Rollup merge of rust-lang#152661 - BartSimpson001:fix-next-solver-from-ice, r=petrochenkov

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

Fixes rust-lang#152518.

Under `-Znext-solver=globally`, `trait_ref.args` may contain fewer
elements than expected. The diagnostic logic in
`fulfillment_errors.rs` assumed at least two elements and
unconditionally called `type_at(1)`, which could lead to an
index out-of-bounds panic during error reporting.

This change adds a defensive check before accessing the second
argument to avoid the ICE. A UI regression test has been added.

Author: Zalathar

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -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 {
+                                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;

tests/ui/traits/next-solver-ice.rs

@@ -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() {}

tests/ui/traits/next-solver-ice.stderr

@@ -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`.