Make TypeFoldable implementors short-circuit on error

Co-authored-by: Alan Egerton <eggyal@gmail.com>

compiler/rustc_data_structures/src/functor.rs

@@ -2,12 +2,16 @@ use rustc_index::vec::{Idx, IndexVec};
 use std::mem;
 use std::ptr;
 
-pub trait IdFunctor {
+pub trait IdFunctor: Sized {
     type Inner;
 
     fn map_id<F>(self, f: F) -> Self
     where
         F: FnMut(Self::Inner) -> Self::Inner;
+
+    fn try_map_id<F, E>(self, f: F) -> Result<Self, E>
+    where
+        F: FnMut(Self::Inner) -> Result<Self::Inner, E>;
 }
 
 impl<T> IdFunctor for Box<T> {
@@ -31,6 +35,25 @@ impl<T> IdFunctor for Box<T> {
             raw.assume_init()
         }
     }
+
+    #[inline]
+    fn try_map_id<F, E>(self, mut f: F) -> Result<Self, E>
+    where
+        F: FnMut(Self::Inner) -> Result<Self::Inner, E>,
+    {
+        let raw = Box::into_raw(self);
+        Ok(unsafe {
+            // SAFETY: The raw pointer points to a valid value of type `T`.
+            let value = ptr::read(raw);
+            // SAFETY: Converts `Box<T>` to `Box<MaybeUninit<T>>` which is the
+            // inverse of `Box::assume_init()` and should be safe.
+            let mut raw: Box<mem::MaybeUninit<T>> = Box::from_raw(raw.cast());
+            // SAFETY: Write the mapped value back into the `Box`.
+            ptr::write(raw.as_mut_ptr(), f(value)?);
+            // SAFETY: We just initialized `raw`.
+            raw.assume_init()
+        })
+    }
 }
 
 impl<T> IdFunctor for Vec<T> {
@@ -55,6 +78,35 @@ impl<T> IdFunctor for Vec<T> {
         }
         self
     }
+
+    #[inline]
+    fn try_map_id<F, E>(mut self, mut f: F) -> Result<Self, E>
+    where
+        F: FnMut(Self::Inner) -> Result<Self::Inner, E>,
+    {
+        // FIXME: We don't really care about panics here and leak
+        // far more than we should, but that should be fine for now.
+        let len = self.len();
+        let mut error = Ok(());
+        unsafe {
+            self.set_len(0);
+            let start = self.as_mut_ptr();
+            for i in 0..len {
+                let p = start.add(i);
+                match f(ptr::read(p)) {
+                    Ok(value) => ptr::write(p, value),
+                    Err(err) => {
+                        error = Err(err);
+                        break;
+                    }
+                }
+            }
+            // Even if we encountered an error, set the len back
+            // so we don't leak memory.
+            self.set_len(len);
+        }
+        error.map(|()| self)
+    }
 }
 
 impl<T> IdFunctor for Box<[T]> {
@@ -67,6 +119,14 @@ impl<T> IdFunctor for Box<[T]> {
     {
         Vec::from(self).map_id(f).into()
     }
+
+    #[inline]
+    fn try_map_id<F, E>(self, f: F) -> Result<Self, E>
+    where
+        F: FnMut(Self::Inner) -> Result<Self::Inner, E>,
+    {
+        Vec::from(self).try_map_id(f).map(Into::into)
+    }
 }
 
 impl<I: Idx, T> IdFunctor for IndexVec<I, T> {
@@ -79,4 +139,12 @@ impl<I: Idx, T> IdFunctor for IndexVec<I, T> {
     {
         IndexVec::from_raw(self.raw.map_id(f))
     }
+
+    #[inline]
+    fn try_map_id<F, E>(self, f: F) -> Result<Self, E>
+    where
+        F: FnMut(Self::Inner) -> Result<Self::Inner, E>,
+    {
+        self.raw.try_map_id(f).map(IndexVec::from_raw)
+    }
 }