rust-lang/cargo · error

`{}` is not supported in build script overrides

Error message

`{}` is not supported in build script overrides

What it means

When parsing `[host]`/`[target.*]` build-script override config (target.rs:130-237), Cargo accepts keys like rustc-link-lib, rustc-link-search, rustc-link-arg*, rustc-cfg, rustc-check-cfg, rustc-env. The keys `warning`, `rerun-if-changed`, and `rerun-if-env-changed` are explicitly rejected because they are runtime directives only a build.rs can emit, not static config. Cargo bails '`<key>` is not supported in build script overrides'.

Source

Thrown at src/context/target.rs:230

                    let args = extra_link_args(LinkArgTarget::Example, key, value)?;
                    output.linker_args.extend(args);
                }
                "rustc-cfg" => {
                    let list = value.string_list(key)?;
                    output.cfgs.extend(list.iter().map(|v| v.0.clone()));
                }
                "rustc-check-cfg" => {
                    let list = value.string_list(key)?;
                    output.check_cfgs.extend(list.iter().map(|v| v.0.clone()));
                }
                "rustc-env" => {
                    for (name, val) in value.table(key)?.0 {
                        let val = val.string(name)?.0;
                        output.env.push((name.clone(), val.to_string()));
                    }
                }
                "warning" | "rerun-if-changed" | "rerun-if-env-changed" => {
                    anyhow::bail!("`{}` is not supported in build script overrides", key);
                }
                _ => {
                    let val = value.string(key)?.0;
                    output.metadata.push((key.clone(), val.to_string()));
                }
            }
        }
        links_overrides.insert(lib_name, output);
    }
    Ok(links_overrides)
}

fn extra_link_args(
    link_type: LinkArgTarget,
    key: &str,
    value: &CV,
) -> CargoResult<Vec<(LinkArgTarget, String)>> {
    let args = value.string_list(key)?;

View on GitHub (pinned to 0e07a15537)

Solutions

1. Remove the `warning`/`rerun-if-changed`/`rerun-if-env-changed` keys from the [target.*]/[host.*] override table.
2. Implement those behaviors in the package's build.rs instead (they are runtime directives).
3. Use only the supported override keys (rustc-link-lib, rustc-link-search, rustc-link-arg*, rustc-cfg, rustc-check-cfg, rustc-env, and metadata).

Example fix

# before (.cargo/config.toml)
[target.x86_64-unknown-linux-gnu]
rustc-link-lib = ["foo"]
rerun-if-changed = ["src/wrapper.rs"]   # not supported -> error
# after
[target.x86_64-unknown-linux-gnu]
rustc-link-lib = ["foo"]
# move 'rerun-if-changed' into build.rs:
#   println!("cargo::rerun-if-changed=src/wrapper.rs");

# Reject unsupported override keys before building:
python3 - <<'EOF'
import tomllib
try: d = tomllib.load(open('.cargo/config.toml','rb'))
except FileNotFoundError: raise SystemExit(0)
bad = {'warning','rerun-if-changed','rerun-if-env-changed'}
for section in ('target','host'):
    for triple, tbl in (d.get(section) or {}).items():
        if isinstance(tbl, dict):
            hit = bad & set(tbl)
            assert not hit, f'{section}.{triple}: unsupported keys {hit}'
EOF

When it happens

Trigger: Placing `warning = [...]`, `rerun-if-changed = [...]`, or `rerun-if-env-changed = [...]` under a `[target.<triple>]` (or `[host.<triple>]`) table in config.toml that is being parsed as a build-script override via links_overrides.

Common situations: Copying build.rs cargo: directives verbatim into config expecting them to work declaratively; misunderstanding the limited set of override keys.

Related errors

• several matching instances of `target.'cfg(..)'.runner` in c
• several matching instances of `target.'cfg(..)'.linker` in c
• the -Zhost-config flag requires the -Ztarget-applies-to-host
• subcommand is required, add a subcommand to the command alia
• alias {} has unresolvable recursive definition: {} -> {}

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/e1909922589d78bb.json. Report an issue: GitHub.