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Rollup merge of rust-lang#107941 - compiler-errors:str-has-u8-slice-f…
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…or-auto, r=lcnr

Treat `str` as containing `[u8]` for auto trait purposes

Wanted to gauge `@rust-lang/lang` and `@rust-lang/types` teams' thoughts on treating `str` as "containing" a `[u8]` slice for auto-trait purposes.

`@dtolnay` brought this up in rust-lang#13231 (comment) as a blocker for future `str` type librarification, and I think it's both a valid concern and very easy to fix. I'm interested in actually doing that `str` type librarification (rust-lang#107939), but this probably should be considered in the mean time regardless of that PR.

r? types for the impl, though this definitely needs an FCP.
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compiler-errors authored Feb 25, 2023
2 parents 9960a3e + e9a8ff7 commit a2329e0
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Showing 5 changed files with 36 additions and 2 deletions.
Original file line number Diff line number Diff line change
Expand Up @@ -20,12 +20,14 @@ pub(super) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Str
| ty::Error(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Never
| ty::Char => Ok(vec![]),

// Treat this like `struct str([u8]);`
ty::Str => Ok(vec![infcx.tcx.mk_slice(infcx.tcx.types.u8)]),

ty::Dynamic(..)
| ty::Param(..)
| ty::Foreign(..)
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Original file line number Diff line number Diff line change
Expand Up @@ -3107,6 +3107,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
self.tcx.def_span(def_id),
"required because it's used within this closure",
),
ty::Str => err.note("`str` is considered to contain a `[u8]` slice for auto trait purposes"),
_ => err.note(&msg),
};
}
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4 changes: 3 additions & 1 deletion compiler/rustc_trait_selection/src/traits/select/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2300,12 +2300,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Str
| ty::Error(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Never
| ty::Char => ty::Binder::dummy(Vec::new()),

// Treat this like `struct str([u8]);`
ty::Str => ty::Binder::dummy(vec![self.tcx().mk_slice(self.tcx().types.u8)]),

ty::Placeholder(..)
| ty::Dynamic(..)
| ty::Param(..)
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13 changes: 13 additions & 0 deletions tests/ui/auto-traits/str-contains-slice-conceptually.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
#![feature(negative_impls)]
#![feature(auto_traits)]

auto trait AutoTrait {}

impl<T> !AutoTrait for [T] {}

fn needs_auto_trait<T: AutoTrait + ?Sized>() {}

fn main() {
needs_auto_trait::<str>();
//~^ ERROR the trait bound `[u8]: AutoTrait` is not satisfied in `str`
}
16 changes: 16 additions & 0 deletions tests/ui/auto-traits/str-contains-slice-conceptually.stderr
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@@ -0,0 +1,16 @@
error[E0277]: the trait bound `[u8]: AutoTrait` is not satisfied in `str`
--> $DIR/str-contains-slice-conceptually.rs:11:22
|
LL | needs_auto_trait::<str>();
| ^^^ within `str`, the trait `AutoTrait` is not implemented for `[u8]`
|
= note: `str` is considered to contain a `[u8]` slice for auto trait purposes
note: required by a bound in `needs_auto_trait`
--> $DIR/str-contains-slice-conceptually.rs:8:24
|
LL | fn needs_auto_trait<T: AutoTrait + ?Sized>() {}
| ^^^^^^^^^ required by this bound in `needs_auto_trait`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

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