Add deprecation lint for duplicated macro_exports

src/librustc/lint/builtin.rs

@@ -17,6 +17,7 @@
 use errors::{Applicability, DiagnosticBuilder};
 use lint::{LintPass, LateLintPass, LintArray};
 use session::Session;
+use syntax::ast;
 use syntax::codemap::Span;
 
 declare_lint! {
@@ -285,6 +286,12 @@ declare_lint! {
     "warns about duplicate associated type bindings in generics"
 }
 
+declare_lint! {
+    pub DUPLICATE_MACRO_EXPORTS,
+    Deny,
+    "detects duplicate macro exports"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -337,6 +344,7 @@ impl LintPass for HardwiredLints {
             ABSOLUTE_PATHS_NOT_STARTING_WITH_CRATE,
             UNSTABLE_NAME_COLLISIONS,
             DUPLICATE_ASSOCIATED_TYPE_BINDINGS,
+            DUPLICATE_MACRO_EXPORTS,
         )
     }
 }
@@ -348,6 +356,7 @@ pub enum BuiltinLintDiagnostics {
     Normal,
     BareTraitObject(Span, /* is_global */ bool),
     AbsPathWithModule(Span),
+    DuplicatedMacroExports(ast::Ident, Span, Span),
 }
 
 impl BuiltinLintDiagnostics {
@@ -380,6 +389,10 @@ impl BuiltinLintDiagnostics {
                 };
                 db.span_suggestion_with_applicability(span, "use `crate`", sugg, app);
             }
+            BuiltinLintDiagnostics::DuplicatedMacroExports(ident, earlier_span, later_span) => {
+                db.span_label(later_span, format!("`{}` already exported", ident));
+                db.span_note(earlier_span, "previous macro export is now shadowed");
+            }
         }
     }
 }

src/librustc_lint/lib.rs

@@ -212,6 +212,11 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             reference: "issue #35203 <https://github.com/rust-lang/rust/issues/35203>",
             edition: None,
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(DUPLICATE_MACRO_EXPORTS),
+            reference: "issue #35896 <https://github.com/rust-lang/rust/issues/35896>",
+            edition: Some(Edition::Edition2018),
+        },
         FutureIncompatibleInfo {
             id: LintId::of(SAFE_EXTERN_STATICS),
             reference: "issue #36247 <https://github.com/rust-lang/rust/issues/36247>",

src/librustc_resolve/resolve_imports.rs

@@ -18,13 +18,14 @@ use {names_to_string, module_to_string};
 use {resolve_error, ResolutionError};
 
 use rustc::ty;
-use rustc::lint::builtin::PUB_USE_OF_PRIVATE_EXTERN_CRATE;
+use rustc::lint::builtin::BuiltinLintDiagnostics;
+use rustc::lint::builtin::{DUPLICATE_MACRO_EXPORTS, PUB_USE_OF_PRIVATE_EXTERN_CRATE};
 use rustc::hir::def_id::{CRATE_DEF_INDEX, DefId};
 use rustc::hir::def::*;
 use rustc::session::DiagnosticMessageId;
 use rustc::util::nodemap::{FxHashMap, FxHashSet};
 
-use syntax::ast::{Ident, Name, NodeId};
+use syntax::ast::{Ident, Name, NodeId, CRATE_NODE_ID};
 use syntax::ext::base::Determinacy::{self, Determined, Undetermined};
 use syntax::ext::hygiene::Mark;
 use syntax::symbol::keywords;
@@ -974,7 +975,16 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
         if module as *const _ == self.graph_root as *const _ {
             let macro_exports = mem::replace(&mut self.macro_exports, Vec::new());
             for export in macro_exports.into_iter().rev() {
-                if exported_macro_names.insert(export.ident.modern(), export.span).is_none() {
+                if let Some(later_span) = exported_macro_names.insert(export.ident.modern(),
+                                                                      export.span) {
+                    self.session.buffer_lint_with_diagnostic(
+                        DUPLICATE_MACRO_EXPORTS,
+                        CRATE_NODE_ID,
+                        later_span,
+                        &format!("a macro named `{}` has already been exported", export.ident),
+                        BuiltinLintDiagnostics::DuplicatedMacroExports(
+                            export.ident, export.span, later_span));
+                } else {
                     reexports.push(export);
                 }
             }

src/test/run-pass/auxiliary/issue_38715.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(duplicate_macro_exports)]
+
 #[macro_export]
 macro_rules! foo { ($i:ident) => {} }
 

src/test/run-pass/auxiliary/two_macros_2.rs

@@ -0,0 +1,13 @@
+// Copyright 2015 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.
+
+macro_rules! macro_one { ($($t:tt)*) => ($($t)*) }
+
+macro_rules! macro_two { ($($t:tt)*) => ($($t)*) }

src/test/run-pass/mod_dir_path.rs

@@ -20,12 +20,12 @@ pub fn main() {
 
     #[path = "auxiliary"]
     mod foo {
-        mod two_macros;
+        mod two_macros_2;
     }
 
     #[path = "auxiliary"]
     mod bar {
-        macro_rules! m { () => { mod two_macros; } }
+        macro_rules! m { () => { mod two_macros_2; } }
         m!();
     }
 }

src/test/ui/issue-38715.rs

@@ -0,0 +1,16 @@
+// Copyright 2016 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.
+
+#[macro_export]
+macro_rules! foo { ($i:ident) => {} }
+
+#[macro_export]
+macro_rules! foo { () => {} } //~ ERROR a macro named `foo` has already been exported
+                              //~| WARN this was previously accepted

src/test/ui/issue-38715.stderr

@@ -0,0 +1,22 @@
+error: a macro named `foo` has already been exported
+  --> $DIR/issue-38715.rs:15:1
+   |
+LL | macro_rules! foo { () => {} } //~ ERROR a macro named `foo` has already been exported
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `foo` already exported
+   |
+   = note: #[deny(duplicate_macro_exports)] on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue #35896 <https://github.com/rust-lang/rust/issues/35896>
+note: previous macro export is now shadowed
+  --> $DIR/issue-38715.rs:12:1
+   |
+LL | macro_rules! foo { ($i:ident) => {} }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0601]: `main` function not found in crate `issue_38715`
+   |
+   = note: consider adding a `main` function to `$DIR/issue-38715.rs`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0601`.