Check entry type as part of item type checking.

compiler/rustc_hir_analysis/src/check/check.rs

@@ -8,7 +8,7 @@ use rustc_attr as attr;
 use rustc_errors::{Applicability, ErrorGuaranteed, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
-use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::{ItemKind, Node, PathSegment};
 use rustc_infer::infer::opaque_types::ConstrainOpaqueTypeRegionVisitor;
@@ -1378,6 +1378,9 @@ pub(super) fn check_mod_item_types(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
     for id in module.items() {
         check_item_type(tcx, id);
     }
+    if module_def_id == CRATE_DEF_ID {
+        super::entry::check_for_entry_fn(tcx);
+    }
 }
 
 fn async_opaque_type_cycle_error(tcx: TyCtxt<'_>, span: Span) -> ErrorGuaranteed {

compiler/rustc_hir_analysis/src/check/entry.rs

@@ -0,0 +1,277 @@
+use rustc_hir as hir;
+use rustc_hir::Node;
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_session::config::EntryFnType;
+use rustc_span::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
+use rustc_span::{symbol::sym, Span};
+use rustc_target::spec::abi::Abi;
+use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
+use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
+
+use std::ops::Not;
+
+use crate::errors;
+use crate::require_same_types;
+
+pub(crate) fn check_for_entry_fn(tcx: TyCtxt<'_>) {
+    match tcx.entry_fn(()) {
+        Some((def_id, EntryFnType::Main { .. })) => check_main_fn_ty(tcx, def_id),
+        Some((def_id, EntryFnType::Start)) => check_start_fn_ty(tcx, def_id),
+        _ => {}
+    }
+}
+
+fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
+    let main_fnsig = tcx.fn_sig(main_def_id).instantiate_identity();
+    let main_span = tcx.def_span(main_def_id);
+
+    fn main_fn_diagnostics_def_id(tcx: TyCtxt<'_>, def_id: DefId, sp: Span) -> LocalDefId {
+        if let Some(local_def_id) = def_id.as_local() {
+            let hir_type = tcx.type_of(local_def_id).instantiate_identity();
+            if !matches!(hir_type.kind(), ty::FnDef(..)) {
+                span_bug!(sp, "main has a non-function type: found `{}`", hir_type);
+            }
+            local_def_id
+        } else {
+            CRATE_DEF_ID
+        }
+    }
+
+    fn main_fn_generics_params_span(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Span> {
+        if !def_id.is_local() {
+            return None;
+        }
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
+        match tcx.hir().find(hir_id) {
+            Some(Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, generics, _), .. })) => {
+                generics.params.is_empty().not().then_some(generics.span)
+            }
+            _ => {
+                span_bug!(tcx.def_span(def_id), "main has a non-function type");
+            }
+        }
+    }
+
+    fn main_fn_where_clauses_span(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Span> {
+        if !def_id.is_local() {
+            return None;
+        }
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
+        match tcx.hir().find(hir_id) {
+            Some(Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, generics, _), .. })) => {
+                Some(generics.where_clause_span)
+            }
+            _ => {
+                span_bug!(tcx.def_span(def_id), "main has a non-function type");
+            }
+        }
+    }
+
+    fn main_fn_asyncness_span(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Span> {
+        if !def_id.is_local() {
+            return None;
+        }
+        Some(tcx.def_span(def_id))
+    }
+
+    fn main_fn_return_type_span(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Span> {
+        if !def_id.is_local() {
+            return None;
+        }
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
+        match tcx.hir().find(hir_id) {
+            Some(Node::Item(hir::Item { kind: hir::ItemKind::Fn(fn_sig, _, _), .. })) => {
+                Some(fn_sig.decl.output.span())
+            }
+            _ => {
+                span_bug!(tcx.def_span(def_id), "main has a non-function type");
+            }
+        }
+    }
+
+    let mut error = false;
+    let main_diagnostics_def_id = main_fn_diagnostics_def_id(tcx, main_def_id, main_span);
+    let main_fn_generics = tcx.generics_of(main_def_id);
+    let main_fn_predicates = tcx.predicates_of(main_def_id);
+    if main_fn_generics.count() != 0 || !main_fnsig.bound_vars().is_empty() {
+        let generics_param_span = main_fn_generics_params_span(tcx, main_def_id);
+        tcx.sess.emit_err(errors::MainFunctionGenericParameters {
+            span: generics_param_span.unwrap_or(main_span),
+            label_span: generics_param_span,
+        });
+        error = true;
+    } else if !main_fn_predicates.predicates.is_empty() {
+        // generics may bring in implicit predicates, so we skip this check if generics is present.
+        let generics_where_clauses_span = main_fn_where_clauses_span(tcx, main_def_id);
+        tcx.sess.emit_err(errors::WhereClauseOnMain {
+            span: generics_where_clauses_span.unwrap_or(main_span),
+            generics_span: generics_where_clauses_span,
+        });
+        error = true;
+    }
+
+    let main_asyncness = tcx.asyncness(main_def_id);
+    if let hir::IsAsync::Async = main_asyncness {
+        let asyncness_span = main_fn_asyncness_span(tcx, main_def_id);
+        tcx.sess.emit_err(errors::MainFunctionAsync { span: main_span, asyncness: asyncness_span });
+        error = true;
+    }
+
+    for attr in tcx.get_attrs(main_def_id, sym::track_caller) {
+        tcx.sess.emit_err(errors::TrackCallerOnMain { span: attr.span, annotated: main_span });
+        error = true;
+    }
+
+    if !tcx.codegen_fn_attrs(main_def_id).target_features.is_empty()
+        // Calling functions with `#[target_feature]` is not unsafe on WASM, see #84988
+        && !tcx.sess.target.is_like_wasm
+        && !tcx.sess.opts.actually_rustdoc
+    {
+        tcx.sess.emit_err(errors::TargetFeatureOnMain { main: main_span });
+        error = true;
+    }
+
+    if error {
+        return;
+    }
+
+    // Main should have no WC, so empty param env is OK here.
+    let param_env = ty::ParamEnv::empty();
+    let expected_return_type;
+    if let Some(term_did) = tcx.lang_items().termination() {
+        let return_ty = main_fnsig.output();
+        let return_ty_span = main_fn_return_type_span(tcx, main_def_id).unwrap_or(main_span);
+        if !return_ty.bound_vars().is_empty() {
+            tcx.sess.emit_err(errors::MainFunctionReturnTypeGeneric { span: return_ty_span });
+            error = true;
+        }
+        let return_ty = return_ty.skip_binder();
+        let infcx = tcx.infer_ctxt().build();
+        let cause = traits::ObligationCause::new(
+            return_ty_span,
+            main_diagnostics_def_id,
+            ObligationCauseCode::MainFunctionType,
+        );
+        let ocx = traits::ObligationCtxt::new(&infcx);
+        let norm_return_ty = ocx.normalize(&cause, param_env, return_ty);
+        ocx.register_bound(cause, param_env, norm_return_ty, term_did);
+        let errors = ocx.select_all_or_error();
+        if !errors.is_empty() {
+            infcx.err_ctxt().report_fulfillment_errors(&errors);
+            error = true;
+        }
+        // now we can take the return type of the given main function
+        expected_return_type = main_fnsig.output();
+    } else {
+        // standard () main return type
+        expected_return_type = ty::Binder::dummy(Ty::new_unit(tcx));
+    }
+
+    if error {
+        return;
+    }
+
+    let se_ty = Ty::new_fn_ptr(
+        tcx,
+        expected_return_type.map_bound(|expected_return_type| {
+            tcx.mk_fn_sig([], expected_return_type, false, hir::Unsafety::Normal, Abi::Rust)
+        }),
+    );
+
+    require_same_types(
+        tcx,
+        &ObligationCause::new(
+            main_span,
+            main_diagnostics_def_id,
+            ObligationCauseCode::MainFunctionType,
+        ),
+        param_env,
+        se_ty,
+        Ty::new_fn_ptr(tcx, main_fnsig),
+    );
+}
+
+fn check_start_fn_ty(tcx: TyCtxt<'_>, start_def_id: DefId) {
+    let start_def_id = start_def_id.expect_local();
+    let start_id = tcx.hir().local_def_id_to_hir_id(start_def_id);
+    let start_span = tcx.def_span(start_def_id);
+    let start_t = tcx.type_of(start_def_id).instantiate_identity();
+    match start_t.kind() {
+        ty::FnDef(..) => {
+            if let Some(Node::Item(it)) = tcx.hir().find(start_id) {
+                if let hir::ItemKind::Fn(sig, generics, _) = &it.kind {
+                    let mut error = false;
+                    if !generics.params.is_empty() {
+                        tcx.sess.emit_err(errors::StartFunctionParameters { span: generics.span });
+                        error = true;
+                    }
+                    if generics.has_where_clause_predicates {
+                        tcx.sess.emit_err(errors::StartFunctionWhere {
+                            span: generics.where_clause_span,
+                        });
+                        error = true;
+                    }
+                    if let hir::IsAsync::Async = sig.header.asyncness {
+                        let span = tcx.def_span(it.owner_id);
+                        tcx.sess.emit_err(errors::StartAsync { span: span });
+                        error = true;
+                    }
+
+                    let attrs = tcx.hir().attrs(start_id);
+                    for attr in attrs {
+                        if attr.has_name(sym::track_caller) {
+                            tcx.sess.emit_err(errors::StartTrackCaller {
+                                span: attr.span,
+                                start: start_span,
+                            });
+                            error = true;
+                        }
+                        if attr.has_name(sym::target_feature)
+                            // Calling functions with `#[target_feature]` is
+                            // not unsafe on WASM, see #84988
+                            && !tcx.sess.target.is_like_wasm
+                            && !tcx.sess.opts.actually_rustdoc
+                        {
+                            tcx.sess.emit_err(errors::StartTargetFeature {
+                                span: attr.span,
+                                start: start_span,
+                            });
+                            error = true;
+                        }
+                    }
+
+                    if error {
+                        return;
+                    }
+                }
+            }
+
+            let se_ty = Ty::new_fn_ptr(
+                tcx,
+                ty::Binder::dummy(tcx.mk_fn_sig(
+                    [tcx.types.isize, Ty::new_imm_ptr(tcx, Ty::new_imm_ptr(tcx, tcx.types.u8))],
+                    tcx.types.isize,
+                    false,
+                    hir::Unsafety::Normal,
+                    Abi::Rust,
+                )),
+            );
+
+            require_same_types(
+                tcx,
+                &ObligationCause::new(
+                    start_span,
+                    start_def_id,
+                    ObligationCauseCode::StartFunctionType,
+                ),
+                ty::ParamEnv::empty(), // start should not have any where bounds.
+                se_ty,
+                Ty::new_fn_ptr(tcx, tcx.fn_sig(start_def_id).instantiate_identity()),
+            );
+        }
+        _ => {
+            span_bug!(start_span, "start has a non-function type: found `{}`", start_t);
+        }
+    }
+}

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -65,6 +65,7 @@ a type parameter).
 mod check;
 mod compare_impl_item;
 pub mod dropck;
+mod entry;
 pub mod intrinsic;
 pub mod intrinsicck;
 mod region;