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Rollup merge of #105839 - LegionMammal978:mut-upvar-not-send, r=lcnr

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Suggest a `T: Send` bound for `&mut T` upvars in `Send` generators

Right now, we suggest a `T: Sync` bound for both `&T` and `&mut T` upvars. A user on URLO [found this confusing](https://users.rust-lang.org/t/error-complains-about-missing-sync-but-send-is-whats-missing/86021), so I wrote this quick fix to look at the mutability before making the suggestion.
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Dylan DPC 2022-12-19 14:41:35 +05:30 • committed by GitHub
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39
compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
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@ -2344,28 +2344,33 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
}
}
GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
// `Some(ref_ty)` if `target_ty` is `&T` and `T` fails to impl `Sync`
let refers_to_non_sync = match target_ty.kind() {
ty::Ref(_, ref_ty, _) => match self.evaluate_obligation(&obligation) {
Ok(eval) if !eval.may_apply() => Some(ref_ty),
// `Some((ref_ty, is_mut))` if `target_ty` is `&T` or `&mut T` and fails to impl `Send`
let non_send = match target_ty.kind() {
ty::Ref(_, ref_ty, mutability) => match self.evaluate_obligation(&obligation) {
Ok(eval) if !eval.may_apply() => Some((ref_ty, mutability.is_mut())),
_ => None,
},
_ => None,
};
let (span_label, span_note) = match refers_to_non_sync {
// if `target_ty` is `&T` and `T` fails to impl `Sync`,
// include suggestions to make `T: Sync` so that `&T: Send`
Some(ref_ty) => (
format!(
"has type `{}` which {}, because `{}` is not `Sync`",
target_ty, trait_explanation, ref_ty
),
format!(
"captured value {} because `&` references cannot be sent unless their referent is `Sync`",
trait_explanation
),
),
let (span_label, span_note) = match non_send {
// if `target_ty` is `&T` or `&mut T` and fails to impl `Send`,
// include suggestions to make `T: Sync` so that `&T: Send`,
// or to make `T: Send` so that `&mut T: Send`
Some((ref_ty, is_mut)) => {
let ref_ty_trait = if is_mut { "Send" } else { "Sync" };
let ref_kind = if is_mut { "&mut" } else { "&" };
(
format!(
"has type `{}` which {}, because `{}` is not `{}`",
target_ty, trait_explanation, ref_ty, ref_ty_trait
),
format!(
"captured value {} because `{}` references cannot be sent unless their referent is `{}`",
trait_explanation, ref_kind, ref_ty_trait
),
)
}
None => (
format!("has type `{}` which {}", target_ty, trait_explanation),
format!("captured value {}", trait_explanation),