Short-circuit calculate_fallback_to_f32 when no float vars

compiler/rustc_hir_typeck/src/fallback.rs

@@ -147,6 +147,14 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
     /// foo(1.0);
     /// ```
     fn calculate_fallback_to_f32(&self, unresolved_variables: &[Ty<'tcx>]) -> UnordSet<FloatVid> {
+        // Short-circuit: if no unresolved variable is a float, no f32 fallback can apply,
+        // so we can skip the (potentially very expensive) work in `from_float_for_f32_root_vids`.
+        // Under the new solver, that function walks `visit_proof_tree` for every pending
+        // obligation, which is O(N × proof_tree_size) and can dominate type-checking on crates
+        // with many large pending obligations and no f32 involvement.
+        if unresolved_variables.iter().all(|ty| ty.float_vid().is_none()) {
+            return UnordSet::new();
+        }
         let roots: UnordSet<ty::FloatVid> = self.from_float_for_f32_root_vids();
         if roots.is_empty() {
             // Most functions have no `f32: From<{float}>` predicates, so short-circuit and return