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Auto merge of #122802 - estebank:unconstrained-generic-const, r=Nadrieril

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Provide structured suggestion for unconstrained generic constant

```
error: unconstrained generic constant
  --> $DIR/const-argument-if-length.rs:18:10
   |
LL |     pad: [u8; is_zst::<T>()],
   |          ^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
   |                                   ++++++++++++++++++++++++++
```

Detect when the constant expression isn't `usize` and suggest casting:

```
error: unconstrained generic constant
 --> f300.rs:6:10
  |
6 |     bb::<{!N}>();
  |          ^^^^
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs:3539:36
  |
help: try adding a `where` bound
  |
5 | fn b<const N: bool>() where [(); {!N} as usize]: {
  |                       ++++++++++++++++++++++++++
```

Fix #122395.
This commit is contained in:
bors 2024-03-25 09:59:37 +00:00
commit dda2372cf3
41 changed files with 351 additions and 111 deletions
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37
compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
View file

@ -3536,12 +3536,39 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
let mut err =
self.dcx().struct_span_err(span, "unconstrained generic constant");
let const_span = self.tcx.def_span(uv.def);
let const_ty = self.tcx.type_of(uv.def).instantiate(self.tcx, uv.args);
let cast = if const_ty != self.tcx.types.usize { " as usize" } else { "" };
let msg = "try adding a `where` bound";
match self.tcx.sess.source_map().span_to_snippet(const_span) {
Ok(snippet) => err.help(format!(
"try adding a `where` bound using this expression: `where [(); {snippet}]:`"
)),
_ => err.help("consider adding a `where` bound using this expression"),
};
Ok(snippet) => {
let code = format!("[(); {snippet}{cast}]:");
let def_id = if let ObligationCauseCode::CompareImplItemObligation {
trait_item_def_id,
..
} = obligation.cause.code()
{
trait_item_def_id.as_local()
} else {
Some(obligation.cause.body_id)
};
if let Some(def_id) = def_id
&& let Some(generics) = self.tcx.hir().get_generics(def_id)
{
err.span_suggestion_verbose(
generics.tail_span_for_predicate_suggestion(),
msg,
format!("{} {code}", generics.add_where_or_trailing_comma()),
Applicability::MaybeIncorrect,
);
} else {
err.help(format!("{msg}: where {code}"));
};
}
_ => {
err.help(msg);
}
};
Ok(err)
}
ty::ConstKind::Expr(_) => {