fix(cargo): preserve compiler warnings in cargo test output on passing runs

What: CargoTestHandler (streaming) and filter_cargo_test (buffered/pipe path)
previously dropped compile-phase 'warning:' blocks entirely on passing runs.
Both now capture and emit warning blocks (mirroring CargoBuildHandler) and
annotate the compact summary with '[N compiler warnings]'. The aggregate
'generated N warnings' count line stays stripped. Buffered path caps blocks
at CAP_WARNINGS with an '+N more' marker and hard-terminates warning blocks
on section markers (failures:/test result:/test /----/error) so short blocks
cannot swallow subsequent sections.

Why: exit-0 runs get no tee log, so dropped warnings were unrecoverable —
an agent reading 'cargo test: N passed' concluded the build was clean when
rustc had emitted warnings. Confirmed behaviorally pre-fix: raw 'cargo test'
showed 3 warning lines; rtk showed none (review lane finding HIGH-2,
docs/research/runs/rtk-review-2026-07-07 in Claude.md-improve).

Verified: cargo test full suite 2394 passed / 0 failed (+ 6 integration
suites green); 2 new tests (buffered + streamed handler) assert warning
survival, warning-count annotation, and aggregate-line stripping; behavioral
A/B on a real crate: passing-with-warnings now shows the block + count
(exit 0), failing run still shows failure detail with exit 101 == raw.

Open risk: pytest wrapper has the analogous success-path loss (warnings
summary + stderr) — tracked as a follow-up, mitigated operationally by
tee.mode=always in local config.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K5bb4GKf9CNKP9uYhFDvsz

Author: 2 people

src/cmds/rust/cargo_cmd.rs

@@ -126,6 +126,8 @@ struct CargoTestHandler {
     in_failure_names: bool,
     summary_lines: Vec<String>,
     has_compile_errors: bool,
+    warnings: usize,
+    in_warning_block: bool,
 }
 
 impl CargoTestHandler {
@@ -135,6 +137,8 @@ impl CargoTestHandler {
             in_failure_names: false,
             summary_lines: Vec::new(),
             has_compile_errors: false,
+            warnings: 0,
+            in_warning_block: false,
         }
     }
 }
@@ -155,6 +159,11 @@ impl BlockHandler for CargoTestHandler {
         if line.starts_with("test ") && line.ends_with("... ok") {
             return true;
         }
+        // Aggregate count line ("warning: `x` generated N warnings"); individual
+        // warning blocks are already streamed via is_block_start below.
+        if line.starts_with("warning:") && line.contains("generated") && line.contains("warning") {
+            return true;
+        }
         // Track compile errors for fallback
         if trimmed.starts_with("error[") || trimmed.starts_with("error:") {
             self.has_compile_errors = true;
@@ -187,10 +196,26 @@ impl BlockHandler for CargoTestHandler {
     }
 
     fn is_block_start(&mut self, line: &str) -> bool {
-        self.in_failure_section && line.starts_with("---- ")
+        if self.in_failure_section && line.starts_with("---- ") {
+            self.in_warning_block = false;
+            return true;
+        }
+        // Compile-phase warnings on a passing run were previously dropped
+        // (exit 0 → no tee → unrecoverable); stream them like CargoBuildHandler.
+        if !self.in_failure_section
+            && (line.starts_with("warning:") || line.starts_with("warning["))
+        {
+            self.warnings += 1;
+            self.in_warning_block = true;
+            return true;
+        }
+        false
     }
 
-    fn is_block_continuation(&mut self, line: &str, _block: &[String]) -> bool {
+    fn is_block_continuation(&mut self, line: &str, block: &[String]) -> bool {
+        if self.in_warning_block {
+            return !(line.trim().is_empty() && block.len() > 3);
+        }
         self.in_failure_section && !line.starts_with("---- ")
     }
 
@@ -234,6 +259,13 @@ impl BlockHandler for CargoTestHandler {
         if all_parsed {
             if let Some(agg) = aggregated {
                 if agg.suites > 0 {
+                    if self.warnings > 0 {
+                        return Some(format!(
+                            "{} [{} compiler warnings]\n",
+                            agg.format_compact(),
+                            self.warnings
+                        ));
+                    }
                     return Some(format!("{}\n", agg.format_compact()));
                 }
             }
@@ -1120,6 +1152,9 @@ pub(crate) fn filter_cargo_test(output: &str) -> String {
     let mut summary_lines: Vec<String> = Vec::new();
     let mut in_failure_section = false;
     let mut current_failure = Vec::new();
+    let mut warnings: Vec<String> = Vec::new();
+    let mut current_warning: Vec<String> = Vec::new();
+    let mut in_warning_block = false;
 
     for line in output.lines() {
         // Skip compilation lines
@@ -1136,6 +1171,48 @@ pub(crate) fn filter_cargo_test(output: &str) -> String {
             continue;
         }
 
+        // Compile-phase warnings on a passing run were previously dropped
+        // (exit 0 → no tee → unrecoverable). Capture the blocks; skip the
+        // aggregate "warning: `x` generated N warnings" count line.
+        if line.starts_with("warning:") && line.contains("generated") && line.contains("warning") {
+            in_warning_block = false;
+            if !current_warning.is_empty() {
+                warnings.push(current_warning.join("\n"));
+                current_warning.clear();
+            }
+            continue;
+        }
+        if !in_failure_section && (line.starts_with("warning:") || line.starts_with("warning[")) {
+            if !current_warning.is_empty() {
+                warnings.push(current_warning.join("\n"));
+                current_warning.clear();
+            }
+            in_warning_block = true;
+            current_warning.push(line.to_string());
+            continue;
+        }
+        if in_warning_block {
+            // A section marker always ends the warning block (falls through to
+            // normal handling); a blank line ends a well-formed block.
+            let is_section = line == "failures:"
+                || line.starts_with("test result:")
+                || line.starts_with("test ")
+                || line.starts_with("---- ")
+                || line.starts_with("error[")
+                || line.starts_with("error:");
+            if is_section || (line.trim().is_empty() && current_warning.len() > 3) {
+                in_warning_block = false;
+                warnings.push(current_warning.join("\n"));
+                current_warning.clear();
+                if !is_section {
+                    continue;
+                }
+            } else {
+                current_warning.push(line.to_string());
+                continue;
+            }
+        }
+
         // Detect failures section
         if line == "failures:" {
             in_failure_section = true;
@@ -1165,8 +1242,24 @@ pub(crate) fn filter_cargo_test(output: &str) -> String {
     if !current_failure.is_empty() {
         failures.push(current_failure.join("\n"));
     }
+    if !current_warning.is_empty() {
+        warnings.push(current_warning.join("\n"));
+    }
+
+    let mut warnings_section = String::new();
+    if !warnings.is_empty() {
+        for block in warnings.iter().take(CAP_WARNINGS) {
+            warnings_section.push_str(block);
+            warnings_section.push('\n');
+        }
+        if warnings.len() > CAP_WARNINGS {
+            warnings_section
+                .push_str(&format!("… +{} more warnings\n", warnings.len() - CAP_WARNINGS));
+        }
+    }
 
     let mut result = String::new();
+    result.push_str(&warnings_section);
 
     if failures.is_empty() && !summary_lines.is_empty() {
         // All passed - try to aggregate
@@ -1190,7 +1283,15 @@ pub(crate) fn filter_cargo_test(output: &str) -> String {
         if all_parsed {
             if let Some(agg) = aggregated {
                 if agg.suites > 0 {
-                    return agg.format_compact();
+                    if warnings.is_empty() {
+                        return agg.format_compact();
+                    }
+                    return format!(
+                        "{}{} [{} compiler warnings]",
+                        warnings_section,
+                        agg.format_compact(),
+                        warnings.len()
+                    );
                 }
             }
         }
@@ -1597,6 +1698,85 @@ test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; fin
         assert!(!result.contains("test utils"));
     }
 
+    #[test]
+    fn test_filter_cargo_test_all_pass_preserves_compiler_warnings() {
+        let output = r#"   Compiling failproj v0.1.0
+warning: function `unused_helper` is never used
+ --> src/lib.rs:9:4
+  |
+9 | fn unused_helper() -> i32 { 42 }
+  |    ^^^^^^^^^^^^^
+  |
+  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
+
+warning: `failproj` (lib) generated 1 warning
+    Finished test [unoptimized + debuginfo] target(s) in 0.05s
+     Running unittests src/lib.rs (target/debug/deps/failproj-abc123)
+
+running 3 tests
+test tests::test_pass ... ok
+test tests::test_pass2 ... ok
+test tests::test_pass3 ... ok
+
+test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
+"#;
+        let result = filter_cargo_test(output);
+        assert!(
+            result.contains("unused_helper"),
+            "compiler warning block must survive a passing run, got: {}",
+            result
+        );
+        assert!(
+            result.contains("[1 compiler warnings]"),
+            "summary must carry the warning count, got: {}",
+            result
+        );
+        assert!(result.contains("3 passed"));
+        // Aggregate count line stays stripped
+        assert!(!result.contains("generated 1 warning"));
+    }
+
+    #[test]
+    fn test_streamed_cargo_test_handler_preserves_compiler_warnings() {
+        use crate::core::stream::{BlockStreamFilter, StreamFilter};
+        let lines = [
+            "   Compiling failproj v0.1.0",
+            "warning: function `unused_helper` is never used",
+            " --> src/lib.rs:9:4",
+            "  |",
+            "9 | fn unused_helper() -> i32 { 42 }",
+            "  |    ^^^^^^^^^^^^^",
+            "",
+            "warning: `failproj` (lib) generated 1 warning",
+            "    Finished test [unoptimized + debuginfo] target(s) in 0.05s",
+            "",
+            "running 3 tests",
+            "test tests::test_pass ... ok",
+            "",
+            "test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s",
+        ];
+        let mut filter = BlockStreamFilter::new(CargoTestHandler::new());
+        let mut emitted = String::new();
+        for line in lines {
+            if let Some(out) = filter.feed_line(line) {
+                emitted.push_str(&out);
+            }
+        }
+        emitted.push_str(&filter.flush());
+        let summary = filter.on_exit(0, "").unwrap_or_default();
+        assert!(
+            emitted.contains("unused_helper"),
+            "warning block must stream through, got: {}",
+            emitted
+        );
+        assert!(
+            summary.contains("[1 compiler warnings]"),
+            "summary must carry warning count, got: {}",
+            summary
+        );
+        assert!(summary.contains("3 passed"));
+    }
+
     #[test]
     fn test_filter_cargo_test_failures() {
         let output = r#"running 5 tests