Auto merge of #91230 - eggyal:fallible-type-fold, r=jackh726

Make `TypeFolder::fold_*` return `Result`

Implements rust-lang/compiler-team#432.

Initially this is just a rebase of `@LeSeulArtichaut's` work in #85469 (abandoned; see #85485 (comment)).  At that time, it caused a regression in performance that required some further exploration... with this rebased PR bors can hopefully report some perf analysis from which we can investigate further (if the regression is indeed still present).

r? `@jackh726` cc `@nikomatsakis`

Author: bors

compiler/rustc_const_eval/src/lib.rs

@@ -23,6 +23,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(trusted_len)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
+#![feature(unwrap_infallible)]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_const_eval/src/transform/validate.rs

@@ -95,7 +95,8 @@ pub fn equal_up_to_regions(
                 // Leave consts and types unchanged.
                 ct_op: |ct| ct,
                 ty_op: |ty| ty,
-            }),
+            })
+            .into_ok(),
         )
     };
     tcx.infer_ctxt().enter(|infcx| infcx.can_eq(param_env, normalize(src), normalize(dest)).is_ok())