coverage: Never emit improperly-ordered coverage regions

If we emit a coverage region that is improperly ordered (end < start),
`llvm-cov` will fail with `coveragemap_error::malformed`, which is inconvenient
for users and also very hard to debug.

Ideally we would fix the root causes of these situations, but they tend to
occur in very obscure edge-case scenarios (often involving nested macros), and
we don't always have a good MCVE to work from. So it makes sense to also have a
catch-all check that will prevent improperly-ordered regions from ever being
emitted.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -286,13 +286,27 @@ fn make_code_region(
         start_line = source_map.doctest_offset_line(&file.name, start_line);
         end_line = source_map.doctest_offset_line(&file.name, end_line);
     }
-    Some(CodeRegion {
-        file_name,
-        start_line: start_line as u32,
-        start_col: start_col as u32,
-        end_line: end_line as u32,
-        end_col: end_col as u32,
-    })
+
+    // Convert lines/columns to u32 now, so that our checks see the same values
+    // that LLVM will see.
+    let start_line = start_line as u32;
+    let start_col = start_col as u32;
+    let end_line = end_line as u32;
+    let end_col = end_col as u32;
+
+    // If we ever emit a region that is improperly ordered (end < start),
+    // `llvm-cov` will fail with `coveragemap_error::malformed`, which is
+    // inconvenient for users and also very hard to debug.
+    let is_ordered = (start_line, start_col) <= (end_line, end_col);
+    debug_assert!(is_ordered, "{start_line}:{start_col} <= {end_line}:{end_col}");
+    if !is_ordered {
+        debug!(
+            "Skipping improperly-ordered region: {start_line}:{start_col} to {end_line}:{end_col}"
+        );
+        return None;
+    }
+
+    Some(CodeRegion { file_name, start_line, start_col, end_line, end_col })
 }
 
 fn is_eligible_for_coverage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {