rustc_builtin_macros/deriving/

debug.rs

use rustc_ast::{self as ast, EnumDef, ExprKind, MetaItem, Safety, TyKind, token};
use rustc_expand::base::{Annotatable, ExtCtxt};
use rustc_session::config::FmtDebug;
use rustc_span::{Ident, Span, Symbol, sym};
use thin_vec::{ThinVec, thin_vec};

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

pub(crate) fn expand_deriving_debug(
    cx: &ExtCtxt<'_>,
    span: Span,
    mitem: &MetaItem,
    item: &Annotatable,
    push: &mut dyn FnMut(Annotatable),
    is_const: bool,
) {
    // &mut ::std::fmt::Formatter
    let fmtr = Ref(Box::new(Path(path_std!(fmt::Formatter))), ast::Mutability::Mut);

    let trait_def = TraitDef {
        span,
        path: path_std!(fmt::Debug),
        skip_path_as_bound: false,
        needs_copy_as_bound_if_packed: true,
        additional_bounds: Vec::new(),
        supports_unions: false,
        methods: vec![MethodDef {
            name: sym::fmt,
            generics: Bounds::empty(),
            explicit_self: true,
            nonself_args: vec![(fmtr, sym::character('f'))],
            ret_ty: Path(path_std!(fmt::Result)),
            attributes: thin_vec![cx.attr_word(sym::inline, span)],
            fieldless_variants_strategy:
                FieldlessVariantsStrategy::SpecializeIfAllVariantsFieldless,
            combine_substructure: combine_substructure(Box::new(|a, b, c| {
                show_substructure(a, b, c)
            })),
        }],
        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 show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
    // We want to make sure we have the ctxt set so that we can use unstable methods
    let span = cx.with_def_site_ctxt(span);

    let fmt_detail = cx.sess.opts.unstable_opts.fmt_debug;
    if fmt_detail == FmtDebug::None {
        return BlockOrExpr::new_expr(cx.expr_ok(span, cx.expr_tuple(span, ThinVec::new())));
    }

    let (ident, vdata, fields) = match substr.fields {
        Struct(vdata, fields) => (substr.type_ident, *vdata, fields),
        EnumMatching(v, fields) => (v.ident, &v.data, fields),
        AllFieldlessEnum(enum_def) => return show_fieldless_enum(cx, span, enum_def, substr),
        EnumDiscr(..) | StaticStruct(..) | StaticEnum(..) => {
            cx.dcx().span_bug(span, "nonsensical .fields in `#[derive(Debug)]`")
        }
    };

    let name = cx.expr_str(span, ident.name);
    let fmt = substr.nonselflike_args[0].clone();

    // Fieldless enums have been special-cased earlier
    if fmt_detail == FmtDebug::Shallow {
        let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
        let expr = cx.expr_call_global(span, fn_path_write_str, thin_vec![fmt, name]);
        return BlockOrExpr::new_expr(expr);
    }

    // Struct and tuples are similar enough that we use the same code for both,
    // with some extra pieces for structs due to the field names.
    let (is_struct, args_per_field) = match vdata {
        ast::VariantData::Unit(..) => {
            // Special fast path for unit variants.
            assert!(fields.is_empty());
            (false, 0)
        }
        ast::VariantData::Tuple(..) => (false, 1),
        ast::VariantData::Struct { .. } => (true, 2),
    };

    // The number of fields that can be handled without an array.
    const CUTOFF: usize = 5;

    fn expr_for_field(
        cx: &ExtCtxt<'_>,
        field: &FieldInfo,
        index: usize,
        len: usize,
    ) -> Box<ast::Expr> {
        if index < len - 1 {
            field.self_expr.clone()
        } else {
            // Unsized types need an extra indirection, but only the last field
            // may be unsized.
            cx.expr_addr_of(field.span, field.self_expr.clone())
        }
    }

    if fields.is_empty() {
        // Special case for no fields.
        let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
        let expr = cx.expr_call_global(span, fn_path_write_str, thin_vec![fmt, name]);
        BlockOrExpr::new_expr(expr)
    } else if fields.len() <= CUTOFF {
        // Few enough fields that we can use a specific-length method.
        let debug = if is_struct {
            format!("debug_struct_field{}_finish", fields.len())
        } else {
            format!("debug_tuple_field{}_finish", fields.len())
        };
        let fn_path_debug = cx.std_path(&[sym::fmt, sym::Formatter, Symbol::intern(&debug)]);

        let mut args = ThinVec::with_capacity(2 + fields.len() * args_per_field);
        args.extend([fmt, name]);
        for i in 0..fields.len() {
            let field = &fields[i];
            if is_struct {
                let name = cx.expr_str(field.span, field.name.unwrap().name);
                args.push(name);
            }

            let field = expr_for_field(cx, field, i, fields.len());
            args.push(field);
        }
        let expr = cx.expr_call_global(span, fn_path_debug, args);
        BlockOrExpr::new_expr(expr)
    } else {
        // Enough fields that we must use the any-length method.
        let mut name_exprs = ThinVec::with_capacity(fields.len());
        let mut value_exprs = ThinVec::with_capacity(fields.len());

        for i in 0..fields.len() {
            let field = &fields[i];
            if is_struct {
                name_exprs.push(cx.expr_str(field.span, field.name.unwrap().name));
            }

            let field = expr_for_field(cx, field, i, fields.len());
            value_exprs.push(field);
        }

        // `let names: &'static _ = &["field1", "field2"];`
        let names_let = is_struct.then(|| {
            let lt_static = Some(cx.lifetime_static(span));
            let ty_static_ref = cx.ty_ref(span, cx.ty_infer(span), lt_static, ast::Mutability::Not);
            cx.stmt_let_ty(
                span,
                false,
                Ident::new(sym::names, span),
                Some(ty_static_ref),
                cx.expr_array_ref(span, name_exprs),
            )
        });

        // `let values: &[&dyn Debug] = &[&&self.field1, &&self.field2];`
        let path_debug = cx.path_global(span, cx.std_path(&[sym::fmt, sym::Debug]));
        let ty_dyn_debug = cx.ty(
            span,
            ast::TyKind::TraitObject(
                thin_vec![cx.trait_bound(path_debug, false)],
                ast::TraitObjectSyntax::Dyn,
            ),
        );
        let ty_slice = cx.ty(
            span,
            ast::TyKind::Slice(cx.ty_ref(span, ty_dyn_debug, None, ast::Mutability::Not)),
        );
        let values_let = cx.stmt_let_ty(
            span,
            false,
            Ident::new(sym::values, span),
            Some(cx.ty_ref(span, ty_slice, None, ast::Mutability::Not)),
            cx.expr_array_ref(span, value_exprs),
        );

        // `fmt::Formatter::debug_struct_fields_finish(fmt, name, names, values)` or
        // `fmt::Formatter::debug_tuple_fields_finish(fmt, name, values)`
        let sym_debug = if is_struct {
            sym::debug_struct_fields_finish
        } else {
            sym::debug_tuple_fields_finish
        };
        let fn_path_debug_internal = cx.std_path(&[sym::fmt, sym::Formatter, sym_debug]);

        let mut args = ThinVec::with_capacity(4);
        args.push(fmt);
        args.push(name);
        if is_struct {
            args.push(cx.expr_ident(span, Ident::new(sym::names, span)));
        }
        args.push(cx.expr_ident(span, Ident::new(sym::values, span)));
        let expr = cx.expr_call_global(span, fn_path_debug_internal, args);

        let mut stmts = ThinVec::with_capacity(2);
        if is_struct {
            stmts.push(names_let.unwrap());
        }
        stmts.push(values_let);
        BlockOrExpr::new_mixed(stmts, Some(expr))
    }
}

/// Special case for enums with no fields. Builds:
/// ```text
/// impl ::core::fmt::Debug for A {
///     fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
///          ::core::fmt::Formatter::write_str(f,
///             match self {
///                 A::A => "A",
///                 A::B() => "B",
///                 A::C {} => "C",
///             })
///     }
/// }
/// ```
fn show_fieldless_enum(
    cx: &ExtCtxt<'_>,
    span: Span,
    def: &EnumDef,
    substr: &Substructure<'_>,
) -> BlockOrExpr {
    let fmt = substr.nonselflike_args[0].clone();
    if let Some((stmts, expr)) = show_fieldless_enum_concat_str(cx, span, def, fmt.clone()) {
        return BlockOrExpr::new_mixed(stmts, Some(expr));
    }
    let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
    let arms = def
        .variants
        .iter()
        .map(|v| {
            let variant_path = cx.path(span, vec![substr.type_ident, v.ident]);
            let pat = match &v.data {
                ast::VariantData::Tuple(fields, _) => {
                    debug_assert!(fields.is_empty());
                    cx.pat_tuple_struct(span, variant_path, ThinVec::new())
                }
                ast::VariantData::Struct { fields, .. } => {
                    debug_assert!(fields.is_empty());
                    cx.pat_struct(span, variant_path, ThinVec::new())
                }
                ast::VariantData::Unit(_) => cx.pat_path(span, variant_path),
            };
            cx.arm(span, pat, cx.expr_str(span, v.ident.name))
        })
        .collect::<ThinVec<_>>();
    let name = cx.expr_match(span, cx.expr_self(span), arms);
    BlockOrExpr::new_expr(cx.expr_call_global(span, fn_path_write_str, thin_vec![fmt, name]))
}

/// Special case for fieldless enums with no discriminants. Builds
/// ```text
/// impl ::core::fmt::Debug for A {
///     fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
///         static __NAMES: &str = "ABBBCC";
///         static __OFFSET: [usize; 4] =[0, 1, 4, 6];
///         let __d = ::core::intrinsics::discriminant_value(self) as usize;
///         ::core::fmt::Formatter::debug_c_like_enums_write_str(f, __NAMES, &__OFFSET, __d)
///     }
/// }
/// ```
fn show_fieldless_enum_concat_str(
    cx: &ExtCtxt<'_>,
    span: Span,
    def: &EnumDef,
    fmt: Box<ast::Expr>,
) -> Option<(ThinVec<ast::Stmt>, Box<ast::Expr>)> {
    // Minimum variants count where this optimization starts to pay off.
    // See https://github.com/rust-lang/rust/pull/155452 for more details.
    const THRESHOLD: usize = 10;
    let variants_count = def.variants.len();
    if variants_count < THRESHOLD {
        return None;
    }

    let variant_names = def
        .variants
        .iter()
        .map(|v| v.disr_expr.is_none().then_some(v.ident.name.as_str()))
        .collect::<Option<ThinVec<_>>>()?;

    let total_bytes: usize = variant_names.iter().map(|n| n.len()).sum();
    let mut concatenated_names = String::with_capacity(total_bytes);
    let mut offset_indices = Vec::with_capacity(variant_names.len() + 1);
    offset_indices.push(0);

    for name in variant_names.iter() {
        concatenated_names.push_str(name);
        offset_indices.push(concatenated_names.len());
    }

    // Create the constant concatenated string
    let names_ident = Ident::from_str_and_span("__NAMES", span);
    let str_ty = cx.ty(
        span,
        TyKind::Ref(
            None,
            ast::MutTy {
                ty: cx.ty(
                    span,
                    TyKind::Path(None, ast::Path::from_ident(Ident::new(sym::str, span))),
                ),
                mutbl: ast::Mutability::Not,
            },
        ),
    );
    let names_str_body = cx.expr_str(span, Symbol::intern(&concatenated_names));
    let names_static_item =
        cx.item_static(span, names_ident, str_ty, ast::Mutability::Not, names_str_body);

    // Create the constant offset array
    let offset_ident = Ident::from_str_and_span("__OFFSET", span);
    let offset_index_exprs =
        offset_indices.iter().map(|s| cx.expr_usize(span, *s)).collect::<ThinVec<_>>();
    let starts_array_body = cx.expr_array(span, offset_index_exprs);
    let usize_ty =
        cx.ty(span, TyKind::Path(None, ast::Path::from_ident(Ident::new(sym::usize, span))));
    let offset_array_len_expr = cx.anon_const(
        span,
        ExprKind::Lit(token::Lit::new(
            token::LitKind::Integer,
            Symbol::intern(&(variants_count + 1).to_string()),
            None,
        )),
    );
    let offset_static_item = cx.item_static(
        span,
        offset_ident,
        cx.ty(span, TyKind::Array(usize_ty, offset_array_len_expr)),
        ast::Mutability::Not,
        starts_array_body,
    );

    // let __d = ::core::intrinsics::discriminant_value(self) as usize;
    let discriminant_ident = Ident::from_str_and_span("__d", span);
    let discriminant_intrinsic_path = cx.std_path(&[sym::intrinsics, sym::discriminant_value]);
    let discriminant_cast_expr = cx.expr(
        span,
        ast::ExprKind::Cast(
            cx.expr_call_global(span, discriminant_intrinsic_path, thin_vec![cx.expr_self(span)]),
            cx.ty_path(ast::Path::from_ident(Ident::new(sym::usize, span))),
        ),
    );
    let discriminant_let_stmt =
        cx.stmt_let(span, false, discriminant_ident, discriminant_cast_expr);

    // __d expression
    let discriminant_expr = cx.expr_ident(span, discriminant_ident);

    // __NAMES expression
    let names_expr = cx.expr_ident(span, names_ident);

    // &__OFFSET expression
    let offset_ref_expr = cx.expr_addr_of(span, cx.expr_ident(span, offset_ident));

    // ::core::fmt::Formatter::debug_c_like_enum_write_str(f, __NAMES, &__OFFSET, __d)
    let fn_path = cx.std_path(&[sym::fmt, sym::Formatter, sym::debug_c_like_enum_write_str]);
    let call_expr = cx.expr_call_global(
        span,
        fn_path,
        thin_vec![fmt, names_expr, offset_ref_expr, discriminant_expr],
    );

    Some((
        thin_vec![
            cx.stmt_item(span, names_static_item),
            cx.stmt_item(span, offset_static_item),
            discriminant_let_stmt,
        ],
        call_expr,
    ))
}