespace effort, as there it will be required for anyone that needs to customize errors caused by local types and foreign traits.