anyhow

anyhow provides anyhow::Error, a trait-object-based error type that makes error handling in Rust application code effortless. Instead of defining bespoke error enums for every fallible function, you return anyhow::Result<T> and let the ? operator propagate any error that implements std::error::Error.

Beyond simple propagation, anyhow lets you attach human-readable context to errors, automatically captures backtraces on recent Rust versions, and supports downcasting back to concrete error types when you need to branch on the underlying cause. It works in both std and no_std environments and pairs naturally with thiserror for library-defined error types.

What You Get

• A single anyhow::Error type and anyhow::Result<T> alias that absorb any std::error::Error via the ? operator
• The Context / with_context API for attaching descriptive, low-overhead context to errors as they propagate
• The anyhow! and bail! macros for constructing and early-returning ad-hoc errors with format-string interpolation
• Automatic backtrace capture (Rust >= 1.65) and downcasting back to concrete error types by value or reference
• no_std support (with a global allocator) exposing nearly the same API surface

Common Use Cases

• Returning errors from application main, CLI tools, and glue code without defining custom error enums
• Adding context to low-level errors so failures are debuggable (“Failed to read config from …”)
• Propagating heterogeneous errors from many libraries through one uniform result type
• Inspecting or branching on a root cause via downcasting when specific handling is required

Under The Hood

Architecture — The crate centers on a single Error type (src/error.rs) that is a thin, pointer-width wrapper around a heap-allocated ErrorImpl reached through custom fat/thin pointer machinery in src/ptr.rs (Own, Ref, Mut), letting one word carry the error plus a vtable of type-erased operations (display, debug, downcast, backtrace). src/context.rs implements the Context trait that layers messages onto the cause chain, src/chain.rs provides iteration over the Caused by: sources, src/fmt.rs renders the chain and backtrace, and src/backtrace.rs gates capture behind Rust-version and feature cfgs. Tech Stack — Pure Rust (edition 2021, MSRV 1.68), zero required runtime dependencies, with a build.rs probe that feature-detects compiler capabilities (core error trait, generic member access, backtrace support) and sets cfgs so the same source compiles across a wide range of toolchains and std/no_std. Dev-only dependencies (thiserror, trybuild, rustversion, syn, futures) exist purely for the test suite. Code Quality — Exceptionally high: 14 integration test files under tests/ cover downcasting, context, ffi, boxed conversion, fmt, macros, source chains, and drop behavior, plus trybuild compile-fail UI tests that pin exact error messages. The code is unsafe-heavy by necessity (manual pointer tagging) but tightly encapsulated behind safe APIs and maintained by a single expert author with meticulous cfg discipline. API Design — Best-in-class developer experience: getting started needs only use anyhow::Result and the ? operator, context is one .context("...") call away, and the anyhow!/bail! macros read naturally. Documentation is thorough with runnable doctests on every public item, and the API deliberately mirrors familiar Result/Error idioms so the learning curve is minimal.