Implements RFC 1937: ? in main

src/librustc/middle/lang_items.rs

@@ -338,6 +338,8 @@ language_item_table! {
     U128ShloFnLangItem,              "u128_shlo",               u128_shlo_fn;
     I128ShroFnLangItem,              "i128_shro",               i128_shro_fn;
     U128ShroFnLangItem,              "u128_shro",               u128_shro_fn;
+
+    TerminationTraitLangItem,        "termination",             termination;
 }
 
 impl<'a, 'tcx, 'gcx> TyCtxt<'a, 'tcx, 'gcx> {

src/librustc/session/code_stats.rs

@@ -155,11 +155,19 @@ impl CodeStats {
 
                     // Include field alignment in output only if it caused padding injection
                     if min_offset != offset {
-                        let pad = offset - min_offset;
-                        println!("print-type-size {}padding: {} bytes",
-                                 indent, pad);
-                        println!("print-type-size {}field `.{}`: {} bytes, alignment: {} bytes",
-                                 indent, name, size, align);
+                        if offset > min_offset {
+                            let pad = offset - min_offset;
+                            println!("print-type-size {}padding: {} bytes",
+                                     indent, pad);
+                            println!("print-type-size {}field `.{}`: {} bytes, \
+                                      alignment: {} bytes",
+                                     indent, name, size, align);
+                        } else {
+                            println!("print-type-size {}field `.{}`: {} bytes, \
+                                      offset: {} bytes, \
+                                      alignment: {} bytes",
+                                     indent, name, size, offset, align);
+                        }
                     } else {
                         println!("print-type-size {}field `.{}`: {} bytes",
                                  indent, name, size);

src/librustc_mir/monomorphize/collector.rs

@@ -194,11 +194,12 @@ use rustc::hir::itemlikevisit::ItemLikeVisitor;
 use rustc::hir::map as hir_map;
 use rustc::hir::def_id::DefId;
 use rustc::middle::const_val::ConstVal;
-use rustc::middle::lang_items::{ExchangeMallocFnLangItem};
+use rustc::middle::lang_items::{ExchangeMallocFnLangItem, StartFnLangItem};
 use rustc::traits;
-use rustc::ty::subst::Substs;
+use rustc::ty::subst::{Substs, Kind};
 use rustc::ty::{self, TypeFoldable, Ty, TyCtxt};
 use rustc::ty::adjustment::CustomCoerceUnsized;
+use rustc::session::config;
 use rustc::mir::{self, Location};
 use rustc::mir::visit::Visitor as MirVisitor;
 use rustc::mir::mono::MonoItem;
@@ -212,6 +213,8 @@ use rustc_data_structures::bitvec::BitVector;
 
 use syntax::attr;
 
+use std::iter;
+
 #[derive(PartialEq, Eq, Hash, Clone, Copy, Debug)]
 pub enum MonoItemCollectionMode {
     Eager,
@@ -329,6 +332,8 @@ fn collect_roots<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             tcx.hir.local_def_id(node_id)
         });
 
+        debug!("collect_roots: entry_fn = {:?}", entry_fn);
+
         let mut visitor = RootCollector {
             tcx,
             mode,
@@ -951,16 +956,8 @@ impl<'b, 'a, 'v> ItemLikeVisitor<'v> for RootCollector<'b, 'a, 'v> {
                 // actually used somewhere. Just declaring them is insufficient.
             }
             hir::ItemFn(..) => {
-                let tcx = self.tcx;
-                let def_id = tcx.hir.local_def_id(item.id);
-
-                if self.is_root(def_id) {
-                    debug!("RootCollector: ItemFn({})",
-                           def_id_to_string(tcx, def_id));
-
-                    let instance = Instance::mono(tcx, def_id);
-                    self.output.push(MonoItem::Fn(instance));
-                }
+                let def_id = self.tcx.hir.local_def_id(item.id);
+                self.push_if_root(def_id);
             }
         }
     }
@@ -973,16 +970,8 @@ impl<'b, 'a, 'v> ItemLikeVisitor<'v> for RootCollector<'b, 'a, 'v> {
     fn visit_impl_item(&mut self, ii: &'v hir::ImplItem) {
         match ii.node {
             hir::ImplItemKind::Method(hir::MethodSig { .. }, _) => {
-                let tcx = self.tcx;
-                let def_id = tcx.hir.local_def_id(ii.id);
-
-                if self.is_root(def_id) {
-                    debug!("RootCollector: MethodImplItem({})",
-                           def_id_to_string(tcx, def_id));
-
-                    let instance = Instance::mono(tcx, def_id);
-                    self.output.push(MonoItem::Fn(instance));
-                }
+                let def_id = self.tcx.hir.local_def_id(ii.id);
+                self.push_if_root(def_id);
             }
             _ => { /* Nothing to do here */ }
         }
@@ -1003,6 +992,56 @@ impl<'b, 'a, 'v> RootCollector<'b, 'a, 'v> {
             }
         }
     }
+
+    /// If `def_id` represents a root, then push it onto the list of
+    /// outputs. (Note that all roots must be monomorphic.)
+    fn push_if_root(&mut self, def_id: DefId) {
+        if self.is_root(def_id) {
+            debug!("RootCollector::push_if_root: found root def_id={:?}", def_id);
+
+            let instance = Instance::mono(self.tcx, def_id);
+            self.output.push(create_fn_mono_item(instance));
+
+            self.push_extra_entry_roots(def_id);
+        }
+    }
+
+    /// As a special case, when/if we encounter the
+    /// `main()` function, we also have to generate a
+    /// monomorphized copy of the start lang item based on
+    /// the return type of `main`. This is not needed when
+    /// the user writes their own `start` manually.
+    fn push_extra_entry_roots(&mut self, def_id: DefId) {
+        if self.entry_fn != Some(def_id) {
+            return;
+        }
+
+        if self.tcx.sess.entry_type.get() != Some(config::EntryMain) {
+            return;
+        }
+
+        let start_def_id = match self.tcx.lang_items().require(StartFnLangItem) {
+            Ok(s) => s,
+            Err(err) => self.tcx.sess.fatal(&err),
+        };
+        let main_ret_ty = self.tcx.fn_sig(def_id).output();
+
+        // Given that `main()` has no arguments,
+        // then its return type cannot have
+        // late-bound regions, since late-bound
+        // regions must appear in the argument
+        // listing.
+        let main_ret_ty = main_ret_ty.no_late_bound_regions().unwrap();
+
+        let start_instance = Instance::resolve(
+            self.tcx,
+            ty::ParamEnv::empty(traits::Reveal::All),
+            start_def_id,
+            self.tcx.mk_substs(iter::once(Kind::from(main_ret_ty)))
+        ).unwrap();
+
+        self.output.push(create_fn_mono_item(start_instance));
+    }
 }
 
 fn item_has_type_parameters<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) -> bool {

src/librustc_mir/monomorphize/partitioning.rs

@@ -312,7 +312,11 @@ fn place_root_translation_items<'a, 'tcx, I>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                     MonoItem::Fn(ref instance) => {
                         let visibility = match instance.def {
                             InstanceDef::Item(def_id) => {
-                                if def_id.is_local() {
+                                // If we encounter the lang start item, we set the visibility to
+                                // default.
+                                if tcx.lang_items().start_fn() == Some(def_id) {
+                                    Visibility::Default
+                                } else if def_id.is_local() {
                                     if tcx.is_exported_symbol(def_id) {
                                         Visibility::Default
                                     } else {

src/librustc_trans/base.rs

@@ -44,6 +44,7 @@ use rustc::ty::{self, Ty, TyCtxt};
 use rustc::ty::layout::{self, Align, TyLayout, LayoutOf};
 use rustc::ty::maps::Providers;
 use rustc::dep_graph::{DepNode, DepConstructor};
+use rustc::ty::subst::Kind;
 use rustc::middle::cstore::{self, LinkMeta, LinkagePreference};
 use rustc::util::common::{time, print_time_passes_entry};
 use rustc::session::config::{self, NoDebugInfo};
@@ -79,6 +80,7 @@ use std::str;
 use std::sync::Arc;
 use std::time::{Instant, Duration};
 use std::i32;
+use std::iter;
 use std::sync::mpsc;
 use syntax_pos::Span;
 use syntax_pos::symbol::InternedString;
@@ -540,17 +542,28 @@ fn maybe_create_entry_wrapper(ccx: &CrateContext) {
 
     let et = ccx.sess().entry_type.get().unwrap();
     match et {
-        config::EntryMain => create_entry_fn(ccx, span, main_llfn, true),
-        config::EntryStart => create_entry_fn(ccx, span, main_llfn, false),
+        config::EntryMain => create_entry_fn(ccx, span, main_llfn, main_def_id, true),
+        config::EntryStart => create_entry_fn(ccx, span, main_llfn, main_def_id, false),
         config::EntryNone => {}    // Do nothing.
     }
 
-    fn create_entry_fn(ccx: &CrateContext,
+    fn create_entry_fn<'ccx>(ccx: &'ccx CrateContext,
                        sp: Span,
                        rust_main: ValueRef,
+                       rust_main_def_id: DefId,
                        use_start_lang_item: bool) {
-        // Signature of native main(), corresponding to C's `int main(int, char **)`
-        let llfty = Type::func(&[Type::c_int(ccx), Type::i8p(ccx).ptr_to()], &Type::c_int(ccx));
+        // The libstd lang_start function does not return anything, while user defined lang start
+        // returns a isize
+        let start_output_ty = if use_start_lang_item { Type::void(ccx) } else { Type::c_int(ccx) };
+        let llfty = Type::func(&[Type::c_int(ccx), Type::i8p(ccx).ptr_to()], &start_output_ty);
+
+        let main_ret_ty = ccx.tcx().fn_sig(rust_main_def_id).output();
+        // Given that `main()` has no arguments,
+        // then its return type cannot have
+        // late-bound regions, since late-bound
+        // regions must appear in the argument
+        // listing.
+        let main_ret_ty = main_ret_ty.no_late_bound_regions().unwrap();
 
         if declare::get_defined_value(ccx, "main").is_some() {
             // FIXME: We should be smart and show a better diagnostic here.
@@ -577,8 +590,8 @@ fn maybe_create_entry_wrapper(ccx: &CrateContext) {
 
         let (start_fn, args) = if use_start_lang_item {
             let start_def_id = ccx.tcx().require_lang_item(StartFnLangItem);
-            let start_instance = Instance::mono(ccx.tcx(), start_def_id);
-            let start_fn = callee::get_fn(ccx, start_instance);
+            let start_fn = callee::resolve_and_get_fn(ccx, start_def_id, ccx.tcx().mk_substs(
+                iter::once(Kind::from(main_ret_ty))));
             (start_fn, vec![bld.pointercast(rust_main, Type::i8p(ccx).ptr_to()),
                             arg_argc, arg_argv])
         } else {
@@ -588,8 +601,11 @@ fn maybe_create_entry_wrapper(ccx: &CrateContext) {
 
         let result = bld.call(start_fn, &args, None);
 
-        // Return rust start function's result from native main()
-        bld.ret(bld.intcast(result, Type::c_int(ccx), true));
+        if use_start_lang_item {
+            bld.ret_void();
+        } else {
+            bld.ret(bld.intcast(result, Type::c_int(ccx), true));
+        }
     }
 }
 

src/librustc_typeck/check/mod.rs

@@ -96,14 +96,14 @@ use rustc::middle::region;
 use rustc::ty::subst::{Kind, Subst, Substs};
 use rustc::traits::{self, FulfillmentContext, ObligationCause, ObligationCauseCode};
 use rustc::ty::{ParamTy, LvaluePreference, NoPreference, PreferMutLvalue};
-use rustc::ty::{self, Ty, TyCtxt, Visibility};
+use rustc::ty::{self, Ty, TyCtxt, Visibility, ToPredicate};
 use rustc::ty::adjustment::{Adjust, Adjustment, AutoBorrow};
 use rustc::ty::fold::TypeFoldable;
 use rustc::ty::maps::Providers;
 use rustc::ty::util::{Representability, IntTypeExt};
 use errors::{DiagnosticBuilder, DiagnosticId};
 use require_c_abi_if_variadic;
-use session::{CompileIncomplete, Session};
+use session::{CompileIncomplete, config, Session};
 use TypeAndSubsts;
 use lint;
 use util::common::{ErrorReported, indenter};
@@ -115,6 +115,7 @@ use std::collections::hash_map::Entry;
 use std::cmp;
 use std::fmt::Display;
 use std::mem::replace;
+use std::iter;
 use std::ops::{self, Deref};
 use syntax::abi::Abi;
 use syntax::ast;
@@ -1064,6 +1065,30 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     }
     fcx.demand_suptype(span, ret_ty, actual_return_ty);
 
+    if fcx.tcx.sess.features.borrow().termination_trait {
+        // If the termination trait language item is activated, check that the main return type
+        // implements the termination trait.
+        if let Some(term_id) = fcx.tcx.lang_items().termination() {
+            if let Some((id, _)) = *fcx.tcx.sess.entry_fn.borrow() {
+                if id == fn_id {
+                    match fcx.sess().entry_type.get() {
+                        Some(config::EntryMain) => {
+                            let substs = fcx.tcx.mk_substs(iter::once(Kind::from(ret_ty)));
+                            let trait_ref = ty::TraitRef::new(term_id, substs);
+                            let cause = traits::ObligationCause::new(
+                                span, fn_id, ObligationCauseCode::MainFunctionType);
+
+                            inherited.register_predicate(
+                                traits::Obligation::new(
+                                    cause, param_env, trait_ref.to_predicate()));
+                        },
+                        _ => {},
+                    }
+                }
+            }
+        }
+    }
+
     (fcx, gen_ty)
 }
 

src/librustc_typeck/lib.rs

@@ -116,6 +116,7 @@ use syntax::abi::Abi;
 use syntax_pos::Span;
 
 use std::iter;
+
 // NB: This module needs to be declared first so diagnostics are
 // registered before they are used.
 mod diagnostics;
@@ -200,10 +201,22 @@ fn check_main_fn_ty<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 }
                 _ => ()
             }
+
+            let actual = tcx.fn_sig(main_def_id);
+            let expected_return_type = if tcx.lang_items().termination().is_some()
+                && tcx.sess.features.borrow().termination_trait {
+                // we take the return type of the given main function, the real check is done
+                // in `check_fn`
+                actual.output().skip_binder()
+            } else {
+                // standard () main return type
+                tcx.mk_nil()
+            };
+
             let se_ty = tcx.mk_fn_ptr(ty::Binder(
                 tcx.mk_fn_sig(
                     iter::empty(),
-                    tcx.mk_nil(),
+                    expected_return_type,
                     false,
                     hir::Unsafety::Normal,
                     Abi::Rust
@@ -214,7 +227,7 @@ fn check_main_fn_ty<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 tcx,
                 &ObligationCause::new(main_span, main_id, ObligationCauseCode::MainFunctionType),
                 se_ty,
-                tcx.mk_fn_ptr(tcx.fn_sig(main_def_id)));
+                tcx.mk_fn_ptr(actual));
         }
         _ => {
             span_bug!(main_span,

src/libstd/lib.rs

@@ -308,6 +308,7 @@
 #![feature(str_char)]
 #![feature(str_internals)]
 #![feature(str_utf16)]
+#![feature(termination_trait)]
 #![feature(test, rustc_private)]
 #![feature(thread_local)]
 #![feature(toowned_clone_into)]
@@ -499,6 +500,11 @@ mod memchr;
 // The runtime entry point and a few unstable public functions used by the
 // compiler
 pub mod rt;
+// The trait to support returning arbitrary types in the main function
+mod termination;
+
+#[unstable(feature = "termination_trait", issue = "43301")]
+pub use self::termination::Termination;
 
 // Include a number of private modules that exist solely to provide
 // the rustdoc documentation for primitive types. Using `include!`