Make the crate compile on stable

Cargo.lock

@@ -4348,6 +4348,7 @@ dependencies = [
  "rustc_target",
  "smallvec",
  "tracing",
+ "typed-arena",
 ]
 
 [[package]]
@@ -5683,6 +5684,12 @@ dependencies = [
  "rustc-hash",
 ]
 
+[[package]]
+name = "typed-arena"
+version = "2.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6af6ae20167a9ece4bcb41af5b80f8a1f1df981f6391189ce00fd257af04126a"
+
 [[package]]
 name = "typenum"
 version = "1.16.0"

compiler/rustc_pattern_analysis/Cargo.toml

@@ -6,24 +6,28 @@ edition = "2021"
 [dependencies]
 # tidy-alphabetical-start
 rustc_apfloat = "0.2.0"
-rustc_arena = { path = "../rustc_arena" }
+rustc_arena = { path = "../rustc_arena", optional = true }
 rustc_data_structures = { path = "../rustc_data_structures", optional = true }
 rustc_errors = { path = "../rustc_errors", optional = true }
 rustc_fluent_macro = { path = "../rustc_fluent_macro", optional = true }
 rustc_hir = { path = "../rustc_hir", optional = true }
-rustc_index = { path = "../rustc_index" }
+rustc_index = { path = "../rustc_index", default-features = false }
 rustc_macros = { path = "../rustc_macros", optional = true }
 rustc_middle = { path = "../rustc_middle", optional = true }
 rustc_session = { path = "../rustc_session", optional = true }
 rustc_span = { path = "../rustc_span", optional = true }
 rustc_target = { path = "../rustc_target", optional = true }
-smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+smallvec = { version = "1.8.1", features = ["union"] }
 tracing = "0.1"
+typed-arena = { version = "2.0.2", optional = true }
 # tidy-alphabetical-end
 
 [features]
 default = ["rustc"]
+# It's not possible to only enable the `typed_arena` dependency when the `rustc` feature is off, so
+# we use another feature instead. The crate won't compile if one of these isn't enabled.
 rustc = [
+    "dep:rustc_arena",
     "dep:rustc_data_structures",
     "dep:rustc_errors",
     "dep:rustc_fluent_macro",
@@ -33,4 +37,9 @@ rustc = [
     "dep:rustc_session",
     "dep:rustc_span",
     "dep:rustc_target",
+    "smallvec/may_dangle",
+    "rustc_index/nightly",
+]
+stable = [
+    "dep:typed-arena",
 ]

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -555,7 +555,12 @@
 use smallvec::{smallvec, SmallVec};
 use std::fmt;
 
+// It's not possible to only enable the `typed_arena` dependency when the `rustc` feature is off, so
+// we use another feature instead. The crate won't compile if one of these isn't enabled.
+#[cfg(feature = "rustc")]
 use rustc_arena::TypedArena;
+#[cfg(feature = "stable")]
+use typed_arena::Arena as TypedArena;
 
 use crate::constructor::{Constructor, ConstructorSet};
 use crate::pat::{DeconstructedPat, WitnessPat};

src/tools/tidy/src/deps.rs

@@ -357,6 +357,7 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "tracing-tree",
     "twox-hash",
     "type-map",
+    "typed-arena",
     "typenum",
     "unic-langid",
     "unic-langid-impl",