Diagnostics

When parsing or validation finds a problem, it does not throw. It records the problem in a Report, and confval renders that report for people to read. Spans are what let each message point at the exact place in the file.

Report and IssueBuilder

Report collects issues. Validators receive &mut Report and emit through a builder:

report
    .error("port must be between 1 and 65535")
    .at(spec.port.span)
    .help("Choose a port in the range 1..=65535.")
    .emit();

• report.error(msg) and report.warning(msg) return an IssueBuilder.
• .at(span) attaches the primary span. Issues without a span render without a source location.
• .help(text) adds a suggestion line.
• .related(span, label) attaches secondary spans, used for messages like "first declared here".
• .emit() finalizes the issue. The builder is #[must_use], so forgetting .emit() is a compile-time warning.

Query methods: has_errors(), has_warnings(), has_issues(). Severity is the two-variant Severity enum (Error, Warning).

Rendering

Renderers write into any fmt::Write sink and take the SourceMap to resolve spans:

Method	Feature gate	Format
render_plain	always available	One line per issue with file:line:col, for CI
render_pretty	color	rustc-style caret output with source excerpts
render_json	serde	Structured JSON for tooling

let mut out = String::new();
report.render_pretty(&sources, &mut out)?;
eprint!("{out}");

Pretty output underlines the offending value in its source line:

error: unknown load_balancing_strategy: failovr
  --> ingress.d/api.hcl:12:31
   |
12 |   load_balancing_strategy = "failovr"
   |                             ^^^^^^^^^
   = help: expected one of: failover, round_robin, request_pressure, sticky_hash, random

Line and column lookups are O(log n) via a per-source line index. Columns count characters, not bytes. An issue records only a severity, message, optional span, optional help, and related spans, and never touches source text until render time.

Spans and source

A span is a byte range inside one registered source:

pub struct Span {
    pub source: SourceId,
    pub start: u32,
    pub end: u32,
}

SourceId is a lightweight handle issued by the SourceMap. Spans are plain data. Resolving them to line and column numbers happens only at render time.

The SourceMap interns source text. Each file (or in-memory string) is registered once and identified by its SourceId:

let mut sources = SourceMap::new();
let id = sources.add("config.hcl", text);

Reports do not own source text. Renderers take &SourceMap so the text is stored exactly once no matter how many issues reference it.