Handle ambiguous projections

Author: smoelius

clippy_lints/src/dereference.rs

@@ -3,7 +3,7 @@ use clippy_utils::mir::{enclosing_mir, expr_local, local_assignments, used_exact
 use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::source::{snippet_with_applicability, snippet_with_context};
 use clippy_utils::sugg::has_enclosing_paren;
-use clippy_utils::ty::{adt_and_variant_of_res, expr_sig, is_copy, peel_mid_ty_refs, ty_sig};
+use clippy_utils::ty::{adt_and_variant_of_res, expr_sig, implements_trait, is_copy, peel_mid_ty_refs, ty_sig};
 use clippy_utils::{
     fn_def_id, get_parent_expr, get_parent_expr_for_hir, is_lint_allowed, path_to_local, walk_to_expr_usage,
 };
@@ -1362,7 +1362,12 @@ fn replace_types<'tcx>(
                     let projection = cx.tcx
                         .mk_projection(assoc_item.def_id, cx.tcx.mk_substs_trait(new_ty, []));
 
-                    if let Ok(projected_ty) = cx.tcx.try_normalize_erasing_regions(cx.param_env, projection)
+                    // Verify that `new_ty` implements the relevant trait before trying to normalize `projection` to
+                    // avoid hitting this `span_bug`:
+                    // https://github.com/rust-lang/rust/blob/695072daa6cc04045f2aa79d751d884ad5263080/compiler/rustc_trait_selection/src/traits/query/normalize.rs#L272-L275
+                    // See: https://github.com/rust-lang/rust/issues/107877
+                    if implements_trait(cx, new_ty, assoc_item.container_id(cx.tcx), List::empty())
+                        && let Ok(projected_ty) = cx.tcx.try_normalize_erasing_regions(cx.param_env, projection)
                         && substs[term_param_ty.index as usize] != ty::GenericArg::from(projected_ty)
                     {
                         deque.push_back((*term_param_ty, projected_ty));

tests/ui/crashes/ice-rust-107877.rs

@@ -0,0 +1,17 @@
+#![allow(dead_code)]
+
+struct Foo;
+
+impl<'a> std::convert::TryFrom<&'a String> for Foo {
+    type Error = std::convert::Infallible;
+
+    fn try_from(_: &'a String) -> Result<Self, Self::Error> {
+        Ok(Foo)
+    }
+}
+
+fn find<E>(_: impl std::convert::TryInto<Foo, Error = E>) {}
+
+fn main() {
+    find(&String::new());
+}