coverage: Store a BCB's list of basic blocks as a boxed slice

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -85,15 +85,15 @@ impl CoverageGraph {
         let mut bb_to_bcb = IndexVec::from_elem_n(None, num_basic_blocks);
 
         let mut add_basic_coverage_block = |basic_blocks: &mut Vec<BasicBlock>| {
-            // Take the accumulated list of blocks, but leave
+            // Take the accumulated list of blocks as a boxed slice, but leave
             // the vector's capacity intact to be reused for subsequent BCBs.
-            let basic_blocks = basic_blocks.split_off(0);
+            let basic_blocks: Box<[BasicBlock]> = basic_blocks.split_off(0).into();
 
             let bcb = bcbs.next_index();
             for &bb in basic_blocks.iter() {
                 bb_to_bcb[bb] = Some(bcb);
             }
-            let bcb_data = BasicCoverageBlockData::from(basic_blocks);
+            let bcb_data = BasicCoverageBlockData::new(basic_blocks);
             debug!("adding bcb{}: {:?}", bcb.index(), bcb_data);
             bcbs.push(bcb_data);
         };
@@ -281,11 +281,11 @@ rustc_index::newtype_index! {
 /// significance.
 #[derive(Debug, Clone)]
 pub(super) struct BasicCoverageBlockData {
-    pub basic_blocks: Vec<BasicBlock>,
+    pub basic_blocks: Box<[BasicBlock]>,
 }
 
 impl BasicCoverageBlockData {
-    pub fn from(basic_blocks: Vec<BasicBlock>) -> Self {
+    pub fn new(basic_blocks: Box<[BasicBlock]>) -> Self {
         assert!(basic_blocks.len() > 0);
         Self { basic_blocks }
     }