From c9ad0e5c95a28440bf841981d07c64184b0d27a5 Mon Sep 17 00:00:00 2001
From: Muhtasim-Rasheed <mutasimos5101@gmail.com>
Date: Sun, 5 Jul 2026 19:53:22 +0600
Subject: [PATCH] Emit a suggestion to cast the never type into a concrete type
 when it fails to satisfy an `impl Trait` bound

---
 .../src/error_reporting/traits/suggestions.rs  | 17 ++++++++++++++---
 tests/ui/impl-trait/return-never-type.rs       | 14 ++++++++++++++
 tests/ui/impl-trait/return-never-type.stderr   | 18 ++++++++++++++++++
 3 files changed, 46 insertions(+), 3 deletions(-)
 create mode 100644 tests/ui/impl-trait/return-never-type.rs
 create mode 100644 tests/ui/impl-trait/return-never-type.stderr

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 139aa6bd77a87..9f5cfb39b9718 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -4479,7 +4479,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             }
             ObligationCauseCode::OpaqueReturnType(expr_info) => {
                 let (expr_ty, expr) = if let Some((expr_ty, hir_id)) = expr_info {
-                    let expr_ty = tcx.short_string(expr_ty, err.long_ty_path());
                     let expr = tcx.hir_expect_expr(hir_id);
                     (expr_ty, expr)
                 } else if let Some(body_id) = tcx.hir_node_by_def_id(body_id).body_id()
@@ -4494,15 +4493,27 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     && let ty::ClauseKind::Trait(pred) = pred.kind().skip_binder()
                     && self.can_eq(param_env, pred.self_ty(), expr_ty)
                 {
-                    let expr_ty = tcx.short_string(expr_ty, err.long_ty_path());
                     (expr_ty, expr)
                 } else {
                     return;
                 };
+                let expr_ty_string = tcx.short_string(expr_ty, err.long_ty_path());
+                if expr_ty.is_never()
+                    && let span = expr.span.source_callsite()
+                    && let Ok(snippet) = tcx.sess.source_map().span_to_snippet(span)
+                    && span != expr.span
+                {
+                    err.span_suggestion(
+                        span,
+                        "`!` can be coerced to any type; consider casting it to a concrete type that implements the trait",
+                        format!("{snippet} as /* Type */"),
+                        Applicability::HasPlaceholders,
+                    );
+                }
                 err.span_label(
                     expr.span,
                     with_forced_trimmed_paths!(format!(
-                        "return type was inferred to be `{expr_ty}` here",
+                        "return type was inferred to be `{expr_ty_string}` here",
                     )),
                 );
                 suggest_remove_deref(err, &expr);
diff --git a/tests/ui/impl-trait/return-never-type.rs b/tests/ui/impl-trait/return-never-type.rs
new file mode 100644
index 0000000000000..97ad2756dce71
--- /dev/null
+++ b/tests/ui/impl-trait/return-never-type.rs
@@ -0,0 +1,14 @@
+//@ edition:2024
+
+#![feature(never_type)]
+
+use std::ops::Add;
+
+fn foo() -> impl Add<u32> {
+    //~^ ERROR cannot add `u32` to `!`
+    //~| HELP the trait `Add<u32>` is not implemented for `!`
+    unimplemented!()
+    //~^ HELP `!` can be coerced to any type; consider casting it to a concrete type that implements the trait
+}
+
+fn main() {}
diff --git a/tests/ui/impl-trait/return-never-type.stderr b/tests/ui/impl-trait/return-never-type.stderr
new file mode 100644
index 0000000000000..dff06d7f41c98
--- /dev/null
+++ b/tests/ui/impl-trait/return-never-type.stderr
@@ -0,0 +1,18 @@
+error[E0277]: cannot add `u32` to `!`
+  --> $DIR/return-never-type.rs:7:13
+   |
+LL | fn foo() -> impl Add<u32> {
+   |             ^^^^^^^^^^^^^ no implementation for `! + u32`
+...
+LL |     unimplemented!()
+   |     ---------------- return type was inferred to be `!` here
+   |
+   = help: the trait `Add<u32>` is not implemented for `!`
+help: `!` can be coerced to any type; consider casting it to a concrete type that implements the trait
+   |
+LL |     unimplemented!() as /* Type */
+   |                      +++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.