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Avoid ICE in From/TryFrom diagnostic under -Znext-solver #152661

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Merged
rust-bors merged 1 commit into from
Feb 17, 2026
+51 −13
Merged

Avoid ICE in From/TryFrom diagnostic under -Znext-solver #152661

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34 changes: 21 additions & 13 deletions compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -282,23 +282,31 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
if self.tcx.is_diagnostic_item(sym::From, trait_def_id)
|| self.tcx.is_diagnostic_item(sym::TryFrom, trait_def_id)
{
let found_ty = leaf_trait_predicate.skip_binder().trait_ref.args.type_at(1);
let ty = main_trait_predicate.skip_binder().self_ty();
if let Some(cast_ty) = self.find_explicit_cast_type(
obligation.param_env,
found_ty,
ty,
) {
let found_ty_str = self.tcx.short_string(found_ty, &mut long_ty_file);
let cast_ty_str = self.tcx.short_string(cast_ty, &mut long_ty_file);
err.help(
format!(
let trait_ref = leaf_trait_predicate.skip_binder().trait_ref;

// Defensive: next-solver may produce fewer args than expected.
if trait_ref.args.len() > 1 {
Comment on lines +287 to +288
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@lcnr lcnr Feb 17, 2026

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It shouldn't. There's a more involved underlying bug here

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@lcnr lcnr Feb 17, 2026

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though actually, this whole diagnostics looks somewhat non-sensical. If the DefId of the root goal is From or TryFrom, this doesn't tell you anything about the leaf_trait_predicate. It could be an arbitrary trait goal?

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@ShoyuVanilla ShoyuVanilla Feb 19, 2026

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Yeah, it's not the next-solver that produces fewer args.
The problem is that obligation, i.e. its trait def just has the fewer args(=1) here and it is ZeroablePrimitive in this case.
I'll look into why we are trying to report a fulfillment error on it.

let found_ty = trait_ref.args.type_at(1);
let ty = main_trait_predicate.skip_binder().self_ty();

if let Some(cast_ty) = self.find_explicit_cast_type(
obligation.param_env,
found_ty,
ty,
) {
let found_ty_str =
self.tcx.short_string(found_ty, &mut long_ty_file);
let cast_ty_str =
self.tcx.short_string(cast_ty, &mut long_ty_file);

err.help(format!(
"consider casting the `{found_ty_str}` value to `{cast_ty_str}`",
),
);
));
}
}
}


*err.long_ty_path() = long_ty_file;

let mut suggested = false;
Expand Down
9 changes: 9 additions & 0 deletions tests/ui/traits/next-solver-ice.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
//@compile-flags: -Znext-solver=globally
//@check-fail

fn check<T: Iterator>() {
<f32 as From<<T as Iterator>::Item>>::from;
//~^ ERROR the trait bound `f32: From<<T as Iterator>::Item>` is not satisfied
}

fn main() {}
21 changes: 21 additions & 0 deletions tests/ui/traits/next-solver-ice.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
error[E0277]: the trait bound `f32: From<<T as Iterator>::Item>` is not satisfied
--> $DIR/next-solver-ice.rs:5:6
|
LL | <f32 as From<<T as Iterator>::Item>>::from;
| ^^^ the nightly-only, unstable trait `ZeroablePrimitive` is not implemented for `f32`
|
= help: the following other types implement trait `ZeroablePrimitive`:
i128
i16
i32
i64
i8
isize
u128
u16
and 4 others
= note: required for `f32` to implement `From<<T as Iterator>::Item>`

error: aborting due to 1 previous error

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