error: a bin target must be available for `cargo run`

error: a bin target must be available for cargo run means the package has no binary target: there is no src/main.rs, no file in src/bin/, and no [[bin]] section in Cargo.toml. This is normal for a library created with cargo new --lib. Add a src/main.rs with fn main(), run an example with cargo run --example NAME, or test the library with cargo test.

Reproduce it

cargo new --lib --vcs none -q mylib
cd mylib
cargo run

error: a bin target must be available for `cargo run`

Cargo exits with code 101, its code for any failed command.

Fix: add a binary next to the library

cargo new --lib --vcs none -q mylib
cd mylib
cat > src/main.rs <<'EOF'
fn main() {
    println!("2 + 2 = {}", mylib::add(2, 2));
}
EOF
cargo run -q

2 + 2 = 4

The library and the binary share the package: the binary imports the library by the package name (mylib::add).

The related error with several binaries

When the package has more than one binary and none is the default, cargo run cannot pick one:

cargo new --vcs none -q multi
cd multi
mkdir -p src/bin
echo 'fn main() { println!("tool"); }' > src/bin/tool.rs
cargo run -q
echo "--- with --bin"
cargo run -q --bin tool

error: `cargo run` could not determine which binary to run. Use the `--bin` option to specify a binary, or the `default-run` manifest key.
available binaries: multi, tool
--- with --bin
tool

Choose one with --bin NAME, or set a default in Cargo.toml:

[package]
name = "multi"
version = "0.1.0"
edition = "2024"
default-run = "multi"

Where Cargo looks for binaries