feat: improve error printing by m4tx · Pull Request #517 · cot-rs/cot

m4tx · 2026-03-22T15:27:49Z

When using Result::expect(), (e.g. in the default cot::main) errors are printed using Debug printer. This causes very ugly errors to be produced. Instead, when the alternate mode is not being used, just format the error nicely, along with the error source chain.

This roughly duplicates the anyhow::Error error printing logic: https://docs.rs/anyhow/latest/anyhow/struct.Error.html#method.chain

The change turns this:

into this:

When using `Result::expect()`, (e.g. in the default `cot::main`) errors are printed using `Debug` printer. This causes very ugly errors to be produced. Instead, when the alternate mode is not being used, just format the error nicely, along with the error source chain. This roughly duplicates the `anyhow::Error` error printing logic: https://docs.rs/anyhow/latest/anyhow/struct.Error.html#method.chain

Copilot

Pull request overview

Updates cot-core::Error’s Debug formatting so that {:?} produces a human-friendly error message plus its source chain (similar to anyhow), while {:#?} retains the original structured debug output. This improves the default panic output from Result::unwrap() / expect() in cot::main.

Changes:

Implement non-alternate Debug formatting to print the error via Display plus a “caused by” source chain.
Preserve the prior Debug output behind alternate formatting ({:#?}).

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Copilot · 2026-03-22T15:31:30Z

cot-core/src/error/error_impl.rs

+            Display::fmt(&self.repr.inner, f)?;
+
+            writeln!(f)?;
+            writeln!(f)?;
+            writeln!(f, "caused by:")?;
+            let mut source = self.source();
+            while let Some(e) = source {
+                writeln!(f, "    {e}")?;
+


The source-chain printing here will duplicate the top-level message for errors created via Error::with_status/Error::internal: the outer Error displays the inner Error via WithStatusCode, and then self.source() returns that same inner Error as the first source, so the first "caused by" entry repeats the root error. Consider printing from self.inner() (both the root message and the chain), or otherwise skipping the first source when it’s the status-propagation Error wrapper.

Suggested change

Display::fmt(&self.repr.inner, f)?;

writeln!(f)?;

writeln!(f)?;

writeln!(f, "caused by:")?;

let mut source = self.source();

while let Some(e) = source {

writeln!(f, " {e}")?;

// Print the root error message.

Display::fmt(&self.repr.inner, f)?;

// Prepare to print the source chain.

let root_msg = format!("{}", &self.repr.inner);

writeln!(f)?;

writeln!(f)?;

writeln!(f, "caused by:")?;

let mut source = self.source();

let mut is_first = true;

while let Some(e) = source {

let msg = format!("{}", e);

// For status-propagation wrappers, the first source may have the

// same message as the root error; skip it to avoid duplication.

if !is_first || msg != root_msg {

writeln!(f, " {msg}")?;

}

is_first = false;

Copilot · 2026-03-22T15:31:31Z

cot-core/src/error/error_impl.rs

+            writeln!(f)?;
+            writeln!(f)?;
+            writeln!(f, "caused by:")?;
+            let mut source = self.source();
+            while let Some(e) = source {
+                writeln!(f, "    {e}")?;
+
+                source = e.source();


This always prints a blank-line separator and a "caused by:" section even when self.source() is None, which will produce output ending with an empty "caused by:" block. Consider only emitting the extra newlines and the "caused by" header when at least one source exists.

Suggested change

writeln!(f)?;

writeln!(f)?;

writeln!(f, "caused by:")?;

let mut source = self.source();

while let Some(e) = source {

writeln!(f, " {e}")?;

source = e.source();

let mut source = self.source();

if source.is_some() {

writeln!(f)?;

writeln!(f)?;

writeln!(f, "caused by:")?;

while let Some(e) = source {

writeln!(f, " {e}")?;

source = e.source();

}

Copilot · 2026-03-22T15:31:31Z

cot-core/src/error/error_impl.rs

 impl Debug for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        Debug::fmt(&self.repr, f)
+        // If the alternate (`{:#?}`) formatting has been specified, print out the
+        // `Debug` formatting normally. If not, (which is the case when using
+        // `Result::unwrap()` or `Result::expect()`) pretty print the error.
+        if f.alternate() {
+            Debug::fmt(&self.repr, f)
+        } else {
+            Display::fmt(&self.repr.inner, f)?;
+
+            writeln!(f)?;
+            writeln!(f)?;
+            writeln!(f, "caused by:")?;
+            let mut source = self.source();
+            while let Some(e) = source {
+                writeln!(f, "    {e}")?;
+
+                source = e.source();
+            }
+
+            Ok(())
+        }


The pretty-printing logic in Debug::fmt is new user-facing behavior but there are no tests asserting the {:#?} vs {:?} formatting, the presence/absence of the "caused by" block, and the formatting of multi-level source chains. Adding unit tests in this file’s tests module would help prevent regressions (especially around Error::internal/Error::with_status).

github-actions · 2026-03-22T15:32:15Z

Bencher Report

Branch	improve-errors
Testbed	github-ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	Benchmark Result microseconds (µs) (Result Δ%)	Upper Boundary microseconds (µs) (Limit %)
empty_router/empty_router	📈 view plot 🚷 view threshold	6,532.60 µs (+9.28%) Baseline: 5,977.82 µs	7,170.38 µs (91.11%)
json_api/json_api	📈 view plot 🚷 view threshold	1,089.80 µs (+5.49%) Baseline: 1,033.12 µs	1,191.23 µs (91.49%)
nested_routers/nested_routers	📈 view plot 🚷 view threshold	991.90 µs (+4.15%) Baseline: 952.34 µs	1,092.51 µs (90.79%)
single_root_route/single_root_route	📈 view plot 🚷 view threshold	964.80 µs (+5.60%) Baseline: 913.60 µs	1,051.21 µs (91.78%)
single_root_route_burst/single_root_route_burst	📈 view plot 🚷 view threshold	16,516.00 µs (-6.22%) Baseline: 17,610.86 µs	21,143.00 µs (78.12%)

🐰 View full continuous benchmarking report in Bencher

seqre · 2026-03-22T15:32:16Z

cot-core/src/error/error_impl.rs

+            writeln!(f)?;
+            writeln!(f)?;
+            writeln!(f, "caused by:")?;


I think that should be within the while loop to show the chain of causality

The Rust stack traces are printed with 0:, 1:, 2:, etc. at the beginning of each frame. Maybe that would do? I don't think I'd like to inflate the error messages with excessive newlines.

codecov · 2026-03-22T15:59:33Z

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cot-core/src/error/error_impl.rs	54.54%	0 Missing and 5 partials ⚠️

Flag	Coverage Δ
rust	`89.97% <54.54%> (-0.02%)`	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cot-core/src/error/error_impl.rs	`93.42% <54.54%> (-3.06%)`	⬇️

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Copilot AI review requested due to automatic review settings March 22, 2026 15:27

Copilot started reviewing on behalf of m4tx March 22, 2026 15:28 View session

m4tx force-pushed the improve-errors branch from 0748bd1 to add338d Compare March 22, 2026 15:28

Copilot AI reviewed Mar 22, 2026

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seqre reviewed Mar 22, 2026

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