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Diagnostics

Table of contents

Overview

The diagnostic code is used by the toolchain to produce output.

DiagnosticEmitter

DiagnosticEmitters handle the main formatting of a message. It's parameterized on a location type, for which a DiagnosticLocationTranslator must be provided that can translate the location type into a standardized DiagnosticLocation of file, line, and column.

When emitting, the resulting formatted message is passed to a DiagnosticConsumer.

DiagnosticConsumers

DiagnosticConsumers handle output of diagnostic messages after they've been formatted by an Emitter. Important consumers are:

Note that SortingDiagnosticConsumer is used by default by carbon compile. In cases where one error leads to another error at an earlier location, for example if an error in a function call argument leads to an error in the function call, this can result in confusing diagnostic output where a consequence of the error is reported before the cause. Usually this should be handled by tracking that an error occurred and suppressing the follow-on diagnostic. During toolchain development, it can be useful to disable the sorting so that the diagnostic order matches the order in which the file was processed. This can be done using carbon compile –stream-errors.

Producing diagnostics

Diagnostics are used to surface issues from compilation. A simple diagnostic looks like:

CARBON_DIAGNOSTIC(InvalidCode, Error, "code is invalid");
emitter.Emit(location, InvalidCode);

Here, CARBON_DIAGNOSTIC defines a static instance of a diagnostic named InvalidCode with the associated severity (Error or Warning).

The Emit call produces a single instance of the diagnostic. When emitted, "Code is invalid" will be the message used. The type of location depends on the DiagnosticEmitter.

A diagnostic with an argument looks like:

CARBON_DIAGNOSTIC(InvalidCharacter, Error, "invalid character {0}", char);
emitter.Emit(location, InvalidCharacter, invalid_char);

Here, the additional char argument to CARBON_DIAGNOSTIC specifies the type of an argument to expect for message formatting. The invalid_char argument to Emit provides the matching value. It's then passed along with the diagnostic message format to llvm::formatv to produce the final diagnostic message.

Diagnostic registry

There is a registry which all diagnostics must be added to. Each diagnostic has a line like:

CARBON_DIAGNOSTIC_KIND(InvalidCode)

This produces a central enumeration of all diagnostics. The eventual intent is to require tests for every diagnostic that can be produced, but that isn't currently implemented.

CARBON_DIAGNOSTIC placement

Idiomatically, CARBON_DIAGNOSTIC will be adjacent to the Emit call. However, this is only because many diagnostics can only be produced in one code location. If they can be produced in multiple locations, they will be at a higher scope so that multiple Emit calls can reference them. When in a function, CARBON_DIAGNOSTIC should be placed as close as possible to the usage so that it's easier to see the associated output.

Diagnostic context

Diagnostics can provide additional context for errors by attaching notes, which have their own location information. A diagnostic with a note looks like:

CARBON_DIAGNOSTIC(CallArgCountMismatch, Error,
                  "{0} argument(s) passed to function expecting "
                  "{1} argument(s).",
                  int, int);
CARBON_DIAGNOSTIC(InCallToFunction, Note,
                  "calling function declared here");
context.emitter()
    .Build(call_parse_node, CallArgCountMismatch, arg_refs.size(),
           param_refs.size())
    .Note(param_parse_node, InCallToFunction)
    .Emit();

The error and the note are registered as two separate diagnostics, but a single overall diagnostic object is built and emitted, so that the error and the note can be treated as a single unit.

Diagnostic context information can also be registered in a scope, so that all diagnostics produced in that scope attach a specific note. For example:

DiagnosticAnnotationScope annotate_diagnostics(
    &context.emitter(), [&](auto& builder) {
      CARBON_DIAGNOSTIC(
          InCallToFunctionParam, Note,
          "initializing parameter {0} of function declared here", int);
      builder.Note(param_parse_node, InCallToFunctionParam,
                   diag_param_index + 1);
    });

This is useful when delegating to another part of Check that may produce many different kinds of diagnostic.

Diagnostic parameter types

Diagnostic parameters should have informative types. We rely on three different methods for formatting arguments:

  • Builtin llvm::formatv support.
    • This includes char and integer types (int, int32_t, and so on).
    • String types can be added as needed, but stringifying values using the methods noted below is preferred.
      • Use llvm::StringLiteral where appropriate; use std::string when allocations are required.
      • llvm::StringRef is disallowed due to lifetime issues.
  • llvm::format_provider<...> specializations.
    • This can be used when formatting the parameter doesn't require additional context.
    • For example, Lex::TokenKind and Parse::RelativeLoc provide diagnostic formatting this way.
  • DiagnosticConverter::ConvertArg overrides.
    • This can provide additional context to a formatter.
    • For example, formatting SemIR::NameId accesses the IR's name list.

Diagnostic message style guide

We want Carbon's diagnostics to be helpful for developers when they run into an error, and phrased consistently across diagnostics. In addition, Carbon diagnostics may be mixed with Clang diagnostics when compiling interoperable code, so we are borrowing some features of Clang's Diagnostic Wording. Carbon's diagnostic style aims to balance these concerns. Our style is:

  • Start diagnostics with a lower case letter or quoted code, and omit trailing periods.

  • Quoted code should be enclosed in backticks, for example: "`{0}` is bad"

  • Phrase diagnostics as bullet points rather than full sentences. Leave out articles unless they're necessary for clarity.

    • Semicolons can be used to separate sentence fragments.

  • Diagnostics should describe the situation the toolchain observed. The language rule violated can be mentioned if it wouldn't otherwise be clear. For example:

    • "redeclaration of X" describes the situation and implies that redeclarations are not permitted.

    • "`self` declared in invalid context; can only be declared in implicit parameter list" describes the language rule.

    • It's OK for a diagnostic to guess at the developer's intent and provide a hint after explaining the situation and the rule, but not as a substitute for that. For example, "add `as String` to convert `i32` to `String`" is not sufficient as an error message, but "cannot implicitly convert `i32` to `String`; add `as String` for explicit conversion" could be acceptable.

  • Use "cannot" if needed, but try to use phrasing that doesn't require it. Avoid "allowed", "legal", "permitted", "valid", and related wording. For example:

    • "`export` in `impl` file" rather than "`export` is only allowed in API files".
    • "`extern library` specifies current library" rather than "`extern library` cannot specify the current library".

  • Try to structure diagnostics such that inputs can be extracted without string parsing; prefer typed parameters. We would like to keep a path for diagnostics to be an API. There can be exceptions where this is particularly difficult.

  • TODO: Should diagnostics be atemporal and non-sequential ("multiple declarations of X", "additional declaration here"), present tense but sequential ("redeclaration of X", "previous declaration is here"), or temporal ("redeclaration of X", "previous declaration was here")? We could try to sidestep difference between the latter two by avoiding verbs with tense ("previously declared here", "Y declared here", with no is/was).

  • TODO: When do we put identifiers or expressions in diagnostics, versus requiring notes pointing at relevant code? Is it only avoided for values, or only allowed for types?

  • TODO: Lots more things to decide, give examples.