Issue #51022: Improve E0131 message when lifetimes are involved.

src/librustc_typeck/lib.rs

@@ -188,10 +188,18 @@ fn check_main_fn_ty<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                         hir::ItemFn(.., ref generics, _) => {
                             let mut error = false;
                             if !generics.params.is_empty() {
-                                struct_span_err!(tcx.sess, generics.span, E0131,
-                                    "`main` function is not allowed to have type parameters")
-                                    .span_label(generics.span,
-                                                "`main` cannot have type parameters")
+                                let param_type = if generics.is_lt_parameterized() {
+                                    "lifetime"
+                                } else {
+                                    "type"
+                                };
+                                let msg =
+                                    format!("`main` function is not allowed to have {} parameters",
+                                            param_type);
+                                let label =
+                                    format!("`main` cannot have {} parameters", param_type);
+                                struct_span_err!(tcx.sess, generics.span, E0131, "{}", msg)
+                                    .span_label(generics.span, label)
                                     .emit();
                                 error = true;
                             }

src/test/compile-fail/issue-51022.rs

@@ -0,0 +1,12 @@
+// 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.
+
+// error-pattern: `main` function is not allowed to have lifetime parameters
+fn main<'a>() { }