rust/crates/hir_ty/src/display.rs

//! FIXME: write short doc here

use std::fmt;

use crate::{
    db::HirDatabase, utils::generics, ApplicationTy, CallableDefId, FnSig, GenericPredicate,
    Lifetime, Obligation, OpaqueTyId, ProjectionTy, Substs, TraitRef, Ty, TypeCtor,
};
use hir_def::{
    find_path, generics::TypeParamProvenance, item_scope::ItemInNs, AdtId, AssocContainerId,
    Lookup, ModuleId,
};
use hir_expand::name::Name;

pub struct HirFormatter<'a> {
    pub db: &'a dyn HirDatabase,
    fmt: &'a mut dyn fmt::Write,
    buf: String,
    curr_size: usize,
    pub(crate) max_size: Option<usize>,
    omit_verbose_types: bool,
    display_target: DisplayTarget,
}

pub trait HirDisplay {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError>;

    /// Returns a `Display`able type that is human-readable.
    fn into_displayable<'a>(
        &'a self,
        db: &'a dyn HirDatabase,
        max_size: Option<usize>,
        omit_verbose_types: bool,
        display_target: DisplayTarget,
    ) -> HirDisplayWrapper<'a, Self>
    where
        Self: Sized,
    {
        HirDisplayWrapper { db, t: self, max_size, omit_verbose_types, display_target }
    }

    /// Returns a `Display`able type that is human-readable.
    /// Use this for showing types to the user (e.g. diagnostics)
    fn display<'a>(&'a self, db: &'a dyn HirDatabase) -> HirDisplayWrapper<'a, Self>
    where
        Self: Sized,
    {
        HirDisplayWrapper {
            db,
            t: self,
            max_size: None,
            omit_verbose_types: false,
            display_target: DisplayTarget::Diagnostics,
        }
    }

    /// Returns a `Display`able type that is human-readable and tries to be succinct.
    /// Use this for showing types to the user where space is constrained (e.g. doc popups)
    fn display_truncated<'a>(
        &'a self,
        db: &'a dyn HirDatabase,
        max_size: Option<usize>,
    ) -> HirDisplayWrapper<'a, Self>
    where
        Self: Sized,
    {
        HirDisplayWrapper {
            db,
            t: self,
            max_size,
            omit_verbose_types: true,
            display_target: DisplayTarget::Diagnostics,
        }
    }

    /// Returns a String representation of `self` that can be inserted into the given module.
    /// Use this when generating code (e.g. assists)
    fn display_source_code<'a>(
        &'a self,
        db: &'a dyn HirDatabase,
        module_id: ModuleId,
    ) -> Result<String, DisplaySourceCodeError> {
        let mut result = String::new();
        match self.hir_fmt(&mut HirFormatter {
            db,
            fmt: &mut result,
            buf: String::with_capacity(20),
            curr_size: 0,
            max_size: None,
            omit_verbose_types: false,
            display_target: DisplayTarget::SourceCode { module_id },
        }) {
            Ok(()) => {}
            Err(HirDisplayError::FmtError) => panic!("Writing to String can't fail!"),
            Err(HirDisplayError::DisplaySourceCodeError(e)) => return Err(e),
        };
        Ok(result)
    }

    /// Returns a String representation of `self` for test purposes
    fn display_test<'a>(&'a self, db: &'a dyn HirDatabase) -> HirDisplayWrapper<'a, Self>
    where
        Self: Sized,
    {
        HirDisplayWrapper {
            db,
            t: self,
            max_size: None,
            omit_verbose_types: false,
            display_target: DisplayTarget::Test,
        }
    }
}

impl<'a> HirFormatter<'a> {
    pub fn write_joined<T: HirDisplay>(
        &mut self,
        iter: impl IntoIterator<Item = T>,
        sep: &str,
    ) -> Result<(), HirDisplayError> {
        let mut first = true;
        for e in iter {
            if !first {
                write!(self, "{}", sep)?;
            }
            first = false;
            e.hir_fmt(self)?;
        }
        Ok(())
    }

    /// This allows using the `write!` macro directly with a `HirFormatter`.
    pub fn write_fmt(&mut self, args: fmt::Arguments) -> Result<(), HirDisplayError> {
        // We write to a buffer first to track output size
        self.buf.clear();
        fmt::write(&mut self.buf, args)?;
        self.curr_size += self.buf.len();

        // Then we write to the internal formatter from the buffer
        self.fmt.write_str(&self.buf).map_err(HirDisplayError::from)
    }

    pub fn should_truncate(&self) -> bool {
        if let Some(max_size) = self.max_size {
            self.curr_size >= max_size
        } else {
            false
        }
    }

    pub fn omit_verbose_types(&self) -> bool {
        self.omit_verbose_types
    }
}

#[derive(Clone, Copy)]
pub enum DisplayTarget {
    /// Display types for inlays, doc popups, autocompletion, etc...
    /// Showing `{unknown}` or not qualifying paths is fine here.
    /// There's no reason for this to fail.
    Diagnostics,
    /// Display types for inserting them in source files.
    /// The generated code should compile, so paths need to be qualified.
    SourceCode { module_id: ModuleId },
    /// Only for test purpose to keep real types
    Test,
}

impl DisplayTarget {
    fn is_source_code(&self) -> bool {
        matches!(self, Self::SourceCode {..})
    }
    fn is_test(&self) -> bool {
        matches!(self, Self::Test)
    }
}

#[derive(Debug)]
pub enum DisplaySourceCodeError {
    PathNotFound,
    UnknownType,
}

pub enum HirDisplayError {
    /// Errors that can occur when generating source code
    DisplaySourceCodeError(DisplaySourceCodeError),
    /// `FmtError` is required to be compatible with std::fmt::Display
    FmtError,
}
impl From<fmt::Error> for HirDisplayError {
    fn from(_: fmt::Error) -> Self {
        Self::FmtError
    }
}

pub struct HirDisplayWrapper<'a, T> {
    db: &'a dyn HirDatabase,
    t: &'a T,
    max_size: Option<usize>,
    omit_verbose_types: bool,
    display_target: DisplayTarget,
}

impl<'a, T> fmt::Display for HirDisplayWrapper<'a, T>
where
    T: HirDisplay,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.t.hir_fmt(&mut HirFormatter {
            db: self.db,
            fmt: f,
            buf: String::with_capacity(20),
            curr_size: 0,
            max_size: self.max_size,
            omit_verbose_types: self.omit_verbose_types,
            display_target: self.display_target,
        }) {
            Ok(()) => Ok(()),
            Err(HirDisplayError::FmtError) => Err(fmt::Error),
            Err(HirDisplayError::DisplaySourceCodeError(_)) => {
                // This should never happen
                panic!("HirDisplay failed when calling Display::fmt!")
            }
        }
    }
}

const TYPE_HINT_TRUNCATION: &str = "…";

impl HirDisplay for &Ty {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        HirDisplay::hir_fmt(*self, f)
    }
}

impl HirDisplay for ApplicationTy {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        if f.should_truncate() {
            return write!(f, "{}", TYPE_HINT_TRUNCATION);
        }

        match self.ctor {
            TypeCtor::Bool => write!(f, "bool")?,
            TypeCtor::Char => write!(f, "char")?,
            TypeCtor::Int(t) => write!(f, "{}", t)?,
            TypeCtor::Float(t) => write!(f, "{}", t)?,
            TypeCtor::Str => write!(f, "str")?,
            TypeCtor::Slice => {
                let t = self.parameters.as_single();
                write!(f, "[")?;
                t.hir_fmt(f)?;
                write!(f, "]")?;
            }
            TypeCtor::Array => {
                let t = self.parameters.as_single();
                write!(f, "[")?;
                t.hir_fmt(f)?;
                write!(f, "; _]")?;
            }
            TypeCtor::RawPtr(m) => {
                let t = self.parameters.as_single();

                write!(f, "*{}", m.as_keyword_for_ptr())?;
                if matches!(t, Ty::Dyn(predicates) if predicates.len() > 1) {
                    write!(f, "(")?;
                    t.hir_fmt(f)?;
                    write!(f, ")")?;
                } else {
                    t.hir_fmt(f)?;
                }
            }
            TypeCtor::Ref(m) => {
                let t = self.parameters.as_single();
                let ty_display =
                    t.into_displayable(f.db, f.max_size, f.omit_verbose_types, f.display_target);

                write!(f, "&{}", m.as_keyword_for_ref())?;
                if matches!(t, Ty::Dyn(predicates) if predicates.len() > 1) {
                    write!(f, "(")?;
                    write!(f, "{}", ty_display)?;
                    write!(f, ")")?;
                } else {
                    write!(f, "{}", ty_display)?;
                }
            }
            TypeCtor::Never => write!(f, "!")?,
            TypeCtor::Tuple { .. } => {
                let ts = &self.parameters;
                if ts.len() == 1 {
                    write!(f, "(")?;
                    ts[0].hir_fmt(f)?;
                    write!(f, ",)")?;
                } else {
                    write!(f, "(")?;
                    f.write_joined(&*ts.0, ", ")?;
                    write!(f, ")")?;
                }
            }
            TypeCtor::FnPtr { is_varargs, .. } => {
                let sig = FnSig::from_fn_ptr_substs(&self.parameters, is_varargs);
                sig.hir_fmt(f)?;
            }
            TypeCtor::FnDef(def) => {
                let sig = f.db.callable_item_signature(def).subst(&self.parameters);
                match def {
                    CallableDefId::FunctionId(ff) => {
                        write!(f, "fn {}", f.db.function_data(ff).name)?
                    }
                    CallableDefId::StructId(s) => write!(f, "{}", f.db.struct_data(s).name)?,
                    CallableDefId::EnumVariantId(e) => {
                        write!(f, "{}", f.db.enum_data(e.parent).variants[e.local_id].name)?
                    }
                };
                if self.parameters.len() > 0 {
                    let generics = generics(f.db.upcast(), def.into());
                    let (parent_params, self_param, type_params, _impl_trait_params) =
                        generics.provenance_split();
                    let total_len = parent_params + self_param + type_params;
                    // We print all params except implicit impl Trait params. Still a bit weird; should we leave out parent and self?
                    if total_len > 0 {
                        write!(f, "<")?;
                        f.write_joined(&self.parameters.0[..total_len], ", ")?;
                        write!(f, ">")?;
                    }
                }
                write!(f, "(")?;
                f.write_joined(sig.params(), ", ")?;
                write!(f, ")")?;
                let ret = sig.ret();
                if *ret != Ty::unit() {
                    let ret_display = ret.into_displayable(
                        f.db,
                        f.max_size,
                        f.omit_verbose_types,
                        f.display_target,
                    );

                    write!(f, " -> {}", ret_display)?;
                }
            }
            TypeCtor::Adt(def_id) => {
                match f.display_target {
                    DisplayTarget::Diagnostics | DisplayTarget::Test => {
                        let name = match def_id {
                            AdtId::StructId(it) => f.db.struct_data(it).name.clone(),
                            AdtId::UnionId(it) => f.db.union_data(it).name.clone(),
                            AdtId::EnumId(it) => f.db.enum_data(it).name.clone(),
                        };
                        write!(f, "{}", name)?;
                    }
                    DisplayTarget::SourceCode { module_id } => {
                        if let Some(path) = find_path::find_path(
                            f.db.upcast(),
                            ItemInNs::Types(def_id.into()),
                            module_id,
                        ) {
                            write!(f, "{}", path)?;
                        } else {
                            return Err(HirDisplayError::DisplaySourceCodeError(
                                DisplaySourceCodeError::PathNotFound,
                            ));
                        }
                    }
                }

                if self.parameters.len() > 0 {
                    let parameters_to_write =
                        if f.display_target.is_source_code() || f.omit_verbose_types() {
                            match self
                                .ctor
                                .as_generic_def()
                                .map(|generic_def_id| f.db.generic_defaults(generic_def_id))
                                .filter(|defaults| !defaults.is_empty())
                            {
                                None => self.parameters.0.as_ref(),
                                Some(default_parameters) => {
                                    let mut default_from = 0;
                                    for (i, parameter) in self.parameters.iter().enumerate() {
                                        match (parameter, default_parameters.get(i)) {
                                            (&Ty::Unknown, _) | (_, None) => {
                                                default_from = i + 1;
                                            }
                                            (_, Some(default_parameter)) => {
                                                let actual_default = default_parameter
                                                    .clone()
                                                    .subst(&self.parameters.prefix(i));
                                                if parameter != &actual_default {
                                                    default_from = i + 1;
                                                }
                                            }
                                        }
                                    }
                                    &self.parameters.0[0..default_from]
                                }
                            }
                        } else {
                            self.parameters.0.as_ref()
                        };
                    if !parameters_to_write.is_empty() {
                        write!(f, "<")?;
                        f.write_joined(parameters_to_write, ", ")?;
                        write!(f, ">")?;
                    }
                }
            }
            TypeCtor::AssociatedType(type_alias) => {
                let trait_ = match type_alias.lookup(f.db.upcast()).container {
                    AssocContainerId::TraitId(it) => it,
                    _ => panic!("not an associated type"),
                };
                let trait_ = f.db.trait_data(trait_);
                let type_alias_data = f.db.type_alias_data(type_alias);

                // Use placeholder associated types when the target is test (https://rust-lang.github.io/chalk/book/clauses/type_equality.html#placeholder-associated-types)
                if f.display_target.is_test() {
                    write!(f, "{}::{}", trait_.name, type_alias_data.name)?;
                    if self.parameters.len() > 0 {
                        write!(f, "<")?;
                        f.write_joined(&*self.parameters.0, ", ")?;
                        write!(f, ">")?;
                    }
                } else {
                    let projection_ty = ProjectionTy {
                        associated_ty: type_alias,
                        parameters: self.parameters.clone(),
                    };

                    projection_ty.hir_fmt(f)?;
                }
            }
            TypeCtor::ForeignType(type_alias) => {
                let type_alias = f.db.type_alias_data(type_alias);
                write!(f, "{}", type_alias.name)?;
                if self.parameters.len() > 0 {
                    write!(f, "<")?;
                    f.write_joined(&*self.parameters.0, ", ")?;
                    write!(f, ">")?;
                }
            }
            TypeCtor::OpaqueType(opaque_ty_id) => {
                match opaque_ty_id {
                    OpaqueTyId::ReturnTypeImplTrait(func, idx) => {
                        let datas =
                            f.db.return_type_impl_traits(func).expect("impl trait id without data");
                        let data = (*datas)
                            .as_ref()
                            .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
                        let bounds = data.subst(&self.parameters);
                        write!(f, "impl ")?;
                        write_bounds_like_dyn_trait(&bounds.value, f)?;
                        // FIXME: it would maybe be good to distinguish this from the alias type (when debug printing), and to show the substitution
                    }
                    OpaqueTyId::AsyncBlockTypeImplTrait(..) => {
                        write!(f, "impl Future<Output = ")?;
                        self.parameters[0].hir_fmt(f)?;
                        write!(f, ">")?;
                    }
                }
            }
            TypeCtor::Closure { .. } => {
                let sig = self.parameters[0].callable_sig(f.db);
                if let Some(sig) = sig {
                    if sig.params().is_empty() {
                        write!(f, "||")?;
                    } else if f.omit_verbose_types() {
                        write!(f, "|{}|", TYPE_HINT_TRUNCATION)?;
                    } else {
                        write!(f, "|")?;
                        f.write_joined(sig.params(), ", ")?;
                        write!(f, "|")?;
                    };

                    let ret_display = sig.ret().into_displayable(
                        f.db,
                        f.max_size,
                        f.omit_verbose_types,
                        f.display_target,
                    );
                    write!(f, " -> {}", ret_display)?;
                } else {
                    write!(f, "{{closure}}")?;
                }
            }
        }
        Ok(())
    }
}

impl HirDisplay for ProjectionTy {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        if f.should_truncate() {
            return write!(f, "{}", TYPE_HINT_TRUNCATION);
        }

        let trait_ = f.db.trait_data(self.trait_(f.db));
        let first_parameter = self.parameters[0].into_displayable(
            f.db,
            f.max_size,
            f.omit_verbose_types,
            f.display_target,
        );
        write!(f, "<{} as {}", first_parameter, trait_.name)?;
        if self.parameters.len() > 1 {
            write!(f, "<")?;
            f.write_joined(&self.parameters[1..], ", ")?;
            write!(f, ">")?;
        }
        write!(f, ">::{}", f.db.type_alias_data(self.associated_ty).name)?;
        Ok(())
    }
}

impl HirDisplay for Ty {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        if f.should_truncate() {
            return write!(f, "{}", TYPE_HINT_TRUNCATION);
        }

        match self {
            Ty::Apply(a_ty) => a_ty.hir_fmt(f)?,
            Ty::Projection(p_ty) => p_ty.hir_fmt(f)?,
            Ty::Placeholder(id) => {
                let generics = generics(f.db.upcast(), id.parent);
                let param_data = &generics.params.types[id.local_id];
                match param_data.provenance {
                    TypeParamProvenance::TypeParamList | TypeParamProvenance::TraitSelf => {
                        write!(f, "{}", param_data.name.clone().unwrap_or_else(Name::missing))?
                    }
                    TypeParamProvenance::ArgumentImplTrait => {
                        write!(f, "impl ")?;
                        let bounds = f.db.generic_predicates_for_param(*id);
                        let substs = Substs::type_params_for_generics(&generics);
                        write_bounds_like_dyn_trait(
                            &bounds.iter().map(|b| b.clone().subst(&substs)).collect::<Vec<_>>(),
                            f,
                        )?;
                    }
                }
            }
            Ty::Bound(idx) => write!(f, "?{}.{}", idx.debruijn.depth(), idx.index)?,
            Ty::Dyn(predicates) => {
                write!(f, "dyn ")?;
                write_bounds_like_dyn_trait(predicates, f)?;
            }
            Ty::Opaque(opaque_ty) => {
                match opaque_ty.opaque_ty_id {
                    OpaqueTyId::ReturnTypeImplTrait(func, idx) => {
                        let datas =
                            f.db.return_type_impl_traits(func).expect("impl trait id without data");
                        let data = (*datas)
                            .as_ref()
                            .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
                        let bounds = data.subst(&opaque_ty.parameters);
                        write!(f, "impl ")?;
                        write_bounds_like_dyn_trait(&bounds.value, f)?;
                    }
                    OpaqueTyId::AsyncBlockTypeImplTrait(..) => {
                        write!(f, "{{async block}}")?;
                    }
                };
            }
            Ty::Unknown => {
                if f.display_target.is_source_code() {
                    return Err(HirDisplayError::DisplaySourceCodeError(
                        DisplaySourceCodeError::UnknownType,
                    ));
                }
                write!(f, "{{unknown}}")?;
            }
            Ty::Infer(..) => write!(f, "_")?,
        }
        Ok(())
    }
}

impl HirDisplay for FnSig {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        write!(f, "fn(")?;
        f.write_joined(self.params(), ", ")?;
        if self.is_varargs {
            if self.params().is_empty() {
                write!(f, "...")?;
            } else {
                write!(f, ", ...")?;
            }
        }
        write!(f, ")")?;
        let ret = self.ret();
        if *ret != Ty::unit() {
            let ret_display =
                ret.into_displayable(f.db, f.max_size, f.omit_verbose_types, f.display_target);
            write!(f, " -> {}", ret_display)?;
        }
        Ok(())
    }
}

pub fn write_bounds_like_dyn_trait(
    predicates: &[GenericPredicate],
    f: &mut HirFormatter,
) -> Result<(), HirDisplayError> {
    // Note: This code is written to produce nice results (i.e.
    // corresponding to surface Rust) for types that can occur in
    // actual Rust. It will have weird results if the predicates
    // aren't as expected (i.e. self types = $0, projection
    // predicates for a certain trait come after the Implemented
    // predicate for that trait).
    let mut first = true;
    let mut angle_open = false;
    for p in predicates.iter() {
        match p {
            GenericPredicate::Implemented(trait_ref) => {
                if angle_open {
                    write!(f, ">")?;
                    angle_open = false;
                }
                if !first {
                    write!(f, " + ")?;
                }
                // We assume that the self type is $0 (i.e. the
                // existential) here, which is the only thing that's
                // possible in actual Rust, and hence don't print it
                write!(f, "{}", f.db.trait_data(trait_ref.trait_).name)?;
                if trait_ref.substs.len() > 1 {
                    write!(f, "<")?;
                    f.write_joined(&trait_ref.substs[1..], ", ")?;
                    // there might be assoc type bindings, so we leave the angle brackets open
                    angle_open = true;
                }
            }
            GenericPredicate::Projection(projection_pred) => {
                // in types in actual Rust, these will always come
                // after the corresponding Implemented predicate
                if angle_open {
                    write!(f, ", ")?;
                } else {
                    write!(f, "<")?;
                    angle_open = true;
                }
                let type_alias = f.db.type_alias_data(projection_pred.projection_ty.associated_ty);
                write!(f, "{} = ", type_alias.name)?;
                projection_pred.ty.hir_fmt(f)?;
            }
            GenericPredicate::Error => {
                if angle_open {
                    // impl Trait<X, {error}>
                    write!(f, ", ")?;
                } else if !first {
                    // impl Trait + {error}
                    write!(f, " + ")?;
                }
                p.hir_fmt(f)?;
            }
        }
        first = false;
    }
    if angle_open {
        write!(f, ">")?;
    }
    Ok(())
}

impl TraitRef {
    fn hir_fmt_ext(&self, f: &mut HirFormatter, use_as: bool) -> Result<(), HirDisplayError> {
        if f.should_truncate() {
            return write!(f, "{}", TYPE_HINT_TRUNCATION);
        }

        self.substs[0].hir_fmt(f)?;
        if use_as {
            write!(f, " as ")?;
        } else {
            write!(f, ": ")?;
        }
        write!(f, "{}", f.db.trait_data(self.trait_).name)?;
        if self.substs.len() > 1 {
            write!(f, "<")?;
            f.write_joined(&self.substs[1..], ", ")?;
            write!(f, ">")?;
        }
        Ok(())
    }
}

impl HirDisplay for TraitRef {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        self.hir_fmt_ext(f, false)
    }
}

impl HirDisplay for &GenericPredicate {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        HirDisplay::hir_fmt(*self, f)
    }
}

impl HirDisplay for GenericPredicate {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        if f.should_truncate() {
            return write!(f, "{}", TYPE_HINT_TRUNCATION);
        }

        match self {
            GenericPredicate::Implemented(trait_ref) => trait_ref.hir_fmt(f)?,
            GenericPredicate::Projection(projection_pred) => {
                write!(f, "<")?;
                projection_pred.projection_ty.trait_ref(f.db).hir_fmt_ext(f, true)?;
                write!(
                    f,
                    ">::{} = ",
                    f.db.type_alias_data(projection_pred.projection_ty.associated_ty).name,
                )?;
                projection_pred.ty.hir_fmt(f)?;
            }
            GenericPredicate::Error => write!(f, "{{error}}")?,
        }
        Ok(())
    }
}

impl HirDisplay for Lifetime {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        match self {
            Lifetime::Parameter(id) => {
                let generics = generics(f.db.upcast(), id.parent);
                let param_data = &generics.params.lifetimes[id.local_id];
                write!(f, "{}", &param_data.name)
            }
            Lifetime::Static => write!(f, "'static"),
        }
    }
}

impl HirDisplay for Obligation {
    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
        match self {
            Obligation::Trait(tr) => {
                write!(f, "Implements(")?;
                tr.hir_fmt(f)?;
                write!(f, ")")
            }
            Obligation::Projection(proj) => {
                write!(f, "Normalize(")?;
                proj.projection_ty.hir_fmt(f)?;
                write!(f, " => ")?;
                proj.ty.hir_fmt(f)?;
                write!(f, ")")
            }
        }
    }
}