Auto merge of #3801 - RalfJung:rustup, r=RalfJung

Rustup

2 files changed, 12 insertions, 3 deletions

diff --git a/compiler/rustc_data_structures/src/steal.rs b/compiler/rustc_data_structures/src/steal.rs
index 9a0fd52677d..0f2c0eee27d 100644
--- a/compiler/rustc_data_structures/src/steal.rs
+++ b/compiler/rustc_data_structures/src/steal.rs
@@ -51,6 +51,15 @@ impl<T> Steal<T> {
         let value = value_ref.take();
         value.expect("attempt to steal from stolen value")
     }
+
+    /// Writers of rustc drivers often encounter stealing issues. This function makes it possible to
+    /// handle these errors gracefully.
+    ///
+    /// This should not be used within rustc as it leaks information not tracked
+    /// by the query system, breaking incremental compilation.
+    pub fn is_stolen(&self) -> bool {
+        self.value.borrow().is_none()
+    }
 }
 
 impl<CTX, T: HashStable<CTX>> HashStable<CTX> for Steal<T> {
diff --git a/compiler/rustc_data_structures/src/transitive_relation.rs b/compiler/rustc_data_structures/src/transitive_relation.rs
index 26b00e0af3a..e81ebb9a4be 100644
--- a/compiler/rustc_data_structures/src/transitive_relation.rs
+++ b/compiler/rustc_data_structures/src/transitive_relation.rs
@@ -203,9 +203,9 @@ impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
     /// exists). See `postdom_upper_bound` for details.
     pub fn mutual_immediate_postdominator(&self, mut mubs: Vec<T>) -> Option<T> {
         loop {
-            match mubs.len() {
-                0 => return None,
-                1 => return Some(mubs[0]),
+            match mubs[..] {
+                [] => return None,
+                [mub] => return Some(mub),
                 _ => {
                     let m = mubs.pop().unwrap();
                     let n = mubs.pop().unwrap();