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