Cargo "failed to run `rustc` to learn about target-specific information"

Before building, Cargo probes rustc to learn the target’s configuration, and that probe failed. The toolchain is broken or mis-set-up, the requested target isn’t installed, or a RUSTC_WRAPPER is interfering.

What this error means

cargo fails very early with failed to run rustc to learn about target-specific information, followed by a Caused by: with the underlying rustc/process error. Nothing compiles because Cargo can’t even query the compiler.

cargo output

error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `rustc - --crate-name ___ --target wasm32-unknown-unknown ...` (exit status: 1)
  --- stderr
  error: Error loading target specification: Could not find specification for target "wasm32-unknown-unknown"

Common causes

Requested target not installed

Building for a --target whose specification/std isn’t installed makes the probe fail because rustc can’t load that target.

Broken toolchain or interfering wrapper

A corrupted rustc, a mismatched RUSTC/RUSTC_WRAPPER (e.g. sccache misconfigured), or PATH pointing at a bad binary makes the probe process exit non-zero.

How to fix it

Install the target you’re building for

Terminal

rustup target add wasm32-unknown-unknown
cargo build --target wasm32-unknown-unknown

Check the toolchain and any rustc wrapper

Run rustc -vV directly - if that fails, the toolchain itself is broken; reinstall it.
Unset or fix RUSTC_WRAPPER/RUSTC if a wrapper (sccache) is misconfigured.
Confirm which rustc and rustup show point at a healthy toolchain.

How to prevent it

Add cross-compile targets with rustup target add before building.
Verify any RUSTC_WRAPPER (sccache) is installed and configured correctly.
Reinstall the toolchain if rustc -vV itself fails.