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::{CrateNum, LocalDefId, CRATE_DEF_INDEX, LOCAL_CRATE};
   4 use rustc_hir::itemlikevisit::ItemLikeVisitor;
   5 use rustc_hir::{ForeignItem, HirId, ImplItem, Item, ItemKind, 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::Session;
  11 use rustc_span::symbol::sym;
  12 use rustc_span::{Span, DUMMY_SP};
  13
  14 struct EntryContext<'a, 'tcx> {
  15     session: &'a Session,
  16
  17     map: Map<'tcx>,
  18
  19     /// The top-level function called `main`.
  20     main_fn: Option<(HirId, Span)>,
  21
  22     /// The function that has attribute named `main`.
  23     attr_main_fn: Option<(HirId, Span)>,
  24
  25     /// The function that has the attribute 'start' on it.
  26     start_fn: Option<(HirId, Span)>,
  27
  28     /// The functions that one might think are `main` but aren't, e.g.
  29     /// main functions not defined at the top level. For diagnostics.
  30     non_main_fns: Vec<(HirId, Span)>,
  31 }
  32
  33 impl<'a, 'tcx> ItemLikeVisitor<'tcx> for EntryContext<'a, 'tcx> {
  34     fn visit_item(&mut self, item: &'tcx Item<'tcx>) {
  35         let def_key = self.map.def_key(item.def_id);
  36         let at_root = def_key.parent == Some(CRATE_DEF_INDEX);
  37         find_item(item, self, at_root);
  38     }
  39
  40     fn visit_trait_item(&mut self, _trait_item: &'tcx TraitItem<'tcx>) {
  41         // Entry fn is never a trait item.
  42     }
  43
  44     fn visit_impl_item(&mut self, _impl_item: &'tcx ImplItem<'tcx>) {
  45         // Entry fn is never a trait item.
  46     }
  47
  48     fn visit_foreign_item(&mut self, _: &'tcx ForeignItem<'tcx>) {
  49         // Entry fn is never a foreign item.
  50     }
  51 }
  52
  53 fn entry_fn(tcx: TyCtxt<'_>, cnum: CrateNum) -> Option<(LocalDefId, EntryFnType)> {
  54     assert_eq!(cnum, LOCAL_CRATE);
  55
  56     let any_exe = tcx.sess.crate_types().iter().any(|ty| *ty == CrateType::Executable);
  57     if !any_exe {
  58         // No need to find a main function.
  59         return None;
  60     }
  61
  62     // If the user wants no main function at all, then stop here.
  63     if tcx.sess.contains_name(&tcx.hir().attrs(CRATE_HIR_ID), sym::no_main) {
  64         return None;
  65     }
  66
  67     let mut ctxt = EntryContext {
  68         session: tcx.sess,
  69         map: tcx.hir(),
  70         main_fn: None,
  71         attr_main_fn: None,
  72         start_fn: None,
  73         non_main_fns: Vec::new(),
  74     };
  75
  76     tcx.hir().krate().visit_all_item_likes(&mut ctxt);
  77
  78     configure_main(tcx, &ctxt)
  79 }
  80
  81 // Beware, this is duplicated in `librustc_builtin_macros/test_harness.rs`
  82 // (with `ast::Item`), so make sure to keep them in sync.
  83 fn entry_point_type(ctxt: &EntryContext<'_, '_>, item: &Item<'_>, at_root: bool) -> EntryPointType {
  84     let attrs = ctxt.map.attrs(item.hir_id());
  85     if ctxt.session.contains_name(attrs, sym::start) {
  86         EntryPointType::Start
  87     } else if ctxt.session.contains_name(attrs, sym::main) {
  88         EntryPointType::MainAttr
  89     } else if item.ident.name == sym::main {
  90         if at_root {
  91             // This is a top-level function so can be `main`.
  92             EntryPointType::MainNamed
  93         } else {
  94             EntryPointType::OtherMain
  95         }
  96     } else {
  97         EntryPointType::None
  98     }
  99 }
 100
 101 fn throw_attr_err(sess: &Session, span: Span, attr: &str) {
 102     sess.struct_span_err(span, &format!("`{}` attribute can only be used on functions", attr))
 103         .emit();
 104 }
 105
 106 fn find_item(item: &Item<'_>, ctxt: &mut EntryContext<'_, '_>, at_root: bool) {
 107     match entry_point_type(ctxt, item, at_root) {
 108         EntryPointType::None => (),
 109         _ if !matches!(item.kind, ItemKind::Fn(..)) => {
 110             let attrs = ctxt.map.attrs(item.hir_id());
 111             if let Some(attr) = ctxt.session.find_by_name(attrs, sym::start) {
 112                 throw_attr_err(&ctxt.session, attr.span, "start");
 113             }
 114             if let Some(attr) = ctxt.session.find_by_name(attrs, sym::main) {
 115                 throw_attr_err(&ctxt.session, attr.span, "main");
 116             }
 117         }
 118         EntryPointType::MainNamed => {
 119             if ctxt.main_fn.is_none() {
 120                 ctxt.main_fn = Some((item.hir_id(), item.span));
 121             } else {
 122                 struct_span_err!(ctxt.session, item.span, E0136, "multiple `main` functions")
 123                     .emit();
 124             }
 125         }
 126         EntryPointType::OtherMain => {
 127             ctxt.non_main_fns.push((item.hir_id(), item.span));
 128         }
 129         EntryPointType::MainAttr => {
 130             if ctxt.attr_main_fn.is_none() {
 131                 ctxt.attr_main_fn = Some((item.hir_id(), item.span));
 132             } else {
 133                 struct_span_err!(
 134                     ctxt.session,
 135                     item.span,
 136                     E0137,
 137                     "multiple functions with a `#[main]` attribute"
 138                 )
 139                 .span_label(item.span, "additional `#[main]` function")
 140                 .span_label(ctxt.attr_main_fn.unwrap().1, "first `#[main]` function")
 141                 .emit();
 142             }
 143         }
 144         EntryPointType::Start => {
 145             if ctxt.start_fn.is_none() {
 146                 ctxt.start_fn = Some((item.hir_id(), item.span));
 147             } else {
 148                 struct_span_err!(ctxt.session, item.span, E0138, "multiple `start` functions")
 149                     .span_label(ctxt.start_fn.unwrap().1, "previous `#[start]` function here")
 150                     .span_label(item.span, "multiple `start` functions")
 151                     .emit();
 152             }
 153         }
 154     }
 155 }
 156
 157 fn configure_main(
 158     tcx: TyCtxt<'_>,
 159     visitor: &EntryContext<'_, '_>,
 160 ) -> Option<(LocalDefId, EntryFnType)> {
 161     if let Some((hir_id, _)) = visitor.start_fn {
 162         Some((tcx.hir().local_def_id(hir_id), EntryFnType::Start))
 163     } else if let Some((hir_id, _)) = visitor.attr_main_fn {
 164         Some((tcx.hir().local_def_id(hir_id), EntryFnType::Main))
 165     } else if let Some((hir_id, _)) = visitor.main_fn {
 166         Some((tcx.hir().local_def_id(hir_id), EntryFnType::Main))
 167     } else {
 168         no_main_err(tcx, visitor);
 169         None
 170     }
 171 }
 172
 173 fn no_main_err(tcx: TyCtxt<'_>, visitor: &EntryContext<'_, '_>) {
 174     let sp = tcx.hir().krate().item.span;
 175     if *tcx.sess.parse_sess.reached_eof.borrow() {
 176         // There's an unclosed brace that made the parser reach `Eof`, we shouldn't complain about
 177         // the missing `fn main()` then as it might have been hidden inside an unclosed block.
 178         tcx.sess.delay_span_bug(sp, "`main` not found, but expected unclosed brace error");
 179         return;
 180     }
 181
 182     // There is no main function.
 183     let mut err = struct_span_err!(
 184         tcx.sess,
 185         DUMMY_SP,
 186         E0601,
 187         "`main` function not found in crate `{}`",
 188         tcx.crate_name(LOCAL_CRATE)
 189     );
 190     let filename = &tcx.sess.local_crate_source_file;
 191     let note = if !visitor.non_main_fns.is_empty() {
 192         for &(_, span) in &visitor.non_main_fns {
 193             err.span_note(span, "here is a function named `main`");
 194         }
 195         err.note("you have one or more functions named `main` not defined at the crate level");
 196         err.help(
 197             "either move the `main` function definitions or attach the `#[main]` attribute \
 198                   to one of them",
 199         );
 200         // There were some functions named `main` though. Try to give the user a hint.
 201         format!(
 202             "the main function must be defined at the crate level{}",
 203             filename.as_ref().map(|f| format!(" (in `{}`)", f.display())).unwrap_or_default()
 204         )
 205     } else if let Some(filename) = filename {
 206         format!("consider adding a `main` function to `{}`", filename.display())
 207     } else {
 208         String::from("consider adding a `main` function at the crate level")
 209     };
 210     // The file may be empty, which leads to the diagnostic machinery not emitting this
 211     // note. This is a relatively simple way to detect that case and emit a span-less
 212     // note instead.
 213     if tcx.sess.source_map().lookup_line(sp.lo()).is_ok() {
 214         err.set_span(sp);
 215         err.span_label(sp, &note);
 216     } else {
 217         err.note(&note);
 218     }
 219     if tcx.sess.teach(&err.get_code().unwrap()) {
 220         err.note(
 221             "If you don't know the basics of Rust, you can go look to the Rust Book \
 222                   to get started: https://doc.rust-lang.org/book/",
 223         );
 224     }
 225     err.emit();
 226 }
 227
 228 pub fn find_entry_point(tcx: TyCtxt<'_>) -> Option<(LocalDefId, EntryFnType)> {
 229     tcx.entry_fn(LOCAL_CRATE)
 230 }
 231
 232 pub fn provide(providers: &mut Providers) {
 233     *providers = Providers { entry_fn, ..*providers };
 234 }