replace codecov ignores with coverage(off) for the per-package gate

cargo-coverage-gate (the per-package line-coverage gate anvil runs) evaluates
raw lcov and does not read codecov.yml, so files the repo excludes from Codecov
via `ignore` globs were still gated and failed. Translate the two ignores into
mechanisms the gate honors:

- testing_aids: add crate-level `#![cfg_attr(coverage_nightly, coverage(off))]`
  (+ feature gate). It is an internal (publish=false) test-helper crate whose
  utilities are exercised opportunistically by other crates' tests, so
  per-package coverage is not meaningful. Now reports NO DATA -> OK.

- ohno/src/test_util.rs: the existing module-level coverage(off) was inert for
  `assert_error_message!` because a macro body is re-instrumented at each
  expansion site and attributed back to this file. Move the assertion logic
  into `assert_error_message_impl` (a real `#[doc(hidden)] pub fn`); the macro
  now expands to a single call to it. The function is compiled once in ohno
  where coverage(off) applies, so test_util.rs no longer contributes
  instrumented lines. ohno goes 99.3% -> 100%. The function is `#[track_caller]`
  so assertion failures still report at the caller's line (a plain function
  would point into test_util.rs). Macro behavior is unchanged (verified: ohno +
  http_extensions tests, incl. the should_panic, doctest, and panic location).

- codecov.yml: drop both now-redundant `ignore` entries (the lines no longer
  appear in the lcov at all, so Codecov and the gate agree).

NOTE: ohno is published (crates.io v0.3.6); this adds a doc-hidden public helper
(additive, non-breaking) to back the macro.

Remaining: cachet 99.7% (no-default-features-only telemetry fallback arms) is a
genuine gap, not a codecov ignore -- handled separately.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: martin-kolinek

codecov.yml

@@ -18,7 +18,3 @@ coverage:
         target: 100
         threshold: "0"
         base: auto
-ignore:
-- "crates/testing_aids/**/*"
-# Macro expansion causes coverage to be inaccurate in these files, so we ignore them for now. We can re-enable them once we have a better solution for macro coverage.
-- "crates/ohno/src/test_util.rs"

crates/ohno/src/test_util.rs

@@ -1,7 +1,11 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
-#![cfg_attr(coverage_nightly, coverage(off))] // coverage doesn't handle panics well
+// The assertion logic lives in `assert_error_message_impl` (a real function),
+// not inline in the macro body, so this module-level `coverage(off)` actually
+// suppresses it. A macro body is re-instrumented at every expansion site and
+// attributed back to this file, so `coverage(off)` here never reached it.
+#![cfg_attr(coverage_nightly, coverage(off))]
 
 //! Test utilities for the ohno crate.
 //!
@@ -30,24 +34,30 @@
 /// ```
 #[macro_export]
 macro_rules! assert_error_message {
-    ($error:expr, $expected:expr) => {{
-        let error_string = $error.to_string();
-        let expected: &str = $expected;
+    ($error:expr, $expected:expr) => {{ $crate::test_util::assert_error_message_impl(&$error.to_string(), $expected) }};
+}
 
-        let test = move || {
-            if error_string == expected {
-                return ();
-            }
-            if let Some(remainder) = error_string.strip_prefix(expected) {
-                // backtrace, caused by, or error trace indicators
-                if remainder.starts_with("\n\nBacktrace:\n") || remainder.starts_with("\ncaused by: ") || remainder.starts_with("\n> ") {
-                    return ();
-                }
-            }
-            panic!("left : {expected}\nright: {error_string}");
-        };
-        test();
-    }};
+/// Implementation backing [`assert_error_message!`].
+///
+/// Kept as a function (rather than inline in the macro) so that the
+/// module-level `#[coverage(off)]` applies to the assertion's untested
+/// branches; inlined in the macro they would be instrumented at every
+/// expansion site and reported as coverage gaps in this file.
+///
+/// Not part of the public API; invoke [`assert_error_message!`] instead.
+#[doc(hidden)]
+#[track_caller]
+pub fn assert_error_message_impl(error_string: &str, expected: &str) {
+    if error_string == expected {
+        return;
+    }
+    if let Some(remainder) = error_string.strip_prefix(expected) {
+        // backtrace, caused by, or error trace indicators
+        if remainder.starts_with("\n\nBacktrace:\n") || remainder.starts_with("\ncaused by: ") || remainder.starts_with("\n> ") {
+            return;
+        }
+    }
+    panic!("left : {expected}\nright: {error_string}");
 }
 
 #[cfg(test)]

crates/testing_aids/src/lib.rs

@@ -1,6 +1,13 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+// Test-only helper crate: its utilities are exercised opportunistically by
+// other crates' tests, so per-package line coverage is not a meaningful gate
+// here. Exclude the whole crate from instrumentation (mirrors the codecov.yml
+// `ignore` this replaces).
+#![cfg_attr(coverage_nightly, feature(coverage_attribute))]
+#![cfg_attr(coverage_nightly, coverage(off))]
+
 //! An unpublished crate containing testing utilities for use within this repo.
 
 #![allow(clippy::panic, clippy::unwrap_used, missing_docs, reason = "Test code")]