FuelLabs/sway · error · anyhow::Error

Loading gas costs from a JSON file is currently not implemen

Error message

Loading gas costs from a JSON file is currently not implemented.

What it means

GasCostsSource::FromStr treats any string that is not "built-in"/"mainnet"/"testnet" as a file path (Self::File), but provide_gas_costs() immediately errors for File(_) because loading gas costs from JSON is unimplemented (sway#7472). Note the path is never even opened — the variant matches on `Self::File(_file_path)` and discards it.

Source

Thrown at forc-test/src/lib.rs:211

impl GasCostsSource {
    pub fn provide_gas_costs(&self) -> Result<GasCostsValues, anyhow::Error> {
        match self {
            // Values in the `gas_costs_values.json` are taken from the `chain-configuration` repository:
            //      chain-configuration/upgradelog/ignition/consensus_parameters/<version>.json
            // Update these values when there are changes to the on-chain gas costs.
            Self::BuiltIn => Ok(serde_json::from_str(include_str!(
                "../gas_costs_values.json"
            ))?),
            // TODO: (GAS-COSTS) Fetch actual gas costs from mainnet/testnet and JSON file.
            //       See: https://github.com/FuelLabs/sway/issues/7472
            Self::Mainnet => Err(anyhow::anyhow!(
                "Fetching gas costs from mainnet is currently not implemented."
            )),
            Self::Testnet => Err(anyhow::anyhow!(
                "Fetching gas costs from testnet is currently not implemented."
            )),
            Self::File(_file_path) => Err(anyhow::anyhow!(
                "Loading gas costs from a JSON file is currently not implemented."
            )),
        }
    }
}

impl FromStr for GasCostsSource {
    type Err = anyhow::Error;

    fn from_str(value: &str) -> Result<Self, Self::Err> {
        match value {
            "built-in" => Ok(Self::BuiltIn),
            "mainnet" => Ok(Self::Mainnet),
            "testnet" => Ok(Self::Testnet),
            file_path => Ok(Self::File(file_path.to_string())),
        }
    }
}

View on GitHub (pinned to 47e5e902fa)

Solutions

  1. Use `--gas-costs built-in` until file loading is implemented
  2. If you control the version of gas_costs_values.json you need, rebuild forc with an updated bundled JSON rather than passing a file
  3. Upgrade forc once sway#7472 lands the File arm

Example fix

# before
forc test --gas-costs ./my_gas_costs.json

# after
forc test --gas-costs built-in
Defensive patterns

Strategy: validation

Validate before calling

fn parse_gas_costs_source(s: &str) -> anyhow::Result<GasCostsSource> {
    match s {
        "built-in" => Ok(GasCostsSource::BuiltIn),
        other => anyhow::bail!(
            "'{other}' would be treated as a file path, which is not implemented; use 'built-in'"
        ),
    }
}

Type guard

fn is_implemented(src: &GasCostsSource) -> bool {
    matches!(src, GasCostsSource::BuiltIn)
}

Try / catch

let values = match source.provide_gas_costs() {
    Ok(v) => v,
    Err(e) if e.to_string().contains("JSON file") => {
        log::warn!("file-based gas costs unsupported; using built-in");
        GasCostsSource::BuiltIn.provide_gas_costs()?
    }
    Err(e) => return Err(e),
};

Prevention

When it happens

Trigger: `forc test --gas-costs path/to/gas_costs.json`, or GasCostsSource::from_str("my.json") then provide_gas_costs(). Any unrecognized token becomes File and hits the stub.

Common situations: Developers trying to supply custom/chain-specific gas cost values, or who typo an intended keyword (e.g. "builtin" without hyphen) and are surprised it is silently interpreted as a file path that then errors.

Related errors

AI-assisted analysis of FuelLabs/sway@47e5e902fa (2026-08-16). Data as JSON: /api/errors/f660b6368fbea778. Report an issue: GitHub.