Add some more comments

src/librustc_mir_build/build/matches/test.rs

@@ -439,7 +439,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     literal: method,
                 }),
                 args: vec![val, expect],
-                destination: Some((eq_result.clone(), eq_block)),
+                destination: Some((eq_result, eq_block)),
                 cleanup: None,
                 from_hir_call: false,
             },

src/librustc_mir_build/build/scope.rs

@@ -237,6 +237,9 @@ trait DropTreeBuilder<'tcx> {
 
 impl DropTree {
     fn new() -> Self {
+        // The root node of the tree doesn't represent a drop, but instead
+        // represents the block in the tree that should be jumped to once all
+        // of the required drops have been performed.
         let fake_source_info = SourceInfo::outermost(DUMMY_SP);
         let fake_data =
             DropData { source_info: fake_source_info, local: Local::MAX, kind: DropKind::Storage };
@@ -258,6 +261,10 @@ impl DropTree {
         self.entry_points.push((to, from));
     }
 
+    /// Builds the MIR for a given drop tree.
+    ///
+    /// `blocks` should have the same length as `self.drops`, and may have its
+    /// first value set to some already existing block.
     fn build_mir<'tcx, T: DropTreeBuilder<'tcx>>(
         &mut self,
         cfg: &mut CFG<'tcx>,
@@ -1344,10 +1351,16 @@ impl<'tcx> DropTreeBuilder<'tcx> for GeneratorDrop {
         cfg.start_new_block()
     }
     fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
-        let kind = &mut cfg.block_data_mut(from).terminator_mut().kind;
-        if let TerminatorKind::Yield { drop, .. } = kind {
+        let term = cfg.block_data_mut(from).terminator_mut();
+        if let TerminatorKind::Yield { ref mut drop, .. } = term.kind {
             *drop = Some(to);
-        };
+        } else {
+            span_bug!(
+                term.source_info.span,
+                "cannot enter generator drop tree from {:?}",
+                term.kind
+            )
+        }
     }
 }
 
@@ -1358,8 +1371,8 @@ impl<'tcx> DropTreeBuilder<'tcx> for Unwind {
         cfg.start_new_cleanup_block()
     }
     fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
-        let term = &mut cfg.block_data_mut(from).terminator_mut().kind;
-        match term {
+        let term = &mut cfg.block_data_mut(from).terminator_mut();
+        match &mut term.kind {
             TerminatorKind::Drop { unwind, .. }
             | TerminatorKind::DropAndReplace { unwind, .. }
             | TerminatorKind::FalseUnwind { unwind, .. }
@@ -1375,7 +1388,9 @@ impl<'tcx> DropTreeBuilder<'tcx> for Unwind {
             | TerminatorKind::Unreachable
             | TerminatorKind::Yield { .. }
             | TerminatorKind::GeneratorDrop
-            | TerminatorKind::FalseEdges { .. } => bug!("cannot unwind from {:?}", term),
+            | TerminatorKind::FalseEdges { .. } => {
+                span_bug!(term.source_info.span, "cannot unwind from {:?}", term.kind)
+            }
         }
     }
 }