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/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,