vectordotdev/vector · info

Fixed template

Error message

Fixed template

What it means

LogToMetricConfig::generate_config builds a sample config and converts the fixed template string "field_name" via try_into().expect("Fixed template"). Template parsing only fails on invalid template syntax (e.g. unbalanced {{ }}), which the hard-coded literal cannot contain, so this expect is unreachable by design. It signals that a developer-introduced change to the literal broke the sample config generation.

Solutions

1. No action needed while the literal is a valid template.
2. If editing the literal, verify it parses as a template (balanced {{ }} delimiters) before committing.
3. Run `cargo vdev check` / config generation tests after changing generate_config to catch the panic in CI.
4. If a dynamic field name is needed, use Template::try_from(...)? and propagate the error instead of expect.

Example fix

// before
field: "field_name".try_into().expect("Fixed template"),
// after (only if literal might change; propagate instead of panic)
field: "field_name".try_into().expect("Fixed template"), // keep literal valid: no unbalanced {{ }}

// validate a template literal before editing generate_config
let _: Template = "field_name".try_into()?; // must parse cleanly

When it happens

Trigger: Only reachable if the string literal passed to try_into is edited to something that is not a valid Vector template (e.g. "{{field" with unbalanced delimiters); then any call to generate_config (used by `vector generate` and docs tooling) panics.

Common situations: Editing the generated sample config for log_to_metric and accidentally introducing invalid template syntax; users would see a panic when running config generation or docs checks.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• UNSUPPORTED_DYNAMIC_FEATURE_SELECTOR
• a record with a next ID must have an event count
• a valid HTTP/1 URI is valid as an HTTP URI
• a validated HTTP endpoint is a valid `http 1` URI
• all json-file keys should be matched

AI-assisted analysis of vectordotdev/vector@bdb87aeaa4 (2026-09-16). Data as JSON: /api/errors/52d1899b3a0dcb67. Report an issue: GitHub.

Appendix: source

Thrown at src/transforms/log_to_metric.rs:182

const fn default_increment_by_value() -> bool {
    false
}

const fn default_kind() -> MetricKind {
    MetricKind::Incremental
}

#[derive(Debug, Clone)]
pub struct LogToMetric {
    pub metrics: Vec<MetricConfig>,
    pub all_metrics: bool,
}

impl GenerateConfig for LogToMetricConfig {
    fn generate_config() -> serde_json::Value {
        serde_json::to_value(Self {
            metrics: Some(vec![MetricConfig {
                field: "field_name".try_into().expect("Fixed template"),
                name: None,
                namespace: None,
                tags: None,
                metric: MetricTypeConfig::Counter(CounterConfig {
                    increment_by_value: false,
                    kind: MetricKind::Incremental,
                }),
            }]),
            all_metrics: Some(true),
        })
        .unwrap()
    }
}

#[async_trait::async_trait]
#[typetag::serde(name = "log_to_metric")]
impl TransformConfig for LogToMetricConfig {
    async fn build(&self, _context: &TransformContext) -> crate::Result<Transform> {

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