diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 9d66863ef70e1..cef0ae65f658d 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -611,11 +611,18 @@ fn get_flow_results<'a, 'tcx>(
 ) -> Results<'tcx, Borrowck<'a, 'tcx>> {
     // We compute these three analyses individually, but them combine them into
     // a single results so that `mbcx` can visit them all together.
-    let borrows = Borrows::new(tcx, body, regioncx, borrow_set).iterate_to_fixpoint(
-        tcx,
-        body,
-        Some("borrowck"),
-    );
+    let borrows_analysis = Borrows::new(tcx, body, regioncx, borrow_set);
+    let borrows = if borrow_set.location_map().is_empty() {
+        // A body without borrows has an empty dataflow domain: every entry
+        // state is the bottom value, so skip running the fixpoint engine.
+        let bottom = borrows_analysis.bottom_value(body);
+        Results {
+            analysis: borrows_analysis,
+            entry_states: IndexVec::from_elem_n(bottom, body.basic_blocks.len()),
+        }
+    } else {
+        borrows_analysis.iterate_to_fixpoint(tcx, body, Some("borrowck"))
+    };
     let uninits = MaybeUninitializedPlaces::new(tcx, body, move_data).iterate_to_fixpoint(
         tcx,
         body,
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index ff6b15b173ed1..bcd90eb77bb6c 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -532,9 +532,11 @@ impl<'a> TraitDef<'a> {
                     _ => unreachable!(),
                 };
                 // Keep the lint attributes of the previous item to control how the
-                // generated implementations are linted
-                let mut attrs = newitem.attrs.clone();
-                attrs.extend(
+                // generated implementations are linted. `newitem` is already
+                // owned, so extend its attributes in place rather than deep-
+                // cloning the whole generated impl.
+                let mut newitem = newitem;
+                newitem.attrs.extend(
                     item.attrs
                         .iter()
                         .filter(|a| {
@@ -549,7 +551,7 @@ impl<'a> TraitDef<'a> {
                         })
                         .cloned(),
                 );
-                push(Annotatable::Item(Box::new(ast::Item { attrs, ..(*newitem).clone() })))
+                push(Annotatable::Item(newitem))
             }
             _ => unreachable!(),
         }
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index c6bbeda36e930..05756f0d9786b 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -2186,6 +2186,12 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
             }
         }
 
+        // The overwhelming majority of nodes have no cfg or macro attribute
+        // to strip; skip the mutable attribute pass entirely for them.
+        if cfg_pos.is_none() && attr_pos.is_none() {
+            return None;
+        }
+
         item.visit_attrs(|attrs| {
             attr = Some(match (cfg_pos, attr_pos) {
                 (Some(pos), _) => (attrs.remove(pos), pos, Vec::new()),
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index bed2be51a3468..8dff1e7be62f8 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -277,14 +277,10 @@ impl<'a> MetaDecoder for &'a MetadataBlob {
 
     fn decoder(self, pos: usize) -> Self::Context {
         BlobDecodeContext {
-            // FIXME: This unwrap should never panic because we check that it won't when creating
-            // `MetadataBlob`. Ideally we'd just have a `MetadataDecoder` and hand out subslices of
-            // it as we do elsewhere in the compiler using `MetadataDecoder::split_at`. But we own
-            // the data for the decoder so holding onto the `MemDecoder` too would make us a
-            // self-referential struct which is downright goofy because `MetadataBlob` is already
-            // self-referential. Probably `MemDecoder` should contain an `OwnedSlice`, but that
-            // demands a significant refactoring due to our crate graph.
-            opaque: MemDecoder::new(self, pos).unwrap(),
+            // The blob passed full `MemDecoder` validation when the
+            // `MetadataBlob` was created, so skip re-validating the magic
+            // suffix here: this runs for every lazily-decoded item.
+            opaque: MemDecoder::new_prevalidated(self, pos),
             lazy_state: LazyState::NoNode,
             blob: self.blob(),
         }
diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index 4a644d3bb6eac..e9a899ef58771 100644
--- a/compiler/rustc_serialize/src/opaque.rs
+++ b/compiler/rustc_serialize/src/opaque.rs
@@ -293,6 +293,18 @@ impl<'a> MemDecoder<'a> {
         Ok(MemDecoder { start, current: data[position..].as_ptr(), end, _marker: PhantomData })
     }
 
+    /// Like [`MemDecoder::new`], but skips re-validating the `MAGIC_END_BYTES`
+    /// suffix. Callers must have validated `data` before (e.g. with a prior
+    /// successful `MemDecoder::new`); this is worthwhile because a decoder is
+    /// constructed for every lazily-decoded metadata item.
+    #[inline]
+    pub fn new_prevalidated(data: &'a [u8], position: usize) -> MemDecoder<'a> {
+        debug_assert!(data.ends_with(MAGIC_END_BYTES));
+        let data = &data[..data.len() - MAGIC_END_BYTES.len()];
+        let Range { start, end } = data.as_ptr_range();
+        MemDecoder { start, current: data[position..].as_ptr(), end, _marker: PhantomData }
+    }
+
     #[inline]
     pub fn split_at(&self, position: usize) -> MemDecoder<'a> {
         assert!(position <= self.len());