feat: improve error printing#517
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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
Debugformatting to print the error viaDisplayplus a “caused by” source chain. - Preserve the prior
Debugoutput behind alternate formatting ({:#?}).
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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 | 6,480.70 µs(+6.36%)Baseline: 6,093.20 µs | 7,677.76 µs (84.41%) |
| json_api/json_api | 📈 view plot 🚷 view threshold | 1,053.70 µs(-1.92%)Baseline: 1,074.37 µs | 1,323.65 µs (79.61%) |
| nested_routers/nested_routers | 📈 view plot 🚷 view threshold | 971.58 µs(-2.09%)Baseline: 992.35 µs | 1,204.08 µs (80.69%) |
| single_root_route/single_root_route | 📈 view plot 🚷 view threshold | 939.38 µs(-1.62%)Baseline: 954.82 µs | 1,167.50 µs (80.46%) |
| single_root_route_burst/single_root_route_burst | 📈 view plot 🚷 view threshold | 16,969.00 µs(-4.26%)Baseline: 17,724.56 µs | 21,190.21 µs (80.08%) |
seqre
reviewed
Mar 22, 2026
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| writeln!(f)?; | ||
| writeln!(f)?; | ||
| writeln!(f, "caused by:")?; |
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I think that should be within the while loop to show the chain of causality
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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.
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seqre
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Mar 29, 2026
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Please make cargo insta test for this functionality
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
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| let error = self.inner(); | ||
| Display::fmt(error, f)?; | ||
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| if let Some(source) = error.source() { |
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| if f.alternate() { | ||
| Debug::fmt(&self.repr, f) |
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| let mut source = Some(source); | ||
| let mut i = 0; | ||
| while let Some(e) = source { | ||
| writeln!(f, "{i:4}: {e}")?; | ||
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| source = e.source(); | ||
| i += 1; |
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| // `Debug` formatting normally. If not, (which is the case when using | ||
| // `Result::unwrap()` or `Result::expect()`) pretty print the error. |
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When using
Result::expect(), (e.g. in the defaultcot::main) errors are printed usingDebugprinter. 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::Errorerror printing logic: https://docs.rs/anyhow/latest/anyhow/struct.Error.html#method.chainThe change turns this:
into this:
