compiler/rustc_passes/src/entry.rs

   1 use rustc_ast::entry::EntryPointType;
   2 use rustc_errors::struct_span_err;
   3 use rustc_hir::def_id::{DefId, CRATE_DEF_INDEX, LOCAL_CRATE};
   4 use rustc_hir::itemlikevisit::ItemLikeVisitor;
   5 use rustc_hir::{ForeignItem, HirId, ImplItem, Item, ItemKind, Node, TraitItem, CRATE_HIR_ID};
   6 use rustc_middle::hir::map::Map;
   7 use rustc_middle::ty::query::Providers;
   8 use rustc_middle::ty::TyCtxt;
   9 use rustc_session::config::{CrateType, EntryFnType};
  10 use rustc_session::parse::feature_err;
  11 use rustc_session::Session;
  12 use rustc_span::symbol::sym;
  13 use rustc_span::{Span, DUMMY_SP};
  14
  15 struct EntryContext<'a, 'tcx> {
  16     session: &'a Session,
  17
  18     map: Map<'tcx>,
  19
  20     /// The function that has attribute named `main`.
  21     attr_main_fn: Option<(HirId, Span)>,
  22
  23     /// The function that has the attribute 'start' on it.
  24     start_fn: Option<(HirId, Span)>,
  25
  26     /// The functions that one might think are `main` but aren't, e.g.
  27     /// main functions not defined at the top level. For diagnostics.
  28     non_main_fns: Vec<(HirId, Span)>,
  29 }
  30
  31 impl<'a, 'tcx> ItemLikeVisitor<'tcx> for EntryContext<'a, 'tcx> {
  32     fn visit_item(&mut self, item: &'tcx Item<'tcx>) {
  33         let def_key = self.map.def_key(item.def_id);
  34         let at_root = def_key.parent == Some(CRATE_DEF_INDEX);
  35         find_item(item, self, at_root);
  36     }
  37
  38     fn visit_trait_item(&mut self, _trait_item: &'tcx TraitItem<'tcx>) {
  39         // Entry fn is never a trait item.
  40     }
  41
  42     fn visit_impl_item(&mut self, _impl_item: &'tcx ImplItem<'tcx>) {
  43         // Entry fn is never a trait item.
  44     }
  45
  46     fn visit_foreign_item(&mut self, _: &'tcx ForeignItem<'tcx>) {
  47         // Entry fn is never a foreign item.
  48     }
  49 }
  50
  51 fn entry_fn(tcx: TyCtxt<'_>, (): ()) -> Option<(DefId, EntryFnType)> {
  52     let any_exe = tcx.sess.crate_types().iter().any(|ty| *ty == CrateType::Executable);
  53     if !any_exe {
  54         // No need to find a main function.
  55         return None;
  56     }
  57
  58     // If the user wants no main function at all, then stop here.
  59     if tcx.sess.contains_name(&tcx.hir().attrs(CRATE_HIR_ID), sym::no_main) {
  60         return None;
  61     }
  62
  63     let mut ctxt = EntryContext {
  64         session: tcx.sess,
  65         map: tcx.hir(),
  66         attr_main_fn: None,
  67         start_fn: None,
  68         non_main_fns: Vec::new(),
  69     };
  70
  71     tcx.hir().krate().visit_all_item_likes(&mut ctxt);
  72
  73     configure_main(tcx, &ctxt)
  74 }
  75
  76 // Beware, this is duplicated in `librustc_builtin_macros/test_harness.rs`
  77 // (with `ast::Item`), so make sure to keep them in sync.
  78 fn entry_point_type(ctxt: &EntryContext<'_, '_>, item: &Item<'_>, at_root: bool) -> EntryPointType {
  79     let attrs = ctxt.map.attrs(item.hir_id());
  80     if ctxt.session.contains_name(attrs, sym::start) {
  81         EntryPointType::Start
  82     } else if ctxt.session.contains_name(attrs, sym::rustc_main) {
  83         EntryPointType::MainAttr
  84     } else if item.ident.name == sym::main {
  85         if at_root {
  86             // This is a top-level function so can be `main`.
  87             EntryPointType::MainNamed
  88         } else {
  89             EntryPointType::OtherMain
  90         }
  91     } else {
  92         EntryPointType::None
  93     }
  94 }
  95
  96 fn throw_attr_err(sess: &Session, span: Span, attr: &str) {
  97     sess.struct_span_err(span, &format!("`{}` attribute can only be used on functions", attr))
  98         .emit();
  99 }
 100
 101 fn find_item(item: &Item<'_>, ctxt: &mut EntryContext<'_, '_>, at_root: bool) {
 102     match entry_point_type(ctxt, item, at_root) {
 103         EntryPointType::None => (),
 104         _ if !matches!(item.kind, ItemKind::Fn(..)) => {
 105             let attrs = ctxt.map.attrs(item.hir_id());
 106             if let Some(attr) = ctxt.session.find_by_name(attrs, sym::start) {
 107                 throw_attr_err(&ctxt.session, attr.span, "start");
 108             }
 109             if let Some(attr) = ctxt.session.find_by_name(attrs, sym::rustc_main) {
 110                 throw_attr_err(&ctxt.session, attr.span, "rustc_main");
 111             }
 112         }
 113         EntryPointType::MainNamed => (),
 114         EntryPointType::OtherMain => {
 115             ctxt.non_main_fns.push((item.hir_id(), item.span));
 116         }
 117         EntryPointType::MainAttr => {
 118             if ctxt.attr_main_fn.is_none() {
 119                 ctxt.attr_main_fn = Some((item.hir_id(), item.span));
 120             } else {
 121                 struct_span_err!(
 122                     ctxt.session,
 123                     item.span,
 124                     E0137,
 125                     "multiple functions with a `#[main]` attribute"
 126                 )
 127                 .span_label(item.span, "additional `#[main]` function")
 128                 .span_label(ctxt.attr_main_fn.unwrap().1, "first `#[main]` function")
 129                 .emit();
 130             }
 131         }
 132         EntryPointType::Start => {
 133             if ctxt.start_fn.is_none() {
 134                 ctxt.start_fn = Some((item.hir_id(), item.span));
 135             } else {
 136                 struct_span_err!(ctxt.session, item.span, E0138, "multiple `start` functions")
 137                     .span_label(ctxt.start_fn.unwrap().1, "previous `#[start]` function here")
 138                     .span_label(item.span, "multiple `start` functions")
 139                     .emit();
 140             }
 141         }
 142     }
 143 }
 144
 145 fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_, '_>) -> Option<(DefId, EntryFnType)> {
 146     if let Some((hir_id, _)) = visitor.start_fn {
 147         Some((tcx.hir().local_def_id(hir_id).to_def_id(), EntryFnType::Start))
 148     } else if let Some((hir_id, _)) = visitor.attr_main_fn {
 149         Some((tcx.hir().local_def_id(hir_id).to_def_id(), EntryFnType::Main))
 150     } else {
 151         if let Some(main_def) = tcx.resolutions(()).main_def {
 152             if let Some(def_id) = main_def.opt_fn_def_id() {
 153                 // non-local main imports are handled below
 154                 if def_id.is_local() {
 155                     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
 156                     if matches!(tcx.hir().find(hir_id), Some(Node::ForeignItem(_))) {
 157                         tcx.sess
 158                             .struct_span_err(
 159                                 tcx.hir().span(hir_id),
 160                                 "the `main` function cannot be declared in an `extern` block",
 161                             )
 162                             .emit();
 163                         return None;
 164                     }
 165                 }
 166
 167                 if main_def.is_import && !tcx.features().imported_main {
 168                     let span = main_def.span;
 169                     feature_err(
 170                         &tcx.sess.parse_sess,
 171                         sym::imported_main,
 172                         span,
 173                         "using an imported function as entry point `main` is experimental",
 174                     )
 175                     .emit();
 176                 }
 177                 return Some((def_id, EntryFnType::Main));
 178             }
 179         }
 180         no_main_err(tcx, visitor);
 181         None
 182     }
 183 }
 184
 185 fn no_main_err(tcx: TyCtxt<'_>, visitor: &EntryContext<'_, '_>) {
 186     let sp = tcx.hir().krate().item.inner;
 187     if *tcx.sess.parse_sess.reached_eof.borrow() {
 188         // There's an unclosed brace that made the parser reach `Eof`, we shouldn't complain about
 189         // the missing `fn main()` then as it might have been hidden inside an unclosed block.
 190         tcx.sess.delay_span_bug(sp, "`main` not found, but expected unclosed brace error");
 191         return;
 192     }
 193
 194     // There is no main function.
 195     let mut err = struct_span_err!(
 196         tcx.sess,
 197         DUMMY_SP,
 198         E0601,
 199         "`main` function not found in crate `{}`",
 200         tcx.crate_name(LOCAL_CRATE)
 201     );
 202     let filename = &tcx.sess.local_crate_source_file;
 203     let note = if !visitor.non_main_fns.is_empty() {
 204         for &(_, span) in &visitor.non_main_fns {
 205             err.span_note(span, "here is a function named `main`");
 206         }
 207         err.note("you have one or more functions named `main` not defined at the crate level");
 208         err.help("consider moving the `main` function definitions");
 209         // There were some functions named `main` though. Try to give the user a hint.
 210         format!(
 211             "the main function must be defined at the crate level{}",
 212             filename.as_ref().map(|f| format!(" (in `{}`)", f.display())).unwrap_or_default()
 213         )
 214     } else if let Some(filename) = filename {
 215         format!("consider adding a `main` function to `{}`", filename.display())
 216     } else {
 217         String::from("consider adding a `main` function at the crate level")
 218     };
 219     // The file may be empty, which leads to the diagnostic machinery not emitting this
 220     // note. This is a relatively simple way to detect that case and emit a span-less
 221     // note instead.
 222     if tcx.sess.source_map().lookup_line(sp.lo()).is_ok() {
 223         err.set_span(sp);
 224         err.span_label(sp, &note);
 225     } else {
 226         err.note(&note);
 227     }
 228
 229     if let Some(main_def) = tcx.resolutions(()).main_def {
 230         if main_def.opt_fn_def_id().is_none() {
 231             // There is something at `crate::main`, but it is not a function definition.
 232             err.span_label(main_def.span, &format!("non-function item at `crate::main` is found"));
 233         }
 234     }
 235
 236     if tcx.sess.teach(&err.get_code().unwrap()) {
 237         err.note(
 238             "If you don't know the basics of Rust, you can go look to the Rust Book \
 239                   to get started: https://doc.rust-lang.org/book/",
 240         );
 241     }
 242     err.emit();
 243 }
 244
 245 pub fn provide(providers: &mut Providers) {
 246     *providers = Providers { entry_fn, ..*providers };
 247 }