Dogfood {exclusive,half-open} ranges in compiler (nfc)

In particular, this allows us to write more explicit matches that
avoid the pitfalls of using a fully general fall-through case, yet
remain fairly ergonomic. Less logic is in guard cases, more is in
the actual exhaustive case analysis.

No functional changes.

2 files changed, 5 insertions, 3 deletions

diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 1db59bfc39d..d036fbbb67d 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -35,6 +35,8 @@
 #![feature(never_type)]
 #![feature(nll)]
 #![feature(or_patterns)]
+#![feature(half_open_range_patterns)]
+#![feature(exclusive_range_pattern)]
 #![recursion_limit = "256"]
 
 #[macro_use]
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index 2409069031d..17f0d5632e6 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -250,13 +250,13 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                     has_emitted
                 }
                 ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
+                    // If the array is empty we don't lint, to avoid false positives
+                    Some(0) | None => false,
                     // If the array is definitely non-empty, we can do `#[must_use]` checking.
-                    Some(n) if n != 0 => {
+                    Some(n) => {
                         let descr_pre = &format!("{}array{} of ", descr_pre, plural_suffix,);
                         check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, n as usize + 1)
                     }
-                    // Otherwise, we don't lint, to avoid false positives.
-                    _ => false,
                 },
                 ty::Closure(..) => {
                     cx.struct_span_lint(UNUSED_MUST_USE, span, |lint| {