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 ) {
  20     item.visit_with(&mut DetectNonVariantDefaultAttr { cx });
  21
  22     let inline = cx.meta_word(span, sym::inline);
  23     let attrs = thin_vec![cx.attribute(inline)];
  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         generics: Bounds::empty(),
  30         supports_unions: false,
  31         methods: vec![MethodDef {
  32             name: kw::Default,
  33             generics: Bounds::empty(),
  34             explicit_self: false,
  35             nonself_args: Vec::new(),
  36             ret_ty: Self_,
  37             attributes: attrs,
  38             unify_fieldless_variants: false,
  39             combine_substructure: combine_substructure(Box::new(|cx, trait_span, substr| {
  40                 match substr.fields {
  41                     StaticStruct(_, fields) => {
  42                         default_struct_substructure(cx, trait_span, substr, fields)
  43                     }
  44                     StaticEnum(enum_def, _) => default_enum_substructure(cx, trait_span, enum_def),
  45                     _ => cx.span_bug(trait_span, "method in `derive(Default)`"),
  46                 }
  47             })),
  48         }],
  49         associated_types: Vec::new(),
  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(ref fields, is_tuple) => {
  66             if !is_tuple {
  67                 cx.expr_ident(trait_span, substr.type_ident)
  68             } else {
  69                 let exprs = fields.iter().map(|sp| default_call(*sp)).collect();
  70                 cx.expr_call_ident(trait_span, substr.type_ident, exprs)
  71             }
  72         }
  73         Named(ref fields) => {
  74             let default_fields = fields
  75                 .iter()
  76                 .map(|&(ident, span)| cx.field_imm(span, ident, default_call(span)))
  77                 .collect();
  78             cx.expr_struct_ident(trait_span, substr.type_ident, default_fields)
  79         }
  80     };
  81     BlockOrExpr::new_expr(expr)
  82 }
  83
  84 fn default_enum_substructure(
  85     cx: &mut ExtCtxt<'_>,
  86     trait_span: Span,
  87     enum_def: &EnumDef,
  88 ) -> BlockOrExpr {
  89     let expr = if let Ok(default_variant) = extract_default_variant(cx, enum_def, trait_span)
  90         && let Ok(_) = validate_default_attribute(cx, default_variant)
  91     {
  92         // We now know there is exactly one unit variant with exactly one `#[default]` attribute.
  93         cx.expr_path(cx.path(
  94             default_variant.span,
  95             vec![Ident::new(kw::SelfUpper, default_variant.span), default_variant.ident],
  96         ))
  97     } else {
  98         DummyResult::raw_expr(trait_span, true)
  99     };
 100     BlockOrExpr::new_expr(expr)
 101 }
 102
 103 fn extract_default_variant<'a>(
 104     cx: &mut ExtCtxt<'_>,
 105     enum_def: &'a EnumDef,
 106     trait_span: Span,
 107 ) -> Result<&'a rustc_ast::Variant, ()> {
 108     let default_variants: SmallVec<[_; 1]> = enum_def
 109         .variants
 110         .iter()
 111         .filter(|variant| cx.sess.contains_name(&variant.attrs, kw::Default))
 112         .collect();
 113
 114     let variant = match default_variants.as_slice() {
 115         [variant] => variant,
 116         [] => {
 117             let possible_defaults = enum_def
 118                 .variants
 119                 .iter()
 120                 .filter(|variant| matches!(variant.data, VariantData::Unit(..)))
 121                 .filter(|variant| !cx.sess.contains_name(&variant.attrs, sym::non_exhaustive));
 122
 123             let mut diag = cx.struct_span_err(trait_span, "no default declared");
 124             diag.help("make a unit variant default by placing `#[default]` above it");
 125             for variant in possible_defaults {
 126                 // Suggest making each unit variant default.
 127                 diag.tool_only_span_suggestion(
 128                     variant.span,
 129                     &format!("make `{}` default", variant.ident),
 130                     format!("#[default] {}", variant.ident),
 131                     Applicability::MaybeIncorrect,
 132                 );
 133             }
 134             diag.emit();
 135
 136             return Err(());
 137         }
 138         [first, rest @ ..] => {
 139             let mut diag = cx.struct_span_err(trait_span, "multiple declared defaults");
 140             diag.span_label(first.span, "first default");
 141             diag.span_labels(rest.iter().map(|variant| variant.span), "additional default");
 142             diag.note("only one variant can be default");
 143             for variant in &default_variants {
 144                 // Suggest making each variant already tagged default.
 145                 let suggestion = default_variants
 146                     .iter()
 147                     .filter_map(|v| {
 148                         if v.ident == variant.ident {
 149                             None
 150                         } else {
 151                             Some((cx.sess.find_by_name(&v.attrs, kw::Default)?.span, String::new()))
 152                         }
 153                     })
 154                     .collect();
 155
 156                 diag.tool_only_multipart_suggestion(
 157                     &format!("make `{}` default", variant.ident),
 158                     suggestion,
 159                     Applicability::MaybeIncorrect,
 160                 );
 161             }
 162             diag.emit();
 163
 164             return Err(());
 165         }
 166     };
 167
 168     if !matches!(variant.data, VariantData::Unit(..)) {
 169         cx.struct_span_err(
 170             variant.ident.span,
 171             "the `#[default]` attribute may only be used on unit enum variants",
 172         )
 173         .help("consider a manual implementation of `Default`")
 174         .emit();
 175
 176         return Err(());
 177     }
 178
 179     if let Some(non_exhaustive_attr) = cx.sess.find_by_name(&variant.attrs, sym::non_exhaustive) {
 180         cx.struct_span_err(variant.ident.span, "default variant must be exhaustive")
 181             .span_label(non_exhaustive_attr.span, "declared `#[non_exhaustive]` here")
 182             .help("consider a manual implementation of `Default`")
 183             .emit();
 184
 185         return Err(());
 186     }
 187
 188     Ok(variant)
 189 }
 190
 191 fn validate_default_attribute(
 192     cx: &mut ExtCtxt<'_>,
 193     default_variant: &rustc_ast::Variant,
 194 ) -> Result<(), ()> {
 195     let attrs: SmallVec<[_; 1]> =
 196         cx.sess.filter_by_name(&default_variant.attrs, kw::Default).collect();
 197
 198     let attr = match attrs.as_slice() {
 199         [attr] => attr,
 200         [] => cx.bug(
 201             "this method must only be called with a variant that has a `#[default]` attribute",
 202         ),
 203         [first, rest @ ..] => {
 204             let suggestion_text =
 205                 if rest.len() == 1 { "try removing this" } else { "try removing these" };
 206
 207             cx.struct_span_err(default_variant.ident.span, "multiple `#[default]` attributes")
 208                 .note("only one `#[default]` attribute is needed")
 209                 .span_label(first.span, "`#[default]` used here")
 210                 .span_label(rest[0].span, "`#[default]` used again here")
 211                 .span_help(rest.iter().map(|attr| attr.span).collect::<Vec<_>>(), suggestion_text)
 212                 // This would otherwise display the empty replacement, hence the otherwise
 213                 // repetitive `.span_help` call above.
 214                 .tool_only_multipart_suggestion(
 215                     suggestion_text,
 216                     rest.iter().map(|attr| (attr.span, String::new())).collect(),
 217                     Applicability::MachineApplicable,
 218                 )
 219                 .emit();
 220
 221             return Err(());
 222         }
 223     };
 224     if !attr.is_word() {
 225         cx.struct_span_err(attr.span, "`#[default]` attribute does not accept a value")
 226             .span_suggestion_hidden(
 227                 attr.span,
 228                 "try using `#[default]`",
 229                 "#[default]",
 230                 Applicability::MaybeIncorrect,
 231             )
 232             .emit();
 233
 234         return Err(());
 235     }
 236     Ok(())
 237 }
 238
 239 struct DetectNonVariantDefaultAttr<'a, 'b> {
 240     cx: &'a ExtCtxt<'b>,
 241 }
 242
 243 impl<'a, 'b> rustc_ast::visit::Visitor<'a> for DetectNonVariantDefaultAttr<'a, 'b> {
 244     fn visit_attribute(&mut self, attr: &'a rustc_ast::Attribute) {
 245         if attr.has_name(kw::Default) {
 246             self.cx
 247                 .struct_span_err(
 248                     attr.span,
 249                     "the `#[default]` attribute may only be used on unit enum variants",
 250                 )
 251                 .emit();
 252         }
 253
 254         rustc_ast::visit::walk_attribute(self, attr);
 255     }
 256     fn visit_variant(&mut self, v: &'a rustc_ast::Variant) {
 257         self.visit_ident(v.ident);
 258         self.visit_vis(&v.vis);
 259         self.visit_variant_data(&v.data);
 260         walk_list!(self, visit_anon_const, &v.disr_expr);
 261         for attr in &v.attrs {
 262             rustc_ast::visit::walk_attribute(self, attr);
 263         }
 264     }
 265 }
 266
 267 fn has_a_default_variant(item: &Annotatable) -> bool {
 268     struct HasDefaultAttrOnVariant {
 269         found: bool,
 270     }
 271
 272     impl<'ast> rustc_ast::visit::Visitor<'ast> for HasDefaultAttrOnVariant {
 273         fn visit_variant(&mut self, v: &'ast rustc_ast::Variant) {
 274             if v.attrs.iter().any(|attr| attr.has_name(kw::Default)) {
 275                 self.found = true;
 276             }
 277             // no need to subrecurse.
 278         }
 279     }
 280
 281     let mut visitor = HasDefaultAttrOnVariant { found: false };
 282     item.visit_with(&mut visitor);
 283     visitor.found
 284 }