Rollup merge of #124555 - Zalathar:init-coverage, r=nnethercote

coverage: Clean up creation of MC/DC condition bitmaps

This PR improves the code for creating and initializing [MC/DC](https://en.wikipedia.org/wiki/Modified_condition/decision_coverage) condition bitmap variables, as introduced by #123409 and modified by #124255.

- The condition bitmap variables are now created eagerly at the start of per-function codegen, via a new `init_coverage` method in `CoverageInfoBuilderMethods`. This avoids having to retroactively create the bitmaps while doing codegen for an individual coverage statement.
- As a result, we can now create and initialize those bitmaps using existing safe APIs, instead of having to perform our own unsafe call to `llvm::LLVMBuildAlloca`.
- This PR also tweaks the way we count the number of condition bitmaps needed, by tracking the total number of bitmaps needed (max depth + 1), instead of only tracking the maximum depth. This reduces the potential for subtle off-by-one confusion.

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -102,7 +102,7 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
 
     inject_mcdc_statements(mir_body, &basic_coverage_blocks, &coverage_spans);
 
-    let mcdc_max_decision_depth = coverage_spans
+    let mcdc_num_condition_bitmaps = coverage_spans
         .mappings
         .iter()
         .filter_map(|bcb_mapping| match bcb_mapping.kind {
@@ -110,7 +110,7 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
             _ => None,
         })
         .max()
-        .unwrap_or(0);
+        .map_or(0, |max| usize::from(max) + 1);
 
     mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
         function_source_hash: hir_info.function_source_hash,
@@ -118,7 +118,7 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
         mcdc_bitmap_bytes: coverage_spans.test_vector_bitmap_bytes(),
         expressions: coverage_counters.into_expressions(),
         mappings,
-        mcdc_max_decision_depth,
+        mcdc_num_condition_bitmaps,
     }));
 }