compiler/rustc_builtin_macros/src/deriving/default.rs

use core::ops::ControlFlow;

use rustc_ast::visit::visit_opt;
use rustc_ast::{self as ast, EnumDef, Safety, VariantData, attr};
use rustc_expand::base::{Annotatable, DummyResult, ExtCtxt};
use rustc_span::{ErrorGuaranteed, Ident, Span, kw, sym};
use smallvec::SmallVec;
use thin_vec::{ThinVec, thin_vec};

use crate::deriving::generic::ty::*;
use crate::deriving::generic::*;
use crate::diagnostics;

pub(crate) fn expand_deriving_default(
    cx: &ExtCtxt<'_>,
    span: Span,
    mitem: &ast::MetaItem,
    item: &Annotatable,
    push: &mut dyn FnMut(Annotatable),
    is_const: bool,
) {
    item.visit_with(&mut DetectNonVariantDefaultAttr { cx });

    let trait_def = TraitDef {
        span,
        path: Path::new(vec![kw::Default, sym::Default]),
        skip_path_as_bound: has_a_default_variant(item),
        needs_copy_as_bound_if_packed: false,
        additional_bounds: Vec::new(),
        supports_unions: false,
        methods: vec![MethodDef {
            name: kw::Default,
            generics: Bounds::empty(),
            explicit_self: false,
            nonself_args: Vec::new(),
            ret_ty: Self_,
            attributes: thin_vec![cx.attr_word(sym::inline, span)],
            fieldless_variants_strategy: FieldlessVariantsStrategy::Default,
            combine_substructure: combine_substructure(Box::new(|cx, trait_span, substr| {
                match substr.fields {
                    StaticStruct(_, fields) => {
                        default_struct_substructure(cx, trait_span, substr, fields)
                    }
                    StaticEnum(enum_def) => {
                        default_enum_substructure(cx, trait_span, enum_def, item.span())
                    }
                    _ => cx.dcx().span_bug(trait_span, "method in `derive(Default)`"),
                }
            })),
        }],
        associated_types: Vec::new(),
        is_const,
        is_staged_api_crate: cx.ecfg.features.staged_api(),
        safety: Safety::Default,
        document: true,
    };
    trait_def.expand(cx, mitem, item, push)
}

fn default_call(cx: &ExtCtxt<'_>, span: Span) -> Box<ast::Expr> {
    // Note that `kw::Default` is "default" and `sym::Default` is "Default"!
    let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
    cx.expr_call_global(span, default_ident, ThinVec::new())
}

fn default_struct_substructure(
    cx: &ExtCtxt<'_>,
    trait_span: Span,
    substr: &Substructure<'_>,
    summary: &StaticFields<'_>,
) -> BlockOrExpr {
    let expr = match summary {
        Unnamed(_, IsTuple::No) => cx.expr_ident(trait_span, substr.type_ident),
        Unnamed(fields, IsTuple::Yes) => {
            let exprs = fields.iter().map(|sp| default_call(cx, *sp)).collect();
            cx.expr_call_ident(trait_span, substr.type_ident, exprs)
        }
        Named(fields) => {
            let default_fields = fields
                .iter()
                .map(|&(ident, span, default_val)| {
                    let value = match default_val {
                        // We use `Default::default()`.
                        None => default_call(cx, span),
                        // We use the field default const expression.
                        Some(val) => {
                            cx.expr(val.value.span, ast::ExprKind::ConstBlock(val.clone()))
                        }
                    };
                    cx.field_imm(span, ident, value)
                })
                .collect();
            cx.expr_struct_ident(trait_span, substr.type_ident, default_fields)
        }
    };
    BlockOrExpr::new_expr(expr)
}

fn default_enum_substructure(
    cx: &ExtCtxt<'_>,
    trait_span: Span,
    enum_def: &EnumDef,
    item_span: Span,
) -> BlockOrExpr {
    let expr = match try {
        let default_variant = extract_default_variant(cx, enum_def, trait_span, item_span)?;
        validate_default_attribute(cx, default_variant)?;
        default_variant
    } {
        Ok(default_variant) => {
            // We now know there is exactly one unit variant with exactly one `#[default]` attribute.
            match &default_variant.data {
                VariantData::Unit(_) => cx.expr_path(cx.path(
                    default_variant.span,
                    vec![Ident::new(kw::SelfUpper, default_variant.span), default_variant.ident],
                )),
                VariantData::Struct { fields, .. } => {
                    // This only happens if `#![feature(default_field_values)]`. We have validated
                    // all fields have default values in the definition.
                    let default_fields = fields
                        .iter()
                        .map(|field| {
                            cx.field_imm(
                                field.span,
                                field.ident.unwrap(),
                                match &field.default {
                                    // We use `Default::default()`.
                                    None => default_call(cx, field.span),
                                    // We use the field default const expression.
                                    Some(val) => cx.expr(
                                        val.value.span,
                                        ast::ExprKind::ConstBlock(val.clone()),
                                    ),
                                },
                            )
                        })
                        .collect();
                    let path = cx.path(
                        default_variant.span,
                        vec![
                            Ident::new(kw::SelfUpper, default_variant.span),
                            default_variant.ident,
                        ],
                    );
                    cx.expr_struct(default_variant.span, path, default_fields)
                }
                // Logic error in `extract_default_variant`.
                VariantData::Tuple(..) => {
                    cx.dcx().bug("encountered tuple variant annotated with `#[default]`")
                }
            }
        }
        Err(guar) => DummyResult::raw_expr(trait_span, Some(guar)),
    };
    BlockOrExpr::new_expr(expr)
}

fn extract_default_variant<'a>(
    cx: &ExtCtxt<'_>,
    enum_def: &'a EnumDef,
    trait_span: Span,
    item_span: Span,
) -> Result<&'a rustc_ast::Variant, ErrorGuaranteed> {
    let default_variants: SmallVec<[_; 1]> = enum_def
        .variants
        .iter()
        .filter(|variant| attr::contains_name(&variant.attrs, kw::Default))
        .collect();

    let variant = match default_variants.as_slice() {
        [variant] => variant,
        [] => {
            let possible_defaults = enum_def
                .variants
                .iter()
                .filter(|variant| matches!(variant.data, VariantData::Unit(..)))
                .filter(|variant| !attr::contains_name(&variant.attrs, sym::non_exhaustive));

            let suggs = possible_defaults
                .map(|v| diagnostics::NoDefaultVariantSugg { span: v.span.shrink_to_lo() })
                .collect();
            let guar = cx.dcx().emit_err(diagnostics::NoDefaultVariant {
                span: trait_span,
                item_span,
                suggs,
            });

            return Err(guar);
        }
        [first, rest @ ..] => {
            let suggs = default_variants
                .iter()
                .filter_map(|variant| {
                    let keep = attr::find_by_name(&variant.attrs, kw::Default)?.span;
                    let spans: Vec<Span> = default_variants
                        .iter()
                        .flat_map(|v| {
                            attr::filter_by_name(&v.attrs, kw::Default)
                                .filter_map(|attr| (attr.span != keep).then_some(attr.span))
                        })
                        .collect();
                    (!spans.is_empty()).then_some(diagnostics::MultipleDefaultsSugg {
                        spans,
                        ident: variant.ident,
                    })
                })
                .collect();
            let guar = cx.dcx().emit_err(diagnostics::MultipleDefaults {
                span: trait_span,
                first: first.span,
                additional: rest.iter().map(|v| v.span).collect(),
                suggs,
            });
            return Err(guar);
        }
    };

    if cx.ecfg.features.default_field_values()
        && let VariantData::Struct { fields, .. } = &variant.data
        && fields.iter().all(|f| f.default.is_some())
        // Disallow `#[default] Variant {}`
        && !fields.is_empty()
    {
        // Allowed
    } else if !matches!(variant.data, VariantData::Unit(..)) {
        let post = if cx.ecfg.features.default_field_values() {
            " or variants where every field has a default value"
        } else {
            ""
        };
        let guar =
            cx.dcx().emit_err(diagnostics::NonUnitDefault { span: variant.ident.span, post });
        return Err(guar);
    }

    if let Some(non_exhaustive_attr) = attr::find_by_name(&variant.attrs, sym::non_exhaustive) {
        let guar = cx.dcx().emit_err(diagnostics::NonExhaustiveDefault {
            span: variant.ident.span,
            non_exhaustive: non_exhaustive_attr.span,
        });

        return Err(guar);
    }

    Ok(variant)
}

fn validate_default_attribute(
    cx: &ExtCtxt<'_>,
    default_variant: &rustc_ast::Variant,
) -> Result<(), ErrorGuaranteed> {
    let attrs: SmallVec<[_; 1]> =
        attr::filter_by_name(&default_variant.attrs, kw::Default).collect();

    let attr = match attrs.as_slice() {
        [attr] => attr,
        [] => cx.dcx().bug(
            "this method must only be called with a variant that has a `#[default]` attribute",
        ),
        [first, rest @ ..] => {
            let sugg = diagnostics::MultipleDefaultAttrsSugg {
                spans: rest.iter().map(|attr| attr.span).collect(),
            };
            let guar = cx.dcx().emit_err(diagnostics::MultipleDefaultAttrs {
                span: default_variant.ident.span,
                first: first.span,
                first_rest: rest[0].span,
                rest: rest.iter().map(|attr| attr.span).collect::<Vec<_>>().into(),
                only_one: rest.len() == 1,
                sugg,
            });

            return Err(guar);
        }
    };
    if !attr.is_word() {
        let guar = cx.dcx().emit_err(diagnostics::DefaultHasArg { span: attr.span });

        return Err(guar);
    }
    Ok(())
}

struct DetectNonVariantDefaultAttr<'a, 'b> {
    cx: &'a ExtCtxt<'b>,
}

impl<'a, 'b> rustc_ast::visit::Visitor<'a> for DetectNonVariantDefaultAttr<'a, 'b> {
    fn visit_attribute(&mut self, attr: &'a rustc_ast::Attribute) {
        if attr.has_name(kw::Default) {
            let post = if self.cx.ecfg.features.default_field_values() {
                " or variants where every field has a default value"
            } else {
                ""
            };
            self.cx.dcx().emit_err(diagnostics::NonUnitDefault { span: attr.span, post });
        }

        rustc_ast::visit::walk_attribute(self, attr);
    }
    fn visit_variant(&mut self, v: &'a rustc_ast::Variant) {
        self.visit_ident(&v.ident);
        self.visit_vis(&v.vis);
        self.visit_variant_data(&v.data);
        visit_opt!(self, visit_anon_const, &v.disr_expr);
        for attr in &v.attrs {
            rustc_ast::visit::walk_attribute(self, attr);
        }
    }
}

fn has_a_default_variant(item: &Annotatable) -> bool {
    struct HasDefaultAttrOnVariant;

    impl<'ast> rustc_ast::visit::Visitor<'ast> for HasDefaultAttrOnVariant {
        type Result = ControlFlow<()>;
        fn visit_variant(&mut self, v: &'ast rustc_ast::Variant) -> ControlFlow<()> {
            if v.attrs.iter().any(|attr| attr.has_name(kw::Default)) {
                ControlFlow::Break(())
            } else {
                // no need to walk the variant, we are only looking for top level variants
                ControlFlow::Continue(())
            }
        }
    }

    item.visit_with(&mut HasDefaultAttrOnVariant).is_break()
}