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Emitting Diagnostics

A lot of effort has been put into making rustc have great error messages. This chapter is about how to emit compile errors and lints from the compiler.

`Span`

Span is the primary data structure in rustc used to represent a location in the code being compiled. Spans are attached to most constructs in HIR and MIR, allowing for more informative error reporting.

A Span can be looked up in a CodeMap to get a "snippet" useful for displaying errors with span_to_snippet and other similar methods on the CodeMap.

Error messages

The rustc_errors crate defines most of the utilities used for reporting errors.

Session and ParseSess have methods (or fields with methods) that allow reporting errors. These methods usually have names like span_err or struct_span_err or span_warn, etc... There are lots of them; they emit different types of "errors", such as warnings, errors, fatal errors, suggestions, etc.

In general, there are two class of such methods: ones that emit an error directly and ones that allow finer control over what to emit. For example, span_err emits the given error message at the given Span, but struct_span_err instead returns a DiagnosticBuilder.

DiagnosticBuilder allows you to add related notes and suggestions to an error before emitting it by calling the emit method. (Failing to either emit or cancel a DiagnosticBuilder will result in an ICE.) See the docs for more info on what you can do.

// Get a DiagnosticBuilder. This does _not_ emit an error yet.
let mut err = sess.struct_span_err(sp, "oh no! this is an error!");

// In some cases, you might need to check if `sp` is generated by a macro to
// avoid printing weird errors about macro-generated code.

if let Ok(snippet) = sess.codemap().span_to_snippet(sp) {
    // Use the snippet to generate a suggested fix
    err.span_suggestion(suggestion_sp, "try using a qux here", format!("qux {}", snip));
} else {
    // If we weren't able to generate a snippet, then emit a "help" message
    // instead of a concrete "suggestion". In practice this is unlikely to be
    // reached.
    err.span_help(suggestion_sp, "you could use a qux here instead");
}

// emit the error
err.emit();

Suggestions

In addition to telling the user exactly why their code is wrong, it's oftentimes furthermore possible to tell them how to fix it. To this end, DiagnosticBuilder offers a structured suggestions API, which formats code suggestions pleasingly in the terminal, or (when the --error-format json flag is passed) as JSON for consumption by tools, most notably the Rust Language Server and rustfix.

Not all suggestions should be applied mechanically. Use the span_suggestion_with_applicability method of DiagnosticBuilder to make a suggestion while providing a hint to tools whether the suggestion is mechanically applicable or not.

For example, to make our qux suggestion machine-applicable, we would do:

let mut err = sess.struct_span_err(sp, "oh no! this is an error!");

if let Ok(snippet) = sess.codemap().span_to_snippet(sp) {
    // Add applicability info!
    err.span_suggestion_with_applicability(
        suggestion_sp,
        "try using a qux here",
        format!("qux {}", snip),
        Applicability::MachineApplicable,
    );
} else {
    err.span_help(suggestion_sp, "you could use a qux here instead");
}

err.emit();

This might emit an error like

$ rustc mycode.rs
error[E0999]: oh no! this is an error!
 --> mycode.rs:3:5
  |
3 |     sad()
  |     ^ help: try using a qux here: `qux sad()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0999`.

In some cases, like when the suggestion spans multiple lines or when there are multiple suggestions, the suggestions are displayed on their own:

error[E0999]: oh no! this is an error!
 --> mycode.rs:3:5
  |
3 |     sad()
  |     ^
help: try using a qux here:
  |
3 |     qux sad()
  |     ^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0999`.

There are a few other Applicability possibilities:

MachineApplicable: Can be applied mechanically.
HasPlaceholders: Cannot be applied mechanically and has placeholder text in the suggestions. For example, "Try adding a type: `let x: <type>`".
MaybeIncorrect: Cannot be applied mechanically because the suggestion may or may not be a good one.
Unspecified: Cannot be applied mechanically because we don't know which of the above cases it falls into.

Lints

The compiler linting infrastructure is defined in the rustc::lint module.

Declaring a lint

The built-in compiler lints are defined in the rustc_lint crate.

Each lint is defined as a struct that implements the LintPass trait. The trait implementation allows you to check certain syntactic constructs the linter walks the source code. You can then choose to emit lints in a very similar way to compile errors. Finally, you register the lint to actually get it to be run by the compiler by using the declare_lint! macro.

For example, the following lint checks for uses of while true { ... } and suggests using loop { ... } instead.

// Declare a lint called `WHILE_TRUE`
declare_lint! {
    WHILE_TRUE,

    // warn-by-default
    Warn,

    // This string is the lint description
    "suggest using `loop { }` instead of `while true { }`"
}

// Define a struct and `impl LintPass` for it.
#[derive(Copy, Clone)]
pub struct WhileTrue;

impl LintPass for WhileTrue {
    fn get_lints(&self) -> LintArray {
        lint_array!(WHILE_TRUE)
    }
}

// LateLintPass has lots of methods. We only override the definition of
// `check_expr` for this lint because that's all we need, but you could
// override other methods for your own lint. See the rustc docs for a full
// list of methods.
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for WhileTrue {
    fn check_expr(&mut self, cx: &LateContext, e: &hir::Expr) {
        if let hir::ExprWhile(ref cond, ..) = e.node {
            if let hir::ExprLit(ref lit) = cond.node {
                if let ast::LitKind::Bool(true) = lit.node {
                    if lit.span.ctxt() == SyntaxContext::empty() {
                        let msg = "denote infinite loops with `loop { ... }`";
                        let condition_span = cx.tcx.sess.codemap().def_span(e.span);
                        let mut err = cx.struct_span_lint(WHILE_TRUE, condition_span, msg);
                        err.span_suggestion_short(condition_span, "use `loop`", "loop".to_owned());
                        err.emit();
                    }
                }
            }
        }
    }
}

Edition-gated Lints

Sometimes we want to change the behavior of a lint in a new edition. To do this, we just add the transition to our invocation of declare_lint!:

declare_lint! {
    pub ANONYMOUS_PARAMETERS,
    Allow,
    "detects anonymous parameters",
    Edition::Edition2018 => Warn,
}

This makes the ANONYMOUS_PARAMETERS lint allow-by-default in the 2015 edition but warn-by-default in the 2018 edition.

Lints that represent an incompatibility (i.e. error) in the upcoming edition should also be registered as FutureIncompatibilityLints in register_builtins function in rustc_lint::lib.

Lint Groups

Lints can be turned on in groups. These groups are declared in the register_builtins function in rustc_lint::lib. The add_lint_group! macro is used to declare a new group.

For example,

    add_lint_group!(sess,
                    "nonstandard_style",
                    NON_CAMEL_CASE_TYPES,
                    NON_SNAKE_CASE,
                    NON_UPPER_CASE_GLOBALS);

This defines the nonstandard_style group which turns on the listed lints. A user can turn on these lints with a !#[warn(nonstandard_style)] attribute in the source code, or by passing -W nonstandard-style on the command line.

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