compiler/rustc_passes/src/entry.rs

   1 use rustc_ast::entry::EntryPointType;
   2 use rustc_errors::struct_span_err;
   3 use rustc_hir::def::DefKind;
   4 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
   5 use rustc_hir::{ItemId, Node, 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 fn entry_fn(tcx: TyCtxt<'_>, (): ()) -> Option<(DefId, EntryFnType)> {
  29     let any_exe = tcx.sess.crate_types().iter().any(|ty| *ty == CrateType::Executable);
  30     if !any_exe {
  31         // No need to find a main function.
  32         return None;
  33     }
  34
  35     // If the user wants no main function at all, then stop here.
  36     if tcx.sess.contains_name(&tcx.hir().attrs(CRATE_HIR_ID), sym::no_main) {
  37         return None;
  38     }
  39
  40     let mut ctxt =
  41         EntryContext { tcx, attr_main_fn: None, start_fn: None, non_main_fns: Vec::new() };
  42
  43     for id in tcx.hir().items() {
  44         find_item(id, &mut ctxt);
  45     }
  46
  47     configure_main(tcx, &ctxt)
  48 }
  49
  50 // Beware, this is duplicated in `librustc_builtin_macros/test_harness.rs`
  51 // (with `ast::Item`), so make sure to keep them in sync.
  52 // A small optimization was added so that hir::Item is fetched only when needed.
  53 // An equivalent optimization was not applied to the duplicated code in test_harness.rs.
  54 fn entry_point_type(ctxt: &EntryContext<'_>, id: ItemId, at_root: bool) -> EntryPointType {
  55     let attrs = ctxt.tcx.hir().attrs(id.hir_id());
  56     if ctxt.tcx.sess.contains_name(attrs, sym::start) {
  57         EntryPointType::Start
  58     } else if ctxt.tcx.sess.contains_name(attrs, sym::rustc_main) {
  59         EntryPointType::RustcMainAttr
  60     } else {
  61         if let Some(name) = ctxt.tcx.opt_item_name(id.def_id.to_def_id())
  62             && name == sym::main {
  63             if at_root {
  64                 // This is a top-level function so can be `main`.
  65                 EntryPointType::MainNamed
  66             } else {
  67                 EntryPointType::OtherMain
  68             }
  69         } else {
  70             EntryPointType::None
  71         }
  72     }
  73 }
  74
  75 fn throw_attr_err(sess: &Session, span: Span, attr: &str) {
  76     sess.struct_span_err(span, &format!("`{}` attribute can only be used on functions", attr))
  77         .emit();
  78 }
  79
  80 fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
  81     let at_root = ctxt.tcx.opt_local_parent(id.def_id) == Some(CRATE_DEF_ID);
  82
  83     match entry_point_type(ctxt, id, at_root) {
  84         EntryPointType::None => (),
  85         _ if !matches!(ctxt.tcx.def_kind(id.def_id), DefKind::Fn) => {
  86             let attrs = ctxt.tcx.hir().attrs(id.hir_id());
  87             if let Some(attr) = ctxt.tcx.sess.find_by_name(attrs, sym::start) {
  88                 throw_attr_err(&ctxt.tcx.sess, attr.span, "start");
  89             }
  90             if let Some(attr) = ctxt.tcx.sess.find_by_name(attrs, sym::rustc_main) {
  91                 throw_attr_err(&ctxt.tcx.sess, attr.span, "rustc_main");
  92             }
  93         }
  94         EntryPointType::MainNamed => (),
  95         EntryPointType::OtherMain => {
  96             ctxt.non_main_fns.push(ctxt.tcx.def_span(id.def_id));
  97         }
  98         EntryPointType::RustcMainAttr => {
  99             if ctxt.attr_main_fn.is_none() {
 100                 ctxt.attr_main_fn = Some((id.def_id, ctxt.tcx.def_span(id.def_id.to_def_id())));
 101             } else {
 102                 struct_span_err!(
 103                     ctxt.tcx.sess,
 104                     ctxt.tcx.def_span(id.def_id.to_def_id()),
 105                     E0137,
 106                     "multiple functions with a `#[rustc_main]` attribute"
 107                 )
 108                 .span_label(
 109                     ctxt.tcx.def_span(id.def_id.to_def_id()),
 110                     "additional `#[rustc_main]` function",
 111                 )
 112                 .span_label(ctxt.attr_main_fn.unwrap().1, "first `#[rustc_main]` function")
 113                 .emit();
 114             }
 115         }
 116         EntryPointType::Start => {
 117             if ctxt.start_fn.is_none() {
 118                 ctxt.start_fn = Some((id.def_id, ctxt.tcx.def_span(id.def_id.to_def_id())));
 119             } else {
 120                 struct_span_err!(
 121                     ctxt.tcx.sess,
 122                     ctxt.tcx.def_span(id.def_id.to_def_id()),
 123                     E0138,
 124                     "multiple `start` functions"
 125                 )
 126                 .span_label(ctxt.start_fn.unwrap().1, "previous `#[start]` function here")
 127                 .span_label(ctxt.tcx.def_span(id.def_id.to_def_id()), "multiple `start` functions")
 128                 .emit();
 129             }
 130         }
 131     }
 132 }
 133
 134 fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_>) -> Option<(DefId, EntryFnType)> {
 135     if let Some((def_id, _)) = visitor.start_fn {
 136         Some((def_id.to_def_id(), EntryFnType::Start))
 137     } else if let Some((def_id, _)) = visitor.attr_main_fn {
 138         Some((def_id.to_def_id(), EntryFnType::Main))
 139     } else {
 140         if let Some(main_def) = tcx.resolutions(()).main_def && let Some(def_id) = main_def.opt_fn_def_id() {
 141             // non-local main imports are handled below
 142             if let Some(def_id) = def_id.as_local() && matches!(tcx.hir().find_by_def_id(def_id), Some(Node::ForeignItem(_))) {
 143                 tcx.sess
 144                     .struct_span_err(
 145                         tcx.def_span(def_id),
 146                         "the `main` function cannot be declared in an `extern` block",
 147                     )
 148                     .emit();
 149                 return None;
 150             }
 151
 152             if main_def.is_import && !tcx.features().imported_main {
 153                 let span = main_def.span;
 154                 feature_err(
 155                     &tcx.sess.parse_sess,
 156                     sym::imported_main,
 157                     span,
 158                     "using an imported function as entry point `main` is experimental",
 159                 )
 160                 .emit();
 161             }
 162             return Some((def_id, EntryFnType::Main));
 163         }
 164         no_main_err(tcx, visitor);
 165         None
 166     }
 167 }
 168
 169 fn no_main_err(tcx: TyCtxt<'_>, visitor: &EntryContext<'_>) {
 170     let sp = tcx.def_span(CRATE_DEF_ID);
 171     if *tcx.sess.parse_sess.reached_eof.borrow() {
 172         // There's an unclosed brace that made the parser reach `Eof`, we shouldn't complain about
 173         // the missing `fn main()` then as it might have been hidden inside an unclosed block.
 174         tcx.sess.delay_span_bug(sp, "`main` not found, but expected unclosed brace error");
 175         return;
 176     }
 177
 178     // There is no main function.
 179     let mut err = struct_span_err!(
 180         tcx.sess,
 181         DUMMY_SP,
 182         E0601,
 183         "`main` function not found in crate `{}`",
 184         tcx.crate_name(LOCAL_CRATE)
 185     );
 186     let filename = &tcx.sess.local_crate_source_file;
 187     let note = if !visitor.non_main_fns.is_empty() {
 188         for &span in &visitor.non_main_fns {
 189             err.span_note(span, "here is a function named `main`");
 190         }
 191         err.note("you have one or more functions named `main` not defined at the crate level");
 192         err.help("consider moving the `main` function definitions");
 193         // There were some functions named `main` though. Try to give the user a hint.
 194         format!(
 195             "the main function must be defined at the crate level{}",
 196             filename.as_ref().map(|f| format!(" (in `{}`)", f.display())).unwrap_or_default()
 197         )
 198     } else if let Some(filename) = filename {
 199         format!("consider adding a `main` function to `{}`", filename.display())
 200     } else {
 201         String::from("consider adding a `main` function at the crate level")
 202     };
 203     // The file may be empty, which leads to the diagnostic machinery not emitting this
 204     // note. This is a relatively simple way to detect that case and emit a span-less
 205     // note instead.
 206     if tcx.sess.source_map().lookup_line(sp.hi()).is_ok() {
 207         err.set_span(sp.shrink_to_hi());
 208         err.span_label(sp.shrink_to_hi(), &note);
 209     } else {
 210         err.note(&note);
 211     }
 212
 213     if let Some(main_def) = tcx.resolutions(()).main_def && main_def.opt_fn_def_id().is_none(){
 214         // There is something at `crate::main`, but it is not a function definition.
 215         err.span_label(main_def.span, "non-function item at `crate::main` is found");
 216     }
 217
 218     if tcx.sess.teach(&err.get_code().unwrap()) {
 219         err.note(
 220             "If you don't know the basics of Rust, you can go look to the Rust Book \
 221                   to get started: https://doc.rust-lang.org/book/",
 222         );
 223     }
 224     err.emit();
 225 }
 226
 227 pub fn provide(providers: &mut Providers) {
 228     *providers = Providers { entry_fn, ..*providers };
 229 }