1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
use crate::deriving::generic::ty::*;
use crate::deriving::generic::*;
use crate::deriving::path_std;

use rustc_ast::{self as ast, MetaItem};
use rustc_expand::base::{Annotatable, ExtCtxt};
use rustc_span::symbol::{sym, Ident, Symbol};
use rustc_span::Span;

pub fn expand_deriving_debug(
    cx: &mut ExtCtxt<'_>,
    span: Span,
    mitem: &MetaItem,
    item: &Annotatable,
    push: &mut dyn FnMut(Annotatable),
) {
    // &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),
        additional_bounds: Vec::new(),
        generics: Bounds::empty(),
        supports_unions: false,
        methods: vec![MethodDef {
            name: sym::fmt,
            generics: Bounds::empty(),
            explicit_self: true,
            nonself_args: vec![(fmtr, sym::f)],
            ret_ty: Path(path_std!(fmt::Result)),
            attributes: ast::AttrVec::new(),
            unify_fieldless_variants: false,
            combine_substructure: combine_substructure(Box::new(|a, b, c| {
                show_substructure(a, b, c)
            })),
        }],
        associated_types: Vec::new(),
    };
    trait_def.expand(cx, mitem, item, push)
}

fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
    let (ident, vdata, fields) = match substr.fields {
        Struct(vdata, fields) => (substr.type_ident, *vdata, fields),
        EnumMatching(_, _, v, fields) => (v.ident, &v.data, fields),
        EnumTag(..) | StaticStruct(..) | StaticEnum(..) => {
            cx.span_bug(span, "nonsensical .fields in `#[derive(Debug)]`")
        }
    };

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

    // 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;

    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, 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 = Vec::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);
            }
            // Use an extra indirection to make sure this works for unsized types.
            let field = cx.expr_addr_of(field.span, field.self_expr.clone());
            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 = Vec::with_capacity(fields.len());
        let mut value_exprs = Vec::with_capacity(fields.len());

        for field in fields {
            if is_struct {
                name_exprs.push(cx.expr_str(field.span, field.name.unwrap().name));
            }

            // Use an extra indirection to make sure this works for unsized types.
            let field = cx.expr_addr_of(field.span, field.self_expr.clone());
            value_exprs.push(field);
        }

        // `let names: &'static _ = &["field1", "field2"];`
        let names_let = if is_struct {
            let lt_static = Some(cx.lifetime_static(span));
            let ty_static_ref =
                cx.ty_rptr(span, cx.ty_infer(span), lt_static, ast::Mutability::Not);
            Some(cx.stmt_let_ty(
                span,
                false,
                Ident::new(sym::names, span),
                Some(ty_static_ref),
                cx.expr_array_ref(span, name_exprs),
            ))
        } else {
            None
        };

        // `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(vec![cx.trait_bound(path_debug)], ast::TraitObjectSyntax::Dyn),
        );
        let ty_slice = cx.ty(
            span,
            ast::TyKind::Slice(cx.ty_rptr(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_rptr(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 = Vec::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 = Vec::with_capacity(3);
        if is_struct {
            stmts.push(names_let.unwrap());
        }
        stmts.push(values_let);
        BlockOrExpr::new_mixed(stmts, Some(expr))
    }
}