rustc -D warnings (RUSTFLAGS="-D warnings"): treat warnings as errors

rustc -D warnings main.rs raises every lint that is currently a warning to an error, so the build fails instead of printing warning: ... emitted and succeeding. In Cargo projects it is usually passed as RUSTFLAGS="-D warnings" cargo build or cargo clippy -- -D warnings, typically in CI. -D is short for --deny; the other lint levels are -A (allow), -W (warn), --force-warn and -F (forbid).

Without and with -D warnings

cat > w.rs <<'EOF'
fn main() {
    let unused = 5;
    let mut n = 1;
    println!("{}", 2);
}
EOF
echo "== default"
rustc w.rs 2>&1 | grep -E "^(warning: unused|warning: variable|warning: [0-9])"; echo "exit: ${PIPESTATUS[0]}"
echo "== -D warnings"
rustc -D warnings w.rs 2>&1 | sed -n '1,9p'; echo "exit: ${PIPESTATUS[0]}"
echo "== -D warnings -A unused-mut -A unused-variables"
rustc -D warnings -A unused-mut -A unused-variables w.rs; echo "exit: $?"
echo "== -A before -D warnings"
rustc -A unused-mut -A unused-variables -D warnings w.rs; echo "exit: $?"

== default
warning: variable does not need to be mutable
warning: unused variable: `unused`
warning: unused variable: `n`
warning: 3 warnings emitted
exit: 0
== -D warnings
error: variable does not need to be mutable
 --> w.rs:3:9
  |
3 |     let mut n = 1;
  |         ----^
  |         |
  |         help: remove this `mut`
  |
  = note: `-D unused-mut` implied by `-D warnings`
exit: 1
== -D warnings -A unused-mut -A unused-variables
exit: 0
== -A before -D warnings
exit: 0

By default rustc prints three warnings and builds the binary (exit 0). With -D warnings the same messages become errors, each with a note saying which lint was denied and how to override it, and the build stops with exit code 1. Allowing specific lints with -A exempts them, whether the -A comes before or after -D warnings: the warnings group only affects lints that would otherwise be reported as warnings.

Lint level flags

warnings is a special group meaning "everything that would be a warning". You can also name a group such as unused or rust-2018-idioms, or a single lint such as unused-variables (dashes and underscores are equivalent).

In Cargo projects

RUSTFLAGS="-D warnings" cargo build            # rustc lints for the whole workspace
cargo clippy --all-targets -- -D warnings      # clippy lints as errors too
RUSTDOCFLAGS="-D warnings" cargo doc --no-deps # broken intra-doc links fail the docs build

Since Rust 1.74 a [lints] table in Cargo.toml sets levels per crate without touching RUSTFLAGS:

[lints.rust]
unsafe_code = "forbid"
unused = "deny"

[lints.clippy]
unwrap_used = "warn"

Pitfalls

• New compilers add warnings. A pinned -D warnings in CI can turn a toolchain update into a red build. Pin the toolchain with rust-toolchain.toml or treat those failures as routine maintenance.
• Allowed lints stay allowed: -D warnings does not re-enable lints switched off with -A or #[allow]. To make a specific lint an error regardless, name it: -D unused-mut.
• Only lints are affected. Hard errors such as E0382 borrow of moved value are errors at every level; -D warnings cannot make them warnings and -A cannot hide them. Use rustc --explain for the error codes.

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