Handle ambiguous projections

Author: smoelius

clippy_lints/src/dereference.rs

@@ -21,7 +21,7 @@ use rustc_hir::{
     TraitItemKind, TyKind, UnOp,
 };
 use rustc_index::bit_set::BitSet;
-use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::infer::{InferOk, TyCtxtInferExt};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::mir::{Rvalue, StatementKind};
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
@@ -31,8 +31,9 @@ use rustc_middle::ty::{
 };
 use rustc_session::{declare_tool_lint, impl_lint_pass};
 use rustc_span::{symbol::sym, Span, Symbol};
-use rustc_trait_selection::infer::InferCtxtExt as _;
-use rustc_trait_selection::traits::{query::evaluate_obligation::InferCtxtExt as _, Obligation, ObligationCause};
+use rustc_trait_selection::infer::{canonical::OriginalQueryValues, InferCtxtExt as _};
+use rustc_trait_selection::traits::query::{evaluate_obligation::InferCtxtExt as _, NormalizationResult};
+use rustc_trait_selection::traits::{Obligation, ObligationCause};
 use std::collections::VecDeque;
 
 declare_clippy_lint! {
@@ -1359,13 +1360,30 @@ fn replace_types<'tcx>(
                 {
                     let item_def_id = projection_predicate.projection_ty.def_id;
                     let assoc_item = cx.tcx.associated_item(item_def_id);
-                    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)
-                        && substs[term_param_ty.index as usize] != ty::GenericArg::from(projected_ty)
+                    let alias_ty = cx
+                        .tcx
+                        .mk_alias_ty(assoc_item.def_id, cx.tcx.mk_substs_trait(new_ty, []));
+
+                    // The following normalization method is based on:
+                    // https://github.com/rust-lang/rust/blob/695072daa6cc04045f2aa79d751d884ad5263080/compiler/rustc_trait_selection/src/traits/query/normalize.rs#L258-L285
+                    // It is meant to address: https://github.com/rust-lang/rust/issues/107877
+                    let infcx = cx.tcx.infer_ctxt().build();
+                    let mut orig_values = OriginalQueryValues::default();
+                    let canonical_ty =
+                        infcx.canonicalize_query_keep_static(cx.param_env.and(alias_ty), &mut orig_values);
+
+                    if let Ok(result) = cx.tcx.normalize_projection_ty(canonical_ty)
+                        && !result.is_ambiguous()
+                        && let Ok(InferOk { value: NormalizationResult { normalized_ty, .. }, .. }) =
+                            infcx.instantiate_query_response_and_region_obligations(
+                                &ObligationCause::dummy(),
+                                cx.param_env,
+                                &orig_values,
+                                result,
+                            )
+                        && substs[term_param_ty.index as usize] != ty::GenericArg::from(normalized_ty)
                     {
-                        deque.push_back((*term_param_ty, projected_ty));
+                        deque.push_back((*term_param_ty, normalized_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 &'a Foo {
+    type Error = ();
+
+    fn try_from(_: &'a String) -> Result<Self, Self::Error> {
+        Ok(&Foo)
+    }
+}
+
+fn find<'a, E>(_: impl TryInto<&'a Foo, Error = E>) {}
+
+fn main() {
+    find(&String::new());
+}