Rollup merge of rust-lang#83020 - hi-rustin:rustin-patch-enum, r=lcnr

Emit the enum range assumption if the range only contains one element

close rust-lang#82871

Author: Dylan-DPC

Diff for: compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -325,7 +325,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
                                 let er = scalar.valid_range_exclusive(bx.cx());
                                 if er.end != er.start
-                                    && scalar.valid_range.end() > scalar.valid_range.start()
+                                    && scalar.valid_range.end() >= scalar.valid_range.start()
                                 {
                                     // We want `table[e as usize ± k]` to not
                                     // have bound checks, and this is the most

Diff for: src/test/codegen/enum-bounds-check-issue-82871.rs

@@ -0,0 +1,16 @@
+// compile-flags: -O
+// min-llvm-version: 11.0
+
+#![crate_type = "lib"]
+
+#[repr(C)]
+pub enum E {
+    A,
+}
+
+// CHECK-LABEL: @index
+#[no_mangle]
+pub fn index(x: &[u32; 3], ind: E) -> u32{
+    // CHECK-NOT: panic_bounds_check
+    x[ind as usize]
+}