error[E0463]: can't find crate for `std` — how to fix

error[E0463]: can't find crate for `std`
  |
  = note: the `wasm32-unknown-unknown` target may not be installed
  = help: consider downloading the target with `rustup target add wasm32-unknown-unknown`
error[E0463]: can't find crate for `serde`

Quick fix: For std/core: install the target's standard library with rustup target add <target> (e.g. rustup target add wasm32-unknown-unknown), and make sure you use the rustup toolchain, not a distro rustc. For other crates: add them to Cargo.toml (cargo add serde) and build with cargo build, not plain rustc.

What it means

E0463 is a linking-stage lookup failure: the compiler needs the compiled metadata (.rlib/.rmeta) of a crate and can't find it in its search paths. There are two very different situations.

Cause 1: can't find crate for std / core when cross-compiling

You built for a target (--target wasm32-unknown-unknown, aarch64-unknown-linux-gnu, thumbv7em-none-eabihf, an Android/iOS target...) whose precompiled standard library isn't installed. rustup only installs the host target by default.

Fix:

rustup target list --installed
rustup target add wasm32-unknown-unknown
# or for a specific toolchain
rustup target add aarch64-unknown-linux-gnu --toolchain nightly

If the project has a rust-toolchain.toml, add the target there so everyone gets it:

[toolchain]
channel = "stable"
targets = ["wasm32-unknown-unknown"]

Note: the toolchain in use may differ from your default (because of rust-toolchain.toml or rustup override). Check with rustup show and add the target to that toolchain.

Cause 2: a rustc that isn't managed by rustup

If which rustc points at /usr/bin/rustc (apt, dnf, Homebrew), rustup target add doesn't affect it, and the distro may not ship std for other targets.

which -a rustc cargo
rustup which rustc

Uninstall the distro package or put ~/.cargo/bin first on PATH.

Cause 3: #![no_std] targets that have no std at all

Bare-metal targets (thumbv*-none-*, riscv32imac-unknown-none-elf) only ship core (and alloc). Any dependency pulling in std triggers E0463 for std. Mark your crate #![no_std] and disable default features of dependencies:

serde = { version = "1", default-features = false, features = ["derive"] }

For targets without any prebuilt library (custom target JSON), build core yourself on nightly: cargo +nightly build -Z build-std=core,alloc --target my-target.json (needs rustup component add rust-src).

Cause 4: can't find crate for serde (or any other crate)

You wrote extern crate serde; or use serde::... but the compiler wasn't given the crate.

• The dependency isn't in Cargo.toml. Add it:

cargo add serde --features derive

• You compiled with rustc main.rs directly. Plain rustc knows nothing about Cargo dependencies; use cargo build / cargo run.
• The crate is a [dev-dependencies] entry used in normal (non-test) code, or a [build-dependencies] entry used outside build.rs. Move it to [dependencies].
• It's an optional dependency whose feature isn't enabled.

In Rust 2018+ editions you rarely need extern crate at all; a use path plus the Cargo.toml entry is enough. A missing crate in a use path usually appears as E0432/E0433 instead.

Cause 5: stale or mixed build artifacts

After switching toolchains, artifacts compiled by a different rustc version can't be loaded. Run cargo clean and rebuild.

Last updated 2026-11-08.