rustc: Prepare to enable ThinLTO by default

This commit prepares to enable ThinLTO and multiple codegen units in release
mode by default. We've still got a debuginfo bug or two to sort out before
actually turning it on by default.

src/libstd/sys_common/backtrace.rs

@@ -252,8 +252,26 @@ fn output_fileline(w: &mut Write,
 // Note that this demangler isn't quite as fancy as it could be. We have lots
 // of other information in our symbols like hashes, version, type information,
 // etc. Additionally, this doesn't handle glue symbols at all.
-pub fn demangle(writer: &mut Write, s: &str, format: PrintFormat) -> io::Result<()> {
-    // First validate the symbol. If it doesn't look like anything we're
+pub fn demangle(writer: &mut Write, mut s: &str, format: PrintFormat) -> io::Result<()> {
+    // During ThinLTO LLVM may import and rename internal symbols, so strip out
+    // those endings first as they're one of the last manglings applied to
+    // symbol names.
+    let llvm = ".llvm.";
+    if let Some(i) = s.find(llvm) {
+        let candidate = &s[i + llvm.len()..];
+        let all_hex = candidate.chars().all(|c| {
+            match c {
+                'A' ... 'F' | '0' ... '9' => true,
+                _ => false,
+            }
+        });
+
+        if all_hex {
+            s = &s[..i];
+        }
+    }
+
+    // Validate the symbol. If it doesn't look like anything we're
     // expecting, we just print it literally. Note that we must handle non-rust
     // symbols because we could have any function in the backtrace.
     let mut valid = true;