Emitting Errors and other 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 SourceMap to get a "snippet" useful for displaying errors with span_to_snippet and other similar methods on the SourceMap.

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.source_map().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 method of DiagnosticBuilder to make a suggestion. The last argument provides 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.source_map().span_to_snippet(sp) {
    err.span_suggestion(
        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`.

The possible values of Applicability are:

  • MachineApplicable: Can be applied mechanically.
  • HasPlaceholders: Cannot be applied mechanically because it 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.

Every lint is implemented via a struct that implements the LintPass trait (you also implement one of the more specific lint pass traits, either EarlyLintPass or LateLintPass). The trait implementation allows you to check certain syntactic constructs as the linter walks the source code. You can then choose to emit lints in a very similar way to compile errors.

You also declare the metadata of a particular lint via the declare_lint! macro. This includes the name, the default level, a short description, and some more details.

Note that the lint and the lint pass must be registered with the compiler.

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;

// This declares a lint pass, providing a list of associated lints.  The
// compiler currently doesn't use the associated lints directly (e.g., to not
// run the pass or otherwise check that the pass emits the appropriate set of
// lints). However, it's good to be accurate here as it's possible that we're
// going to register the lints via the get_lints method on our lint pass (that
// this macro generates).
impl_lint_pass!(
    WhileTrue => [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.source_map().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.

A future-incompatible lint should be declared with the @future_incompatible additional "field":

declare_lint! {
    pub ANONYMOUS_PARAMETERS,
    Allow,
    "detects anonymous parameters",
    @future_incompatible = FutureIncompatibleInfo {
        reference: "issue #41686 <https://github.com/rust-lang/rust/issues/41686>",
        edition: Some(Edition::Edition2018),
    };
}

If you need a combination of options that's not supported by the declare_lint! macro, you can always define your own static with a type of &Lint but this is currently linted against in the compiler tree.

Guidelines for creating a future incompatibility lint

  • Create a lint defaulting to warn as normal, with ideally the same error message you would normally give.
  • Add a suitable reference, typically an RFC or tracking issue. Go ahead and include the full URL, sort items in ascending order of issue numbers.
  • Later, change lint to error.
  • Eventually, remove lint.

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.

Linting early in the compiler

On occasion, you may need to define a lint that runs before the linting system has been initialized (e.g. during parsing or macro expansion). This is problematic because we need to have computed lint levels to know whether we should emit a warning or an error or nothing at all.

To solve this problem, we buffer the lints until the linting system is processed. Session and ParseSess both have buffer_lint methods that allow you to buffer a lint for later. The linting system automatically takes care of handling buffered lints later.

Thus, to define a lint that runs early in the compilation, one defines a lint like normal but invokes the lint with buffer_lint.

Linting even earlier in the compiler

The parser (libsyntax) is interesting in that it cannot have dependencies on any of the other librustc* crates. In particular, it cannot depend on librustc::lint or librustc_lint, where all of the compiler linting infrastructure is defined. That's troublesome!

To solve this, libsyntax defines its own buffered lint type, which ParseSess::buffer_lint uses. After macro expansion, these buffered lints are then dumped into the Session::buffered_lints used by the rest of the compiler.

Usage for buffered lints in libsyntax is pretty much the same as the rest of the compiler with one exception because we cannot import the LintIds for lints we want to emit. Instead, the BufferedEarlyLintId type is used. If you are defining a new lint, you will want to add an entry to this enum. Then, add an appropriate mapping to the body of Lint::from_parser_lint_id.

JSON diagnostic output

The compiler accepts an --error-format json flag to output diagnostics as JSON objects (for the benefit of tools such as cargo fix or the RLS). It looks like this—

$ rustc json_error_demo.rs --error-format json
{"message":"cannot add `&str` to `{integer}`","code":{"code":"E0277","explanation":"\nYou tried to use a type which doesn't implement some trait in a place which\nexpected that trait. Erroneous code example:\n\n```compile_fail,E0277\n// here we declare the Foo trait with a bar method\ntrait Foo {\n    fn bar(&self);\n}\n\n// we now declare a function which takes an object implementing the Foo trait\nfn some_func<T: Foo>(foo: T) {\n    foo.bar();\n}\n\nfn main() {\n    // we now call the method with the i32 type, which doesn't implement\n    // the Foo trait\n    some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied\n}\n```\n\nIn order to fix this error, verify that the type you're using does implement\nthe trait. Example:\n\n```\ntrait Foo {\n    fn bar(&self);\n}\n\nfn some_func<T: Foo>(foo: T) {\n    foo.bar(); // we can now use this method since i32 implements the\n               // Foo trait\n}\n\n// we implement the trait on the i32 type\nimpl Foo for i32 {\n    fn bar(&self) {}\n}\n\nfn main() {\n    some_func(5i32); // ok!\n}\n```\n\nOr in a generic context, an erroneous code example would look like:\n\n```compile_fail,E0277\nfn some_func<T>(foo: T) {\n    println!(\"{:?}\", foo); // error: the trait `core::fmt::Debug` is not\n                           //        implemented for the type `T`\n}\n\nfn main() {\n    // We now call the method with the i32 type,\n    // which *does* implement the Debug trait.\n    some_func(5i32);\n}\n```\n\nNote that the error here is in the definition of the generic function: Although\nwe only call it with a parameter that does implement `Debug`, the compiler\nstill rejects the function: It must work with all possible input types. In\norder to make this example compile, we need to restrict the generic type we're\naccepting:\n\n```\nuse std::fmt;\n\n// Restrict the input type to types that implement Debug.\nfn some_func<T: fmt::Debug>(foo: T) {\n    println!(\"{:?}\", foo);\n}\n\nfn main() {\n    // Calling the method is still fine, as i32 implements Debug.\n    some_func(5i32);\n\n    // This would fail to compile now:\n    // struct WithoutDebug;\n    // some_func(WithoutDebug);\n}\n```\n\nRust only looks at the signature of the called function, as such it must\nalready specify all requirements that will be used for every type parameter.\n"},"level":"error","spans":[{"file_name":"json_error_demo.rs","byte_start":50,"byte_end":51,"line_start":4,"line_end":4,"column_start":7,"column_end":8,"is_primary":true,"text":[{"text":"    a + b","highlight_start":7,"highlight_end":8}],"label":"no implementation for `{integer} + &str`","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"the trait `std::ops::Add<&str>` is not implemented for `{integer}`","code":null,"level":"help","spans":[],"children":[],"rendered":null}],"rendered":"error[E0277]: cannot add `&str` to `{integer}`\n --> json_error_demo.rs:4:7\n  |\n4 |     a + b\n  |       ^ no implementation for `{integer} + &str`\n  |\n  = help: the trait `std::ops::Add<&str>` is not implemented for `{integer}`\n\n"}
{"message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error\n\n"}
{"message":"For more information about this error, try `rustc --explain E0277`.","code":null,"level":"","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0277`.\n"}

Note that the output is a series of lines, each of which is a JSON object, but the series of lines taken together is, unfortunately, not valid JSON, thwarting tools and tricks (such as piping to python3 -m json.tool) that require such. (One speculates that this was intentional for LSP performance purposes, so that each line/object can be sent to RLS as it is flushed?)

Also note the "rendered" field, which contains the "human" output as a string; this was introduced so that UI tests could both make use of the structured JSON and see the "human" output (well, sans colors) without having to compile everything twice.

The JSON emitter currently lives in libsyntax/json.rs. (But arguably it should live in librustc_errors along with the "human" emitter? It's not obvious to the present author why it wasn't moved from libsyntax to librustc_errors at the same time the "human" emitter was moved.)

The JSON emitter defines its own Diagnostic struct (and sub-structs) for the JSON serialization. Don't confuse this with errors::Diagnostic!

#[rustc_on_unimplemented(...)]

The #[rustc_on_unimplemented] attribute allows trait definitions to add specialized notes to error messages when an implementation was expected but not found. You can refer to the trait's generic arguments by name and to the resolved type using Self.

For example:

#![feature(rustc_attrs)]

#[rustc_on_unimplemented="an iterator over elements of type `{A}` \
    cannot be built from a collection of type `{Self}`"]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}

fn iterate_chars<I: MyIterator<char>>(i: I) {
    // ...
}

fn main() {
    iterate_chars(&[1, 2, 3][..]);
}

When the user compiles this, they will see the following;

error[E0277]: the trait bound `&[{integer}]: MyIterator<char>` is not satisfied
  --> <anon>:14:5
   |
14 |     iterate_chars(&[1, 2, 3][..]);
   |     ^^^^^^^^^^^^^ an iterator over elements of type `char` cannot be built from a collection of type `&[{integer}]`
   |
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: required by `iterate_chars`

rustc_on_unimplemented also supports advanced filtering for better targeting of messages, as well as modifying specific parts of the error message. You target the text of:

  • the main error message (message)
  • the label (label)
  • an extra note (note)

For example, the following attribute

#[rustc_on_unimplemented(
    message="message",
    label="label",
    note="note"
)]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}

Would generate the following output:

error[E0277]: message
  --> <anon>:14:5
   |
14 |     iterate_chars(&[1, 2, 3][..]);
   |     ^^^^^^^^^^^^^ label
   |
   = note: note
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: required by `iterate_chars`

To allow more targeted error messages, it is possible to filter the application of these fields based on a variety of attributes when using on:

  • crate_local: whether the code causing the trait bound to not be fulfilled is part of the user's crate. This is used to avoid suggesting code changes that would require modifying a dependency.
  • Any of the generic arguments that can be substituted in the text can be referred by name as well for filtering, like Rhs="i32", except for Self.
  • _Self: to filter only on a particular calculated trait resolution, like Self="std::iter::Iterator<char>". This is needed because Self is a keyword which cannot appear in attributes.
  • direct: user-specified rather than derived obligation.
  • from_method: usable both as boolean (whether the flag is present, like crate_local) or matching against a particular method. Currently used for try.
  • from_desugaring: usable both as boolean (whether the flag is present) or matching against a particular desugaring. The desugaring is identified with its variant name in the DesugaringKind enum.

For example, the Iterator trait can be annotated in the following way:

#[rustc_on_unimplemented(
    on(
        _Self="&str",
        note="call `.chars()` or `.as_bytes()` on `{Self}"
    ),
    message="`{Self}` is not an iterator",
    label="`{Self}` is not an iterator",
    note="maybe try calling `.iter()` or a similar method"
)]
pub trait Iterator {}

Which would produce the following outputs:

error[E0277]: `Foo` is not an iterator
 --> src/main.rs:4:16
  |
4 |     for foo in Foo {}
  |                ^^^ `Foo` is not an iterator
  |
  = note: maybe try calling `.iter()` or a similar method
  = help: the trait `std::iter::Iterator` is not implemented for `Foo`
  = note: required by `std::iter::IntoIterator::into_iter`

error[E0277]: `&str` is not an iterator
 --> src/main.rs:5:16
  |
5 |     for foo in "" {}
  |                ^^ `&str` is not an iterator
  |
  = note: call `.chars()` or `.bytes() on `&str`
  = help: the trait `std::iter::Iterator` is not implemented for `&str`
  = note: required by `std::iter::IntoIterator::into_iter`

If you need to filter on multiple attributes, you can use all, any or not in the following way:

#[rustc_on_unimplemented(
    on(
        all(_Self="&str", T="std::string::String"),
        note="you can coerce a `{T}` into a `{Self}` by writing `&*variable`"
    )
)]
pub trait From<T>: Sized { /* ... */ }