src/librustc/middle/entry.rs

use hir::map as hir_map;
use hir::def_id::{CrateNum, CRATE_DEF_INDEX, DefId, LOCAL_CRATE};
use session::{config, Session};
use session::config::EntryFnType;
use syntax::ast::NodeId;
use syntax::attr;
use syntax::entry::EntryPointType;
use syntax_pos::Span;
use hir::{Item, ItemKind, ImplItem, TraitItem};
use hir::itemlikevisit::ItemLikeVisitor;
use ty::TyCtxt;
use ty::query::Providers;

struct EntryContext<'a, 'tcx: 'a> {
    session: &'a Session,

    map: &'a hir_map::Map<'tcx>,

    // The top-level function called 'main'
    main_fn: Option<(NodeId, Span)>,

    // The function that has attribute named 'main'
    attr_main_fn: Option<(NodeId, Span)>,

    // The function that has the attribute 'start' on it
    start_fn: Option<(NodeId, Span)>,

    // The functions that one might think are 'main' but aren't, e.g.
    // main functions not defined at the top level. For diagnostics.
    non_main_fns: Vec<(NodeId, Span)> ,
}

impl<'a, 'tcx> ItemLikeVisitor<'tcx> for EntryContext<'a, 'tcx> {
    fn visit_item(&mut self, item: &'tcx Item) {
        let def_id = self.map.local_def_id(item.id);
        let def_key = self.map.def_key(def_id);
        let at_root = def_key.parent == Some(CRATE_DEF_INDEX);
        find_item(item, self, at_root);
    }

    fn visit_trait_item(&mut self, _trait_item: &'tcx TraitItem) {
        // entry fn is never a trait item
    }

    fn visit_impl_item(&mut self, _impl_item: &'tcx ImplItem) {
        // entry fn is never an impl item
    }
}

fn entry_fn(tcx: TyCtxt<'_, '_, '_>, cnum: CrateNum) -> Option<(DefId, EntryFnType)> {
    assert_eq!(cnum, LOCAL_CRATE);

    let any_exe = tcx.sess.crate_types.borrow().iter().any(|ty| {
        *ty == config::CrateType::Executable
    });
    if !any_exe {
        // No need to find a main function
        return None;
    }

    // If the user wants no main function at all, then stop here.
    if attr::contains_name(&tcx.hir().krate().attrs, "no_main") {
        return None;
    }

    let mut ctxt = EntryContext {
        session: tcx.sess,
        map: tcx.hir(),
        main_fn: None,
        attr_main_fn: None,
        start_fn: None,
        non_main_fns: Vec::new(),
    };

    tcx.hir().krate().visit_all_item_likes(&mut ctxt);

    configure_main(tcx, &ctxt)
}

// Beware, this is duplicated in `libsyntax/entry.rs`, so make sure to keep
// them in sync.
fn entry_point_type(item: &Item, at_root: bool) -> EntryPointType {
    match item.node {
        ItemKind::Fn(..) => {
            if attr::contains_name(&item.attrs, "start") {
                EntryPointType::Start
            } else if attr::contains_name(&item.attrs, "main") {
                EntryPointType::MainAttr
            } else if item.ident.name == "main" {
                if at_root {
                    // This is a top-level function so can be 'main'.
                    EntryPointType::MainNamed
                } else {
                    EntryPointType::OtherMain
                }
            } else {
                EntryPointType::None
            }
        }
        _ => EntryPointType::None,
    }
}


fn find_item(item: &Item, ctxt: &mut EntryContext<'_, '_>, at_root: bool) {
    match entry_point_type(item, at_root) {
        EntryPointType::MainNamed => {
            if ctxt.main_fn.is_none() {
                ctxt.main_fn = Some((item.id, item.span));
            } else {
                span_err!(ctxt.session, item.span, E0136,
                          "multiple 'main' functions");
            }
        },
        EntryPointType::OtherMain => {
            ctxt.non_main_fns.push((item.id, item.span));
        },
        EntryPointType::MainAttr => {
            if ctxt.attr_main_fn.is_none() {
                ctxt.attr_main_fn = Some((item.id, item.span));
            } else {
                struct_span_err!(ctxt.session, item.span, E0137,
                                 "multiple functions with a #[main] attribute")
                .span_label(item.span, "additional #[main] function")
                .span_label(ctxt.attr_main_fn.unwrap().1, "first #[main] function")
                .emit();
            }
        },
        EntryPointType::Start => {
            if ctxt.start_fn.is_none() {
                ctxt.start_fn = Some((item.id, item.span));
            } else {
                struct_span_err!(ctxt.session, item.span, E0138, "multiple 'start' functions")
                    .span_label(ctxt.start_fn.unwrap().1, "previous `start` function here")
                    .span_label(item.span, "multiple `start` functions")
                    .emit();
            }
        }
        EntryPointType::None => (),
    }
}

fn configure_main(
    tcx: TyCtxt<'_, '_, '_>,
    visitor: &EntryContext<'_, '_>,
) -> Option<(DefId, EntryFnType)> {
    if let Some((node_id, _)) = visitor.start_fn {
        Some((tcx.hir().local_def_id(node_id), EntryFnType::Start))
    } else if let Some((node_id, _)) = visitor.attr_main_fn {
        Some((tcx.hir().local_def_id(node_id), EntryFnType::Main))
    } else if let Some((node_id, _)) = visitor.main_fn {
        Some((tcx.hir().local_def_id(node_id), EntryFnType::Main))
    } else {
        // No main function
        let mut err = struct_err!(tcx.sess, E0601,
            "`main` function not found in crate `{}`", tcx.crate_name(LOCAL_CRATE));
        if !visitor.non_main_fns.is_empty() {
            // There were some functions named 'main' though. Try to give the user a hint.
            err.note("the main function must be defined at the crate level \
                      but you have one or more functions named 'main' that are not \
                      defined at the crate level. Either move the definition or \
                      attach the `#[main]` attribute to override this behavior.");
            for &(_, span) in &visitor.non_main_fns {
                err.span_note(span, "here is a function named 'main'");
            }
            err.emit();
            tcx.sess.abort_if_errors();
        } else {
            if let Some(ref filename) = tcx.sess.local_crate_source_file {
                err.note(&format!("consider adding a `main` function to `{}`", filename.display()));
            }
            if tcx.sess.teach(&err.get_code().unwrap()) {
                err.note("If you don't know the basics of Rust, you can go look to the Rust Book \
                          to get started: https://doc.rust-lang.org/book/");
            }
            err.emit();
        }

        None
    }
}

pub fn find_entry_point(tcx: TyCtxt<'_, '_, '_>) -> Option<(DefId, EntryFnType)> {
    tcx.entry_fn(LOCAL_CRATE)
}

pub fn provide(providers: &mut Providers<'_>) {
    *providers = Providers {
        entry_fn,
        ..*providers
    };
}