Rollup merge of #136610 - Jarcho:range_idx, r=Noratrieb

Allow `IndexSlice` to be indexed by ranges.

This comes with some annoyances as the index type can no longer inferred from indexing expressions. The biggest offender for this is `IndexVec::from_fn_n(|idx| ..., n)` where the index type won't be inferred from the call site or any index expressions inside the closure.

My main use case for this is mapping a `Place` to `Range<Idx>` for value tracking where the range represents all the values the place contains.

compiler/rustc_mir_transform/src/check_pointers.rs

@@ -71,8 +71,7 @@ pub(crate) fn check_pointers<'tcx, F>(
     // statements/blocks after. Iterating or visiting the MIR in order would require updating
     // our current location after every insertion. By iterating backwards, we dodge this issue:
     // The only Locations that an insertion changes have already been handled.
-    for block in (0..basic_blocks.len()).rev() {
-        let block = block.into();
+    for block in basic_blocks.indices().rev() {
         for statement_index in (0..basic_blocks[block].statements.len()).rev() {
             let location = Location { block, statement_index };
             let statement = &basic_blocks[block].statements[statement_index];

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -50,7 +50,7 @@ fn make_node_flow_priority_list(
     // A "reloop" node has exactly one out-edge, which jumps back to the top
     // of an enclosing loop. Reloop nodes are typically visited more times
     // than loop-exit nodes, so try to avoid giving them physical counters.
-    let is_reloop_node = IndexVec::from_fn_n(
+    let is_reloop_node = IndexVec::<BasicCoverageBlock, _>::from_fn_n(
         |node| match graph.successors[node].as_slice() {
             &[succ] => graph.dominates(succ, node),
             _ => false,

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -42,7 +42,7 @@ impl CoverageGraph {
         // `SwitchInt` to have multiple targets to the same destination `BasicBlock`, so
         // de-duplication is required. This is done without reordering the successors.
 
-        let successors = IndexVec::from_fn_n(
+        let successors = IndexVec::<BasicCoverageBlock, _>::from_fn_n(
             |bcb| {
                 let mut seen_bcbs = FxHashSet::default();
                 let terminator = mir_body[bcbs[bcb].last_bb()].terminator();

compiler/rustc_mir_transform/src/gvn.rs

@@ -1259,7 +1259,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
         let layout = self.ecx.layout_of(lhs_ty).ok()?;
 
-        let as_bits = |value| {
+        let as_bits = |value: VnIndex| {
             let constant = self.evaluated[value].as_ref()?;
             if layout.backend_repr.is_scalar() {
                 let scalar = self.ecx.read_scalar(constant).discard_err()?;