rustc_attr_parsing/attributes/diagnostic/

mod.rs

use std::ops::Range;

use rustc_ast::PathSegment;
use rustc_attr_ir::diagnostic::{
    Directive, Filter, FilterFormatString, Flag, FormatArg, FormatString, LitOrArg, Name,
    NameValue, Piece, Predicate,
};
use rustc_errors::{Diagnostic, MultiSpan};
use rustc_lint_defs::LintId;
use rustc_parse_format::{
    Argument, FormatSpec, ParseError, ParseMode, Parser, Piece as RpfPiece, Position,
};
use rustc_session::lint::builtin::{
    MALFORMED_DIAGNOSTIC_ATTRIBUTES, MALFORMED_DIAGNOSTIC_FILTERS,
    MALFORMED_DIAGNOSTIC_FORMAT_LITERALS, UNKNOWN_DIAGNOSTIC_ATTRIBUTES,
};
use rustc_span::edit_distance::find_best_match_for_name;
use rustc_span::{Ident, InnerSpan, Span, Symbol, kw, sym};
use thin_vec::{ThinVec, thin_vec};

use crate::context::AcceptContext;
use crate::diagnostics::{
    FormatWarning, IgnoredDiagnosticOption, InvalidOnClause, MalFormedDiagnosticAttributeLint,
    MissingOptionsForDiagnosticAttribute, NonMetaItemDiagnosticAttribute, WrappedParserError,
};
use crate::parser::{ArgParser, MetaItemListParser, MetaItemOrLitParser, MetaItemParser};
use crate::{EmitAttribute, diagnostics};

pub(crate) mod do_not_recommend;
pub(crate) mod on_const;
pub(crate) mod on_move;
pub(crate) mod on_type_error;
pub(crate) mod on_unimplemented;
pub(crate) mod on_unknown;
pub(crate) mod on_unmatched_args;
pub(crate) mod opaque;

impl<'sess> crate::AttributeParser<'sess> {
    pub(crate) fn unknown_diagnostic_attr(
        &self,
        segment: &PathSegment,
        mut emit_lint: impl FnMut(LintId, MultiSpan, EmitAttribute),
    ) {
        const DIAGNOSTIC_ATTRIBUTES: [(
            Symbol,         /* name */
            Option<Symbol>, /* feature gate */
        ); 8] = [
            (sym::on_unimplemented, None),
            (sym::do_not_recommend, None),
            (sym::on_move, Some(sym::diagnostic_on_move)),
            (sym::on_const, Some(sym::diagnostic_on_const)),
            (sym::on_unknown, Some(sym::diagnostic_on_unknown)),
            (sym::on_unmatched_args, Some(sym::diagnostic_on_unmatched_args)),
            (sym::on_type_error, Some(sym::diagnostic_on_type_error)),
            (sym::opaque, Some(sym::diagnostic_opaque)),
        ];
        // No need to emit a lint if features aren't available.
        let Some(features) = self.features else { return };
        let span = segment.span();
        let candidates = DIAGNOSTIC_ATTRIBUTES
            .iter()
            .filter_map(|(attr, feature)| {
                feature.is_none_or(|f| features.enabled(f)).then_some(*attr)
            })
            .collect::<Vec<_>>();

        let typo = find_best_match_for_name(&candidates, segment.ident.name, None)
            .map(|typo_name| diagnostics::UnknownDiagnosticAttributeTypo { span, typo_name });
        emit_lint(
            LintId::of(UNKNOWN_DIAGNOSTIC_ATTRIBUTES),
            span.into(),
            EmitAttribute(Box::new(move |dcx, level, _| {
                diagnostics::UnknownDiagnosticAttribute { typo }.into_diag(dcx, level)
            })),
        )
    }
}

#[rustc_macro_transparency = "transparent"]
macro gate_diagnostic_attr($feature:ident) {{
    if let Some(features) = cx.features_option()
        && !features.$feature()
    {
        args.ignore_args();
        let nightly_build = cx.sess.is_nightly_build();
        let span = cx.attr_span;
        cx.emit_lint(
            rustc_lint_defs::builtin::UNKNOWN_DIAGNOSTIC_ATTRIBUTES,
            $crate::diagnostics::UnstableDiagnosticAttribute {
                feature: sym::$feature,
                nightly_build,
            },
            span,
        );
        return;
    }
}}

#[derive(Copy, Clone)]
pub(crate) enum Mode {
    /// `#[rustc_on_unimplemented]`
    RustcOnUnimplemented,
    /// `#[diagnostic::on_unimplemented]`
    DiagnosticOnUnimplemented,
    /// `#[diagnostic::on_const]`
    DiagnosticOnConst,
    /// `#[diagnostic::on_move]`
    DiagnosticOnMove,
    /// `#[diagnostic::on_unknown]`
    DiagnosticOnUnknown,
    /// `#[diagnostic::on_unmatched_args]`
    DiagnosticOnUnmatchedArgs,
    /// `#[diagnostic::on_type_error]`
    DiagnosticOnTypeError,
}

impl Mode {
    fn as_str(self) -> &'static str {
        match self {
            Self::RustcOnUnimplemented => "rustc_on_unimplemented",
            Self::DiagnosticOnUnimplemented => "diagnostic::on_unimplemented",
            Self::DiagnosticOnConst => "diagnostic::on_const",
            Self::DiagnosticOnMove => "diagnostic::on_move",
            Self::DiagnosticOnUnknown => "diagnostic::on_unknown",
            Self::DiagnosticOnUnmatchedArgs => "diagnostic::on_unmatched_args",
            Self::DiagnosticOnTypeError => "diagnostic::on_type_error",
        }
    }

    fn expected_options(self) -> &'static str {
        const DEFAULT: &str =
            "at least one of the `message`, `note` and `label` options are expected";
        const DIAGNOSTIC_ON_TYPE_ERROR_EXPECTED_OPTIONS: &str =
            "at least a single `note` option is expected";
        match self {
            Self::RustcOnUnimplemented => {
                "see <https://rustc-dev-guide.rust-lang.org/diagnostics.html#rustc_on_unimplemented>"
            }
            Self::DiagnosticOnUnimplemented
            | Self::DiagnosticOnConst
            | Self::DiagnosticOnMove
            | Self::DiagnosticOnUnknown
            | Self::DiagnosticOnUnmatchedArgs => DEFAULT,
            Self::DiagnosticOnTypeError => DIAGNOSTIC_ON_TYPE_ERROR_EXPECTED_OPTIONS,
        }
    }

    fn allowed_options(self) -> &'static str {
        const DEFAULT: &str = "only `message`, `note` and `label` are allowed as options";
        const DIAGNOSTIC_ON_TYPE_ERROR_ALLOWED_OPTIONS: &str =
            "only `note` is allowed as option for `diagnostic::on_type_error`";
        match self {
            Self::RustcOnUnimplemented => {
                "see <https://rustc-dev-guide.rust-lang.org/diagnostics.html#rustc_on_unimplemented>"
            }
            Self::DiagnosticOnUnimplemented
            | Self::DiagnosticOnConst
            | Self::DiagnosticOnMove
            | Self::DiagnosticOnUnknown
            | Self::DiagnosticOnUnmatchedArgs => DEFAULT,
            Self::DiagnosticOnTypeError => DIAGNOSTIC_ON_TYPE_ERROR_ALLOWED_OPTIONS,
        }
    }

    fn allowed_format_arguments(self) -> &'static str {
        match self {
            Self::RustcOnUnimplemented => {
                "see <https://rustc-dev-guide.rust-lang.org/diagnostics.html#rustc_on_unimplemented> for allowed format arguments"
            }
            Self::DiagnosticOnUnimplemented => {
                "only `Self` and generics of the trait are allowed as a format argument"
            }
            Self::DiagnosticOnConst => {
                "only `Self` and generics of the implementation are allowed as a format argument"
            }
            Self::DiagnosticOnMove => {
                "only `This`, `Self` and generics of the type are allowed as a format argument"
            }
            Self::DiagnosticOnUnknown => {
                "only `This` is allowed as a format argument, referring to the failed import"
            }
            Self::DiagnosticOnUnmatchedArgs => {
                "only `This` is allowed as a format argument, referring to the macro's name"
            }
            Self::DiagnosticOnTypeError => {
                "only `note` is allowed as option for `diagnostic::on_type_error`"
            }
        }
    }
}

fn merge_directives(
    cx: &mut AcceptContext<'_, '_>,
    first: &mut Option<(Span, Directive)>,
    later: (Span, Directive),
) {
    if let Some((_, first)) = first {
        let Directive { is_rustc_attr, filters, message, label, notes, parent_label } = later.1;

        first.is_rustc_attr |= is_rustc_attr;
        first.filters.extend(filters);
        merge(cx, &mut first.message, message, sym::message);
        merge(cx, &mut first.label, label, sym::label);
        first.notes.extend(notes);
        merge(cx, &mut first.parent_label, parent_label, sym::parent_label);
    } else {
        *first = Some(later);
    }
}

fn merge<T>(
    cx: &mut AcceptContext<'_, '_>,
    first: &mut Option<(Span, T)>,
    later: Option<(Span, T)>,
    option_name: Symbol,
) {
    match (first, later) {
        (Some(_) | None, None) => {}
        (Some((first_span, _)), Some((later_span, _))) => {
            let first_span = *first_span;
            cx.emit_lint(
                MALFORMED_DIAGNOSTIC_ATTRIBUTES,
                IgnoredDiagnosticOption { first_span, later_span, option_name },
                later_span,
            );
        }
        (first @ None, Some(later)) => {
            first.get_or_insert(later);
        }
    }
}

fn parse_list<'p>(
    cx: &mut AcceptContext<'_, '_>,
    args: &'p ArgParser,
    mode: Mode,
) -> Option<&'p MetaItemListParser> {
    let span = cx.attr_span;
    match args {
        ArgParser::List(items) if items.len() != 0 => return Some(items),
        ArgParser::List(list) => {
            // We're dealing with `#[diagnostic::attr()]`.
            // This can be because that is what the user typed, but that's also what we'd see
            // if the user used non-metaitem syntax. See `ArgParser::from_attr_args`.
            cx.emit_lint(
                MALFORMED_DIAGNOSTIC_ATTRIBUTES,
                NonMetaItemDiagnosticAttribute,
                list.span,
            );
        }
        ArgParser::NoArgs => {
            cx.emit_lint(
                MALFORMED_DIAGNOSTIC_ATTRIBUTES,
                MissingOptionsForDiagnosticAttribute {
                    attribute: mode.as_str(),
                    options: mode.expected_options(),
                },
                span,
            );
        }
        ArgParser::NameValue(_) => {
            cx.emit_lint(
                MALFORMED_DIAGNOSTIC_ATTRIBUTES,
                MalFormedDiagnosticAttributeLint {
                    attribute: mode.as_str(),
                    options: mode.allowed_options(),
                    span,
                },
                span,
            );
        }
    }
    None
}

fn parse_directive_items<'p>(
    cx: &mut AcceptContext<'_, '_>,
    mode: Mode,
    items: impl Iterator<Item = &'p MetaItemOrLitParser>,
    is_root: bool,
) -> Option<Directive> {
    let mut message: Option<(Span, _)> = None;
    let mut label: Option<(Span, _)> = None;
    let mut notes = ThinVec::new();
    let mut parent_label: Option<(Span, FormatString)> = None;
    let mut filters = ThinVec::new();

    for item in items {
        let span = item.span();

        macro malformed() {{
            cx.emit_lint(
                MALFORMED_DIAGNOSTIC_ATTRIBUTES,
                MalFormedDiagnosticAttributeLint {
                    attribute: mode.as_str(),
                    options: mode.allowed_options(),
                    span,
                },
                span,
            );
            continue;
        }}

        macro or_malformed($($code:tt)*) {{
            let Some(ret) = (
                try {
                    $($code)*
                }
            ) else {
                malformed!()
            };
            ret
        }}

        macro duplicate($name: ident, $($first_span:tt)*) {{
            let first_span = $($first_span)*;
            cx.emit_lint(
                MALFORMED_DIAGNOSTIC_ATTRIBUTES,
                IgnoredDiagnosticOption {
                    first_span,
                    later_span: span,
                    option_name: $name,
                },
                span,
            );
        }}

        let item: &MetaItemParser = or_malformed!(item.meta_item()?);
        let name = or_malformed!(item.ident()?).name;

        // Currently, as of April 2026, all arguments of all diagnostic attrs
        // must have a value, like `message = "message"`. Thus in a well-formed
        // diagnostic attribute this is never `None`.
        //
        // But we don't assert its presence yet because we don't want to mention it
        // if someone does something like `#[diagnostic::on_unimplemented(doesnt_exist)]`.
        // That happens in the big `match` below.
        let value: Option<Ident> = match item.args().as_name_value() {
            Some(nv) => Some(or_malformed!(nv.value_as_ident()?)),
            None => None,
        };

        let mut parse_format = |input: Ident| {
            let snippet = cx.sess.source_map().span_to_snippet(input.span).ok();
            let is_snippet = snippet.is_some();
            match parse_format_string(input.name, snippet, input.span, mode) {
                Ok((f, warnings)) => {
                    for warning in warnings {
                        let (FormatWarning::InvalidSpecifier { span }
                        | FormatWarning::PositionalArgument { span }
                        | FormatWarning::IndexedArgument { span }
                        | FormatWarning::DisallowedPlaceholder { span, .. }) = warning;
                        cx.emit_lint(MALFORMED_DIAGNOSTIC_FORMAT_LITERALS, warning, span);
                    }

                    f
                }
                Err(e) => {
                    cx.emit_lint(
                        MALFORMED_DIAGNOSTIC_FORMAT_LITERALS,
                        WrappedParserError {
                            description: e.description,
                            label: e.label,
                            span: slice_span(input.span, e.span.clone(), is_snippet),
                        },
                        input.span,
                    );
                    // We could not parse the input, just use it as-is.
                    FormatString {
                        input: input.name,
                        span: input.span,
                        pieces: thin_vec![Piece::Lit(input.name)],
                    }
                }
            }
        };
        match (mode, name) {
            (
                Mode::RustcOnUnimplemented
                | Mode::DiagnosticOnUnimplemented
                | Mode::DiagnosticOnConst
                | Mode::DiagnosticOnMove
                | Mode::DiagnosticOnUnknown
                | Mode::DiagnosticOnUnmatchedArgs,
                sym::message,
            ) => {
                let value = or_malformed!(value?);
                if let Some(message) = &message {
                    duplicate!(name, message.0)
                } else {
                    message = Some((item.span(), parse_format(value)));
                }
            }
            (
                Mode::RustcOnUnimplemented
                | Mode::DiagnosticOnUnimplemented
                | Mode::DiagnosticOnConst
                | Mode::DiagnosticOnMove
                | Mode::DiagnosticOnUnknown
                | Mode::DiagnosticOnUnmatchedArgs,
                sym::label,
            ) => {
                let value = or_malformed!(value?);
                if let Some(label) = &label {
                    duplicate!(name, label.0)
                } else {
                    label = Some((item.span(), parse_format(value)));
                }
            }
            (_, sym::note) => {
                let value = or_malformed!(value?);
                notes.push(parse_format(value))
            }
            (Mode::RustcOnUnimplemented, sym::parent_label) => {
                let value = or_malformed!(value?);
                if let Some(parent_label) = &parent_label {
                    duplicate!(name, parent_label.0)
                } else {
                    let format = parse_format(value);
                    parent_label = Some((format.span, format));
                }
            }
            (Mode::RustcOnUnimplemented, sym::on) => {
                if is_root {
                    let items = or_malformed!(item.args().as_list()?);
                    let mut iter = items.mixed();
                    let filter = if let Some(c) = iter.next() {
                        c
                    } else {
                        cx.emit_lint(
                            MALFORMED_DIAGNOSTIC_FILTERS,
                            InvalidOnClause::Empty { span },
                            span,
                        );
                        continue;
                    };

                    let filter = parse_filter(filter);

                    if items.len() < 2 {
                        // Something like `#[rustc_on_unimplemented(on(.., /* nothing */))]`
                        // There's a filter but no directive behind it, this is a mistake.
                        malformed!();
                    }

                    match filter {
                        Ok(filter) => {
                            let directive =
                                or_malformed!(parse_directive_items(cx, mode, iter, false)?);
                            filters.push((filter, directive));
                        }
                        Err(e) => {
                            cx.emit_lint(MALFORMED_DIAGNOSTIC_FILTERS, e, span);
                        }
                    }
                } else {
                    malformed!();
                }
            }
            _other => {
                malformed!();
            }
        }
    }

    Some(Directive {
        is_rustc_attr: matches!(mode, Mode::RustcOnUnimplemented),
        filters,
        message,
        label,
        notes,
        parent_label,
    })
}

pub(crate) fn parse_format_string(
    input: Symbol,
    snippet: Option<String>,
    span: Span,
    mode: Mode,
) -> Result<(FormatString, Vec<FormatWarning>), ParseError> {
    let s = input.as_str();
    let mut parser = Parser::new(s, None, snippet, false, ParseMode::Diagnostic);
    let pieces: Vec<_> = parser.by_ref().collect();

    if let Some(err) = parser.errors.into_iter().next() {
        return Err(err);
    }
    let mut warnings = Vec::new();

    let pieces = pieces
        .into_iter()
        .map(|piece| match piece {
            RpfPiece::Lit(lit) => Piece::Lit(Symbol::intern(lit)),
            RpfPiece::NextArgument(arg) => {
                Piece::Arg(parse_arg(&arg, mode, &mut warnings, span, parser.is_source_literal))
            }
        })
        .collect();

    Ok((FormatString { input, pieces, span }, warnings))
}

fn parse_arg(
    arg: &Argument<'_>,
    mode: Mode,
    warnings: &mut Vec<FormatWarning>,
    input_span: Span,
    is_source_literal: bool,
) -> FormatArg {
    let span = slice_span(input_span, arg.position_span.clone(), is_source_literal);

    let mut check_format = true;

    let ret = match arg.position {
        // Something like "hello {name}"
        Position::ArgumentNamed(name) => match (mode, Symbol::intern(name)) {
            (Mode::RustcOnUnimplemented, sym::ItemContext) => FormatArg::ItemContext,

            // `{This:ty}`
            (Mode::RustcOnUnimplemented, sym::This) => match arg.format.ty {
                "resolved" => {
                    check_format = false;
                    FormatArg::ThisResolved
                }
                "path" => {
                    check_format = false;
                    FormatArg::ThisPath
                }
                _ => FormatArg::This,
            },

            (Mode::DiagnosticOnTypeError, sym::Found) => FormatArg::Found,
            (Mode::DiagnosticOnTypeError, sym::Expected) => FormatArg::Expected,
            (Mode::DiagnosticOnUnknown, sym::Unresolved) => FormatArg::Unresolved,

            // Some diagnostic attributes can use `{This}` to refer to the annotated item.
            // For those that don't, we continue and maybe use it as a generic parameter.
            //
            // FIXME(mejrs) `DiagnosticOnUnimplemented` is intentionally not here;
            // that requires lang approval which is best kept for a standalone PR.
            (
                Mode::DiagnosticOnUnknown
                | Mode::DiagnosticOnMove
                | Mode::DiagnosticOnUnmatchedArgs
                | Mode::DiagnosticOnTypeError,
                sym::This,
            ) => FormatArg::This,

            // `{Self}`; the self type.
            // - For trait declaration attributes that's the type that does not implement it.
            // - for trait impl attributes, the implemented for type.
            // - For ADT attributes, that's the type (which will be identical to `{This}`)
            // - For everything else it doesn't make sense.
            (
                Mode::RustcOnUnimplemented
                | Mode::DiagnosticOnUnimplemented
                | Mode::DiagnosticOnMove
                | Mode::DiagnosticOnConst,
                kw::SelfUpper,
            ) => FormatArg::SelfUpper,

            // Generic parameters.
            // FIXME(mejrs) unfortunately, all the "special" symbols above might fall through,
            // but at this time we are not aware of what generic parameters the trait actually has.
            // If we find `ItemContext` or something we have to assume that's a generic parameter.
            // We lint against that in `check_attr.rs` though.
            (
                Mode::RustcOnUnimplemented
                | Mode::DiagnosticOnUnimplemented
                | Mode::DiagnosticOnMove
                | Mode::DiagnosticOnConst
                | Mode::DiagnosticOnTypeError,
                generic_param,
            ) => FormatArg::GenericParam { generic_param, span },

            // Generics are explicitly not allowed, we print those back as is.
            (Mode::DiagnosticOnUnknown | Mode::DiagnosticOnUnmatchedArgs, as_is) => {
                warnings.push(FormatWarning::DisallowedPlaceholder {
                    span,
                    attr: mode.as_str(),
                    allowed: mode.allowed_format_arguments(),
                });
                FormatArg::AsIs(Symbol::intern(&format!("{{{as_is}}}")))
            }
        },

        // `{1}` and `{}` are ignored
        Position::ArgumentIs(idx) => {
            warnings.push(FormatWarning::IndexedArgument { span });
            FormatArg::AsIs(Symbol::intern(&format!("{{{idx}}}")))
        }
        Position::ArgumentImplicitlyIs(_) => {
            warnings.push(FormatWarning::PositionalArgument { span });
            FormatArg::AsIs(sym::empty_braces)
        }
    };
    if check_format {
        warn_on_format_spec(&arg.format, warnings, input_span, is_source_literal);
    }
    ret
}

/// `#[rustc_on_unimplemented]` and `#[diagnostic::...]` don't actually do anything
/// with specifiers, so emit a warning if they are used.
fn warn_on_format_spec(
    spec: &FormatSpec<'_>,
    warnings: &mut Vec<FormatWarning>,
    input_span: Span,
    is_source_literal: bool,
) {
    if let Some(ty_span) = &spec.ty_span {
        let span = slice_span(input_span, ty_span.clone(), is_source_literal);
        warnings.push(FormatWarning::InvalidSpecifier { span })
    }
}

fn slice_span(input: Span, Range { start, end }: Range<usize>, is_source_literal: bool) -> Span {
    if is_source_literal { input.from_inner(InnerSpan { start, end }) } else { input }
}

pub(crate) fn parse_filter(input: &MetaItemOrLitParser) -> Result<Filter, InvalidOnClause> {
    let span = input.span();
    let pred = parse_predicate(input)?;
    Ok(Filter { span, pred })
}

fn parse_predicate(input: &MetaItemOrLitParser) -> Result<Predicate, InvalidOnClause> {
    let Some(meta_item) = input.meta_item() else {
        return Err(InvalidOnClause::UnsupportedLiteral { span: input.span() });
    };

    let Some(predicate) = meta_item.ident() else {
        return Err(InvalidOnClause::ExpectedIdentifier {
            span: meta_item.path().span(),
            path: meta_item.path().get_attribute_path(),
        });
    };

    match meta_item.args() {
        ArgParser::List(mis) => match predicate.name {
            sym::any => Ok(Predicate::Any(parse_predicate_sequence(mis)?)),
            sym::all => Ok(Predicate::All(parse_predicate_sequence(mis)?)),
            sym::not => {
                if let Some(single) = mis.as_single() {
                    Ok(Predicate::Not(Box::new(parse_predicate(single)?)))
                } else {
                    Err(InvalidOnClause::ExpectedOnePredInNot { span: mis.span })
                }
            }
            invalid_pred => {
                Err(InvalidOnClause::InvalidPredicate { span: predicate.span, invalid_pred })
            }
        },
        ArgParser::NameValue(p) => {
            let Some(value) = p.value_as_ident() else {
                return Err(InvalidOnClause::UnsupportedLiteral { span: p.args_span() });
            };
            let name = parse_name(predicate.name);
            let value = parse_filter_format(value.name);
            let kv = NameValue { name, value };
            Ok(Predicate::Match(kv))
        }
        ArgParser::NoArgs => {
            let flag = parse_flag(predicate)?;
            Ok(Predicate::Flag(flag))
        }
    }
}

fn parse_predicate_sequence(
    sequence: &MetaItemListParser,
) -> Result<ThinVec<Predicate>, InvalidOnClause> {
    sequence.mixed().map(parse_predicate).collect()
}

fn parse_flag(Ident { name, span }: Ident) -> Result<Flag, InvalidOnClause> {
    match name {
        sym::crate_local => Ok(Flag::CrateLocal),
        sym::direct => Ok(Flag::Direct),
        sym::from_desugaring => Ok(Flag::FromDesugaring),
        invalid_flag => Err(InvalidOnClause::InvalidFlag { invalid_flag, span }),
    }
}

fn parse_name(name: Symbol) -> Name {
    match name {
        kw::SelfUpper => Name::SelfUpper,
        sym::from_desugaring => Name::FromDesugaring,
        sym::cause => Name::Cause,
        generic => Name::GenericArg(generic),
    }
}

fn parse_filter_format(input: Symbol) -> FilterFormatString {
    let pieces = Parser::new(input.as_str(), None, None, false, ParseMode::Diagnostic)
        .map(|p| match p {
            RpfPiece::Lit(s) => LitOrArg::Lit(Symbol::intern(s)),
            // We just ignore formatspecs here
            RpfPiece::NextArgument(a) => match a.position {
                // In `TypeErrCtxt::on_unimplemented_note` we substitute `"{integral}"` even
                // if the integer type has been resolved, to allow targeting all integers.
                // `"{integer}"` and `"{float}"` come from numerics that haven't been inferred yet,
                // from the `Display` impl of `InferTy` to be precise.
                // `"{union|enum|struct}"` is used as a special selector for ADTs.
                //
                // Don't try to format these later!
                Position::ArgumentNamed(
                    arg @ ("integer" | "integral" | "float" | "union" | "enum" | "struct"),
                ) => LitOrArg::Lit(Symbol::intern(&format!("{{{arg}}}"))),

                Position::ArgumentNamed(arg) => LitOrArg::Arg(Symbol::intern(arg)),
                Position::ArgumentImplicitlyIs(_) => LitOrArg::Lit(sym::empty_braces),
                Position::ArgumentIs(idx) => LitOrArg::Lit(Symbol::intern(&format!("{{{idx}}}"))),
            },
        })
        .collect();
    FilterFormatString { pieces }
}