compiler/rustc_builtin_macros/src/deriving/default.rs

   1 use crate::deriving::generic::ty::*;
   2 use crate::deriving::generic::*;
   3 use rustc_ast as ast;
   4 use rustc_ast::{walk_list, EnumDef, VariantData};
   5 use rustc_errors::Applicability;
   6 use rustc_expand::base::{Annotatable, DummyResult, ExtCtxt};
   7 use rustc_span::symbol::Ident;
   8 use rustc_span::symbol::{kw, sym};
   9 use rustc_span::Span;
  10 use smallvec::SmallVec;
  11 use thin_vec::thin_vec;
  12
  13 pub fn expand_deriving_default(
  14     cx: &mut ExtCtxt<'_>,
  15     span: Span,
  16     mitem: &ast::MetaItem,
  17     item: &Annotatable,
  18     push: &mut dyn FnMut(Annotatable),
  19     is_const: bool,
  20 ) {
  21     item.visit_with(&mut DetectNonVariantDefaultAttr { cx });
  22
  23     let attrs = thin_vec![cx.attr_word(sym::inline, span)];
  24     let trait_def = TraitDef {
  25         span,
  26         path: Path::new(vec![kw::Default, sym::Default]),
  27         skip_path_as_bound: has_a_default_variant(item),
  28         additional_bounds: Vec::new(),
  29         supports_unions: false,
  30         methods: vec![MethodDef {
  31             name: kw::Default,
  32             generics: Bounds::empty(),
  33             explicit_self: false,
  34             nonself_args: Vec::new(),
  35             ret_ty: Self_,
  36             attributes: attrs,
  37             fieldless_variants_strategy: FieldlessVariantsStrategy::Default,
  38             combine_substructure: combine_substructure(Box::new(|cx, trait_span, substr| {
  39                 match substr.fields {
  40                     StaticStruct(_, fields) => {
  41                         default_struct_substructure(cx, trait_span, substr, fields)
  42                     }
  43                     StaticEnum(enum_def, _) => default_enum_substructure(cx, trait_span, enum_def),
  44                     _ => cx.span_bug(trait_span, "method in `derive(Default)`"),
  45                 }
  46             })),
  47         }],
  48         associated_types: Vec::new(),
  49         is_const,
  50     };
  51     trait_def.expand(cx, mitem, item, push)
  52 }
  53
  54 fn default_struct_substructure(
  55     cx: &mut ExtCtxt<'_>,
  56     trait_span: Span,
  57     substr: &Substructure<'_>,
  58     summary: &StaticFields,
  59 ) -> BlockOrExpr {
  60     // Note that `kw::Default` is "default" and `sym::Default` is "Default"!
  61     let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
  62     let default_call = |span| cx.expr_call_global(span, default_ident.clone(), Vec::new());
  63
  64     let expr = match summary {
  65         Unnamed(_, false) => cx.expr_ident(trait_span, substr.type_ident),
  66         Unnamed(fields, true) => {
  67             let exprs = fields.iter().map(|sp| default_call(*sp)).collect();
  68             cx.expr_call_ident(trait_span, substr.type_ident, exprs)
  69         }
  70         Named(fields) => {
  71             let default_fields = fields
  72                 .iter()
  73                 .map(|&(ident, span)| cx.field_imm(span, ident, default_call(span)))
  74                 .collect();
  75             cx.expr_struct_ident(trait_span, substr.type_ident, default_fields)
  76         }
  77     };
  78     BlockOrExpr::new_expr(expr)
  79 }
  80
  81 fn default_enum_substructure(
  82     cx: &mut ExtCtxt<'_>,
  83     trait_span: Span,
  84     enum_def: &EnumDef,
  85 ) -> BlockOrExpr {
  86     let expr = if let Ok(default_variant) = extract_default_variant(cx, enum_def, trait_span)
  87         && let Ok(_) = validate_default_attribute(cx, default_variant)
  88     {
  89         // We now know there is exactly one unit variant with exactly one `#[default]` attribute.
  90         cx.expr_path(cx.path(
  91             default_variant.span,
  92             vec![Ident::new(kw::SelfUpper, default_variant.span), default_variant.ident],
  93         ))
  94     } else {
  95         DummyResult::raw_expr(trait_span, true)
  96     };
  97     BlockOrExpr::new_expr(expr)
  98 }
  99
 100 fn extract_default_variant<'a>(
 101     cx: &mut ExtCtxt<'_>,
 102     enum_def: &'a EnumDef,
 103     trait_span: Span,
 104 ) -> Result<&'a rustc_ast::Variant, ()> {
 105     let default_variants: SmallVec<[_; 1]> = enum_def
 106         .variants
 107         .iter()
 108         .filter(|variant| cx.sess.contains_name(&variant.attrs, kw::Default))
 109         .collect();
 110
 111     let variant = match default_variants.as_slice() {
 112         [variant] => variant,
 113         [] => {
 114             let possible_defaults = enum_def
 115                 .variants
 116                 .iter()
 117                 .filter(|variant| matches!(variant.data, VariantData::Unit(..)))
 118                 .filter(|variant| !cx.sess.contains_name(&variant.attrs, sym::non_exhaustive));
 119
 120             let mut diag = cx.struct_span_err(trait_span, "no default declared");
 121             diag.help("make a unit variant default by placing `#[default]` above it");
 122             for variant in possible_defaults {
 123                 // Suggest making each unit variant default.
 124                 diag.tool_only_span_suggestion(
 125                     variant.span,
 126                     &format!("make `{}` default", variant.ident),
 127                     format!("#[default] {}", variant.ident),
 128                     Applicability::MaybeIncorrect,
 129                 );
 130             }
 131             diag.emit();
 132
 133             return Err(());
 134         }
 135         [first, rest @ ..] => {
 136             let mut diag = cx.struct_span_err(trait_span, "multiple declared defaults");
 137             diag.span_label(first.span, "first default");
 138             diag.span_labels(rest.iter().map(|variant| variant.span), "additional default");
 139             diag.note("only one variant can be default");
 140             for variant in &default_variants {
 141                 // Suggest making each variant already tagged default.
 142                 let suggestion = default_variants
 143                     .iter()
 144                     .filter_map(|v| {
 145                         if v.span == variant.span {
 146                             None
 147                         } else {
 148                             Some((cx.sess.find_by_name(&v.attrs, kw::Default)?.span, String::new()))
 149                         }
 150                     })
 151                     .collect();
 152
 153                 diag.tool_only_multipart_suggestion(
 154                     &format!("make `{}` default", variant.ident),
 155                     suggestion,
 156                     Applicability::MaybeIncorrect,
 157                 );
 158             }
 159             diag.emit();
 160
 161             return Err(());
 162         }
 163     };
 164
 165     if !matches!(variant.data, VariantData::Unit(..)) {
 166         cx.struct_span_err(
 167             variant.ident.span,
 168             "the `#[default]` attribute may only be used on unit enum variants",
 169         )
 170         .help("consider a manual implementation of `Default`")
 171         .emit();
 172
 173         return Err(());
 174     }
 175
 176     if let Some(non_exhaustive_attr) = cx.sess.find_by_name(&variant.attrs, sym::non_exhaustive) {
 177         cx.struct_span_err(variant.ident.span, "default variant must be exhaustive")
 178             .span_label(non_exhaustive_attr.span, "declared `#[non_exhaustive]` here")
 179             .help("consider a manual implementation of `Default`")
 180             .emit();
 181
 182         return Err(());
 183     }
 184
 185     Ok(variant)
 186 }
 187
 188 fn validate_default_attribute(
 189     cx: &mut ExtCtxt<'_>,
 190     default_variant: &rustc_ast::Variant,
 191 ) -> Result<(), ()> {
 192     let attrs: SmallVec<[_; 1]> =
 193         cx.sess.filter_by_name(&default_variant.attrs, kw::Default).collect();
 194
 195     let attr = match attrs.as_slice() {
 196         [attr] => attr,
 197         [] => cx.bug(
 198             "this method must only be called with a variant that has a `#[default]` attribute",
 199         ),
 200         [first, rest @ ..] => {
 201             let suggestion_text =
 202                 if rest.len() == 1 { "try removing this" } else { "try removing these" };
 203
 204             cx.struct_span_err(default_variant.ident.span, "multiple `#[default]` attributes")
 205                 .note("only one `#[default]` attribute is needed")
 206                 .span_label(first.span, "`#[default]` used here")
 207                 .span_label(rest[0].span, "`#[default]` used again here")
 208                 .span_help(rest.iter().map(|attr| attr.span).collect::<Vec<_>>(), suggestion_text)
 209                 // This would otherwise display the empty replacement, hence the otherwise
 210                 // repetitive `.span_help` call above.
 211                 .tool_only_multipart_suggestion(
 212                     suggestion_text,
 213                     rest.iter().map(|attr| (attr.span, String::new())).collect(),
 214                     Applicability::MachineApplicable,
 215                 )
 216                 .emit();
 217
 218             return Err(());
 219         }
 220     };
 221     if !attr.is_word() {
 222         cx.struct_span_err(attr.span, "`#[default]` attribute does not accept a value")
 223             .span_suggestion_hidden(
 224                 attr.span,
 225                 "try using `#[default]`",
 226                 "#[default]",
 227                 Applicability::MaybeIncorrect,
 228             )
 229             .emit();
 230
 231         return Err(());
 232     }
 233     Ok(())
 234 }
 235
 236 struct DetectNonVariantDefaultAttr<'a, 'b> {
 237     cx: &'a ExtCtxt<'b>,
 238 }
 239
 240 impl<'a, 'b> rustc_ast::visit::Visitor<'a> for DetectNonVariantDefaultAttr<'a, 'b> {
 241     fn visit_attribute(&mut self, attr: &'a rustc_ast::Attribute) {
 242         if attr.has_name(kw::Default) {
 243             self.cx
 244                 .struct_span_err(
 245                     attr.span,
 246                     "the `#[default]` attribute may only be used on unit enum variants",
 247                 )
 248                 .emit();
 249         }
 250
 251         rustc_ast::visit::walk_attribute(self, attr);
 252     }
 253     fn visit_variant(&mut self, v: &'a rustc_ast::Variant) {
 254         self.visit_ident(v.ident);
 255         self.visit_vis(&v.vis);
 256         self.visit_variant_data(&v.data);
 257         walk_list!(self, visit_anon_const, &v.disr_expr);
 258         for attr in &v.attrs {
 259             rustc_ast::visit::walk_attribute(self, attr);
 260         }
 261     }
 262 }
 263
 264 fn has_a_default_variant(item: &Annotatable) -> bool {
 265     struct HasDefaultAttrOnVariant {
 266         found: bool,
 267     }
 268
 269     impl<'ast> rustc_ast::visit::Visitor<'ast> for HasDefaultAttrOnVariant {
 270         fn visit_variant(&mut self, v: &'ast rustc_ast::Variant) {
 271             if v.attrs.iter().any(|attr| attr.has_name(kw::Default)) {
 272                 self.found = true;
 273             }
 274             // no need to subrecurse.
 275         }
 276     }
 277
 278     let mut visitor = HasDefaultAttrOnVariant { found: false };
 279     item.visit_with(&mut visitor);
 280     visitor.found
 281 }