Rollup merge of rust-lang#55755 - ljedrz:a_few_indexvec_tweaks, r=varkor

Improve creation of 3 IndexVecs

- preallocate when the minimal size is known
- use `from_elem_n` instead of `new`+`resize`

Author: kennytm

src/librustc/ty/query/on_disk_cache.rs

@@ -450,8 +450,7 @@ impl<'sess> OnDiskCache<'sess> {
                                      .map(|&(cnum, ..)| cnum)
                                      .max()
                                      .unwrap_or(0) + 1;
-            let mut map = IndexVec::new();
-            map.resize(map_size as usize, None);
+            let mut map = IndexVec::from_elem_n(None, map_size as usize);
 
             for &(prev_cnum, ref crate_name, crate_disambiguator) in prev_cnums {
                 let key = (crate_name.clone(), crate_disambiguator);

src/librustc_mir/shim.rs

@@ -196,7 +196,7 @@ fn build_drop_shim<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let source_info = SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE };
 
     let return_block = BasicBlock::new(1);
-    let mut blocks = IndexVec::new();
+    let mut blocks = IndexVec::with_capacity(2);
     let block = |blocks: &mut IndexVec<_, _>, kind| {
         blocks.push(BasicBlockData {
             statements: vec![],
@@ -768,7 +768,8 @@ fn build_call_shim<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         }));
     }
 
-    let mut blocks = IndexVec::new();
+    let n_blocks = if let Adjustment::RefMut = rcvr_adjustment { 5 } else { 2 };
+    let mut blocks = IndexVec::with_capacity(n_blocks);
     let block = |blocks: &mut IndexVec<_, _>, statements, kind, is_cleanup| {
         blocks.push(BasicBlockData {
             statements,