Prevent main and start from having a where clause.

src/librustc_typeck/diagnostics.rs

@@ -4519,6 +4519,32 @@ impl Foo for () {
 ```
 "##,
 
+E0646: r##"
+It is not possible to define `main` with a where clause.
+Erroneous code example:
+
+```compile_fail,E0646
+fn main() where i32: Copy { // error: main function is not allowed to have
+                            // a where clause
+}
+```
+"##,
+
+E0647: r##"
+It is not possible to define `start` with a where clause.
+Erroneous code example:
+
+```compile_fail,E0647
+#![feature(start)]
+
+#[start]
+fn start(_: isize, _: *const *const u8) -> isize where (): Copy {
+    //^ error: start function is not allowed to have a where clause
+    0
+}
+```
+"##,
+
 E0689: r##"
 This error indicates that the numeric value for the method being passed exists
 but the type of the numeric value or binding could not be identified.

src/librustc_typeck/lib.rs

@@ -196,6 +196,12 @@ fn check_main_fn_ty<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                     .emit();
                                 return;
                             }
+                            if !generics.where_clause.predicates.is_empty() {
+                                struct_span_err!(tcx.sess, main_span, E0646,
+                                         "main function is not allowed to have a where clause")
+                                         .emit();
+                                return;
+                            }
                         }
                         _ => ()
                     }
@@ -247,14 +253,21 @@ fn check_start_fn_ty<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             match tcx.hir.find(start_id) {
                 Some(hir_map::NodeItem(it)) => {
                     match it.node {
-                        hir::ItemFn(..,ref ps,_)
+                        hir::ItemFn(..,ref ps,_) => {
-                        if !ps.params.is_empty() => {
+                            if !ps.params.is_empty() {
-                            struct_span_err!(tcx.sess, ps.span, E0132,
+                                struct_span_err!(tcx.sess, ps.span, E0132,
-                                "start function is not allowed to have type parameters")
+                                    "start function is not allowed to have type parameters")
-                                .span_label(ps.span,
+                                    .span_label(ps.span,
-                                            "start function cannot have type parameters")
+                                                "start function cannot have type parameters")
-                                .emit();
+                                    .emit();
-                            return;
+                                return;
+                            }
+                            if !ps.where_clause.predicates.is_empty() {
+                                struct_span_err!(tcx.sess, start_span, E0647,
+                                            "start function is not allowed to have a where clause")
+                                            .emit();
+                                return;
+                            }
                         }
                         _ => ()
                     }

src/test/ui/error-codes/E0646.rs

@@ -0,0 +1,11 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() where (): Copy {} //~ ERROR [E0646]

src/test/ui/error-codes/E0646.stderr

@@ -0,0 +1,9 @@
+error[E0646]: main function is not allowed to have a where clause
+  --> $DIR/E0646.rs:11:1
+   |
+LL | fn main() where (): Copy {} //~ ERROR [E0646]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0646`.

src/test/ui/error-codes/E0647.rs

@@ -0,0 +1,19 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![no_std]
+#![feature(start)]
+
+extern crate std;
+
+#[start]
+fn start(_: isize, _: *const *const u8) -> isize where (): Copy { //~ ERROR [E0647]
+    0
+}

src/test/ui/error-codes/E0647.stderr

@@ -0,0 +1,11 @@
+error[E0647]: start function is not allowed to have a where clause
+  --> $DIR/E0647.rs:17:1
+   |
+LL | / fn start(_: isize, _: *const *const u8) -> isize where (): Copy { //~ ERROR [E0647]
+LL | |     0
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0647`.

src/test/ui/issue-50714-1.rs

@@ -0,0 +1,22 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for issue 50714, make sure that this isn't a linker error.
+
+#![no_std]
+#![feature(start)]
+
+extern crate std;
+
+#[start]
+fn start(_: isize, _: *const *const u8) -> isize where fn(&()): Eq { //~ ERROR [E0647]
+    0
+}
+

src/test/ui/issue-50714-1.stderr

@@ -0,0 +1,11 @@
+error[E0647]: start function is not allowed to have a where clause
+  --> $DIR/issue-50714-1.rs:19:1
+   |
+LL | / fn start(_: isize, _: *const *const u8) -> isize where fn(&()): Eq { //~ ERROR [E0647]
+LL | |     0
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0647`.

src/test/ui/issue-50714.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for issue 50714, make sure that this isn't a linker error.
+
+fn main() where fn(&()): Eq {} //~ ERROR [E0646]
+

src/test/ui/issue-50714.stderr

@@ -0,0 +1,9 @@
+error[E0646]: main function is not allowed to have a where clause
+  --> $DIR/issue-50714.rs:13:1
+   |
+LL | fn main() where fn(&()): Eq {} //~ ERROR [E0646]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0646`.