From 5768fb7d93414e830c6ca0fbf0f4f8502d2de4f4 Mon Sep 17 00:00:00 2001
From: Trevor Gross <tg@trevorgross.com>
Date: Sat, 7 Feb 2026 05:49:02 -0600
Subject: [PATCH] symcheck: Check for core symbols with the new mangling

The recent switch in default mangling meant that the check was no longer
working correctly. Resolve this by checking for both legacy- and
v0-mangled core symbols to the extent that this is possible.
---
 .../compiler-builtins/crates/symbol-check/Cargo.toml |  1 +
 .../crates/symbol-check/src/main.rs                  | 12 +++++++++++-
 .../crates/symbol-check/tests/all.rs                 |  6 ++++--
 3 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/library/compiler-builtins/crates/symbol-check/Cargo.toml b/library/compiler-builtins/crates/symbol-check/Cargo.toml
index 774d1c31a4b0..6291a0ca7f45 100644
--- a/library/compiler-builtins/crates/symbol-check/Cargo.toml
+++ b/library/compiler-builtins/crates/symbol-check/Cargo.toml
@@ -6,6 +6,7 @@ publish = false
 
 [dependencies]
 object = { version = "0.37.3", features = ["wasm"] }
+regex = "1.12.3"
 serde_json = "1.0.149"
 
 [dev-dependencies]
diff --git a/library/compiler-builtins/crates/symbol-check/src/main.rs b/library/compiler-builtins/crates/symbol-check/src/main.rs
index 1413042e6c70..733d9f4e8bef 100644
--- a/library/compiler-builtins/crates/symbol-check/src/main.rs
+++ b/library/compiler-builtins/crates/symbol-check/src/main.rs
@@ -9,12 +9,14 @@ use std::fs;
 use std::io::{BufRead, BufReader};
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
+use std::sync::LazyLock;
 
 use object::read::archive::ArchiveFile;
 use object::{
     File as ObjFile, Object, ObjectSection, ObjectSymbol, Result as ObjResult, Symbol, SymbolKind,
     SymbolScope,
 };
+use regex::Regex;
 use serde_json::Value;
 
 const CHECK_LIBRARIES: &[&str] = &["compiler_builtins", "builtins_test_intrinsics"];
@@ -255,6 +257,14 @@ fn verify_no_duplicates(archive: &BinFile) {
 
 /// Ensure that there are no references to symbols from `core` that aren't also (somehow) defined.
 fn verify_core_symbols(archive: &BinFile) {
+    // Match both mangling styles:
+    //
+    // * `_ZN4core3str8converts9from_utf817hd4454ac14cbbb790E` (old)
+    // * `_RNvNtNtCscK9O3IwVk7N_4core3str8converts9from_utf8` (v0)
+    //
+    // Also account for the Apple leading `_`.
+    static RE: LazyLock<Regex> = LazyLock::new(|| Regex::new(r"^_?_[RZ].*4core").unwrap());
+
     let mut defined = BTreeSet::new();
     let mut undefined = Vec::new();
     let mut has_symbols = false;
@@ -263,7 +273,7 @@ fn verify_core_symbols(archive: &BinFile) {
         has_symbols = true;
 
         // Find only symbols from `core`
-        if !symbol.name().unwrap().contains("_ZN4core") {
+        if !RE.is_match(symbol.name().unwrap()) {
             return;
         }
 
diff --git a/library/compiler-builtins/crates/symbol-check/tests/all.rs b/library/compiler-builtins/crates/symbol-check/tests/all.rs
index 4ad9509726fb..400469a49e2a 100644
--- a/library/compiler-builtins/crates/symbol-check/tests/all.rs
+++ b/library/compiler-builtins/crates/symbol-check/tests/all.rs
@@ -66,8 +66,10 @@ fn test_core_symbols() {
     let lib_out = dir.path().join("libfoo.rlib");
     rustc_build(&input_dir().join("core_symbols.rs"), &lib_out, |cmd| cmd);
     let assert = cargo_bin_cmd!().arg("check").arg(&lib_out).assert();
-    // FIXME(symcheck): this should fail but we don't detect the new mangling.
-    assert.success();
+    assert
+        .failure()
+        .stderr_contains("found 1 undefined symbols from core")
+        .stderr_contains("from_utf8");
 }
 
 #[test]