Estimate size of InstanceDef::DropGlue more accurately

Also a little bit of clean up.

src/librustc/mir/mono.rs

@@ -157,8 +157,7 @@ impl<'tcx> CodegenUnit<'tcx> {
 
     pub fn size_estimate(&self) -> usize {
         // Should only be called if `estimate_size` has previously been called.
-        assert!(self.size_estimate.is_some());
-        self.size_estimate.unwrap()
+        self.size_estimate.expect("estimate_size must be called before getting a size_estimate")
     }
 
     pub fn modify_size_estimate(&mut self, delta: usize) {
@@ -176,7 +175,8 @@ impl<'tcx> HashStable<StableHashingContext<'tcx>> for CodegenUnit<'tcx> {
         let CodegenUnit {
             ref items,
             name,
-            ..
+            // The size estimate is not relevant to the hash
+            size_estimate: _,
         } = *self;
 
         name.hash_stable(hcx, hasher);

src/librustc/ty/mod.rs

@@ -2673,11 +2673,12 @@ fn instance_def_size_estimate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                         instance_def: InstanceDef<'tcx>)
                                         -> usize {
     match instance_def {
-        InstanceDef::Item(def_id) => {
-            let mir = tcx.optimized_mir(def_id);
+        InstanceDef::Item(..) |
+        InstanceDef::DropGlue(..) => {
+            let mir = tcx.instance_mir(instance_def);
             mir.basic_blocks().iter().map(|bb| bb.statements.len()).sum()
         },
-        // Estimate the size of compiler-generated shims to be 1.
+        // Estimate the size of other compiler-generated shims to be 1.
         _ => 1
     }
 }