Shorten some overlong comment lines.

It's annoying that these wrap in a 100-char terminal window.

Author: nnethercote

compiler/rustc_mir_build/src/build/scope.rs

@@ -760,12 +760,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     ) {
         let (current_root, parent_root) =
             if self.tcx.sess.opts.unstable_opts.maximal_hir_to_mir_coverage {
-                // Some consumers of rustc need to map MIR locations back to HIR nodes. Currently the
-                // the only part of rustc that tracks MIR -> HIR is the `SourceScopeLocalData::lint_root`
-                // field that tracks lint levels for MIR locations. Normally the number of source scopes
-                // is limited to the set of nodes with lint annotations. The -Zmaximal-hir-to-mir-coverage
-                // flag changes this behavior to maximize the number of source scopes, increasing the
-                // granularity of the MIR->HIR mapping.
+                // Some consumers of rustc need to map MIR locations back to HIR nodes. Currently
+                // the the only part of rustc that tracks MIR -> HIR is the
+                // `SourceScopeLocalData::lint_root` field that tracks lint levels for MIR
+                // locations. Normally the number of source scopes is limited to the set of nodes
+                // with lint annotations. The -Zmaximal-hir-to-mir-coverage flag changes this
+                // behavior to maximize the number of source scopes, increasing the granularity of
+                // the MIR->HIR mapping.
                 (current_id, parent_id)
             } else {
                 // Use `maybe_lint_level_root_bounded` with `self.hir_id` as a bound