Auto merge of #53778 - petrochenkov:shadrelax2, r=nikomatsakis

resolve: Relax shadowing restrictions on macro-expanded macros

Previously any macro-expanded macros weren't allowed to shadow macros from outer scopes.
Now only "more macro-expanded" macros cannot shadow "less macro-expanded" macros.
See comments to `fn may_appear_after` and added tests for more details and examples.

The functional changes are a21f6f588fc28c97533130ae44a6957b579ab58c and 46dd365ce9ca0a6b8653849b80267763c542842a, other commits are refactorings.

4 files changed, 246 insertions, 203 deletions

diff --git a/src/librustc_resolve/build_reduced_graph.rs b/src/librustc_resolve/build_reduced_graph.rs
index 92a9057d96e..63f643d7a29 100644
--- a/src/librustc_resolve/build_reduced_graph.rs
+++ b/src/librustc_resolve/build_reduced_graph.rs
@@ -946,7 +946,7 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
 
 pub struct BuildReducedGraphVisitor<'a, 'b: 'a, 'c: 'b> {
     pub resolver: &'a mut Resolver<'b, 'c>,
-    pub legacy_scope: LegacyScope<'b>,
+    pub current_legacy_scope: LegacyScope<'b>,
     pub expansion: Mark,
 }
 
@@ -956,7 +956,8 @@ impl<'a, 'b, 'cl> BuildReducedGraphVisitor<'a, 'b, 'cl> {
         self.resolver.current_module.unresolved_invocations.borrow_mut().insert(mark);
         let invocation = self.resolver.invocations[&mark];
         invocation.module.set(self.resolver.current_module);
-        invocation.legacy_scope.set(self.legacy_scope);
+        invocation.parent_legacy_scope.set(self.current_legacy_scope);
+        invocation.output_legacy_scope.set(self.current_legacy_scope);
         invocation
     }
 }
@@ -982,29 +983,30 @@ impl<'a, 'b, 'cl> Visitor<'a> for BuildReducedGraphVisitor<'a, 'b, 'cl> {
     fn visit_item(&mut self, item: &'a Item) {
         let macro_use = match item.node {
             ItemKind::MacroDef(..) => {
-                self.resolver.define_macro(item, self.expansion, &mut self.legacy_scope);
+                self.resolver.define_macro(item, self.expansion, &mut self.current_legacy_scope);
                 return
             }
             ItemKind::Mac(..) => {
-                self.legacy_scope = LegacyScope::Expansion(self.visit_invoc(item.id));
+                self.current_legacy_scope = LegacyScope::Invocation(self.visit_invoc(item.id));
                 return
             }
             ItemKind::Mod(..) => self.resolver.contains_macro_use(&item.attrs),
             _ => false,
         };
 
-        let (parent, legacy_scope) = (self.resolver.current_module, self.legacy_scope);
+        let orig_current_module = self.resolver.current_module;
+        let orig_current_legacy_scope = self.current_legacy_scope;
         self.resolver.build_reduced_graph_for_item(item, self.expansion);
         visit::walk_item(self, item);
-        self.resolver.current_module = parent;
+        self.resolver.current_module = orig_current_module;
         if !macro_use {
-            self.legacy_scope = legacy_scope;
+            self.current_legacy_scope = orig_current_legacy_scope;
         }
     }
 
     fn visit_stmt(&mut self, stmt: &'a ast::Stmt) {
         if let ast::StmtKind::Mac(..) = stmt.node {
-            self.legacy_scope = LegacyScope::Expansion(self.visit_invoc(stmt.id));
+            self.current_legacy_scope = LegacyScope::Invocation(self.visit_invoc(stmt.id));
         } else {
             visit::walk_stmt(self, stmt);
         }
@@ -1021,11 +1023,12 @@ impl<'a, 'b, 'cl> Visitor<'a> for BuildReducedGraphVisitor<'a, 'b, 'cl> {
     }
 
     fn visit_block(&mut self, block: &'a Block) {
-        let (parent, legacy_scope) = (self.resolver.current_module, self.legacy_scope);
+        let orig_current_module = self.resolver.current_module;
+        let orig_current_legacy_scope = self.current_legacy_scope;
         self.resolver.build_reduced_graph_for_block(block, self.expansion);
         visit::walk_block(self, block);
-        self.resolver.current_module = parent;
-        self.legacy_scope = legacy_scope;
+        self.resolver.current_module = orig_current_module;
+        self.current_legacy_scope = orig_current_legacy_scope;
     }
 
     fn visit_trait_item(&mut self, item: &'a TraitItem) {
diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index 453e7421ff1..5cb615554ee 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -1177,9 +1177,7 @@ struct UseError<'a> {
 }
 
 struct AmbiguityError<'a> {
-    span: Span,
-    name: Name,
-    lexical: bool,
+    ident: Ident,
     b1: &'a NameBinding<'a>,
     b2: &'a NameBinding<'a>,
 }
@@ -1283,6 +1281,26 @@ impl<'a> NameBinding<'a> {
     fn descr(&self) -> &'static str {
         if self.is_extern_crate() { "extern crate" } else { self.def().kind_name() }
     }
+
+    // Suppose that we resolved macro invocation with `invoc_id` to binding `binding` at some
+    // expansion round `max(invoc_id, binding)` when they both emerged from macros.
+    // Then this function returns `true` if `self` may emerge from a macro *after* that
+    // in some later round and screw up our previously found resolution.
+    // See more detailed explanation in
+    // https://github.com/rust-lang/rust/pull/53778#issuecomment-419224049
+    fn may_appear_after(&self, invoc_id: Mark, binding: &NameBinding) -> bool {
+        // self > max(invoc_id, binding) => !(self <= invoc_id || self <= binding)
+        // Expansions are partially ordered, so "may appear after" is an inversion of
+        // "certainly appears before or simultaneously" and includes unordered cases.
+        let self_parent_expansion = self.expansion;
+        let other_parent_expansion = binding.expansion;
+        let invoc_parent_expansion = invoc_id.parent();
+        let certainly_before_other_or_simultaneously =
+            other_parent_expansion.is_descendant_of(self_parent_expansion);
+        let certainly_before_invoc_or_simultaneously =
+            invoc_parent_expansion.is_descendant_of(self_parent_expansion);
+        !(certainly_before_other_or_simultaneously || certainly_before_invoc_or_simultaneously)
+    }
 }
 
 /// Interns the names of the primitive types.
@@ -1416,8 +1434,6 @@ pub struct Resolver<'a, 'b: 'a> {
     proc_mac_errors: Vec<macros::ProcMacError>,
     /// crate-local macro expanded `macro_export` referred to by a module-relative path
     macro_expanded_macro_export_errors: BTreeSet<(Span, Span)>,
-    /// macro-expanded `macro_rules` shadowing existing macros
-    disallowed_shadowing: Vec<&'a LegacyBinding<'a>>,
 
     arenas: &'a ResolverArenas<'a>,
     dummy_binding: &'a NameBinding<'a>,
@@ -1729,7 +1745,6 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
             ambiguity_errors: Vec::new(),
             use_injections: Vec::new(),
             proc_mac_errors: Vec::new(),
-            disallowed_shadowing: Vec::new(),
             macro_expanded_macro_export_errors: BTreeSet::new(),
 
             arenas,
@@ -1815,7 +1830,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         self.arenas.alloc_module(module)
     }
 
-    fn record_use(&mut self, ident: Ident, ns: Namespace, binding: &'a NameBinding<'a>, span: Span)
+    fn record_use(&mut self, ident: Ident, ns: Namespace, binding: &'a NameBinding<'a>)
                   -> bool /* true if an error was reported */ {
         match binding.kind {
             NameBindingKind::Import { directive, binding, ref used }
@@ -1824,13 +1839,11 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
                 directive.used.set(true);
                 self.used_imports.insert((directive.id, ns));
                 self.add_to_glob_map(directive.id, ident);
-                self.record_use(ident, ns, binding, span)
+                self.record_use(ident, ns, binding)
             }
             NameBindingKind::Import { .. } => false,
             NameBindingKind::Ambiguity { b1, b2 } => {
-                self.ambiguity_errors.push(AmbiguityError {
-                    span, name: ident.name, lexical: false, b1, b2,
-                });
+                self.ambiguity_errors.push(AmbiguityError { ident, b1, b2 });
                 true
             }
             _ => false
@@ -2850,7 +2863,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
                             Def::Const(..) if is_syntactic_ambiguity => {
                                 // Disambiguate in favor of a unit struct/variant
                                 // or constant pattern.
-                                self.record_use(ident, ValueNS, binding.unwrap(), ident.span);
+                                self.record_use(ident, ValueNS, binding.unwrap());
                                 Some(PathResolution::new(def))
                             }
                             Def::StructCtor(..) | Def::VariantCtor(..) |
@@ -3484,6 +3497,20 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         path_span: Span,
         crate_lint: CrateLint,
     ) -> PathResult<'a> {
+        self.resolve_path_with_invoc_id(base_module, path, opt_ns, Mark::root(),
+                                        record_used, path_span, crate_lint)
+    }
+
+    fn resolve_path_with_invoc_id(
+        &mut self,
+        base_module: Option<ModuleOrUniformRoot<'a>>,
+        path: &[Ident],
+        opt_ns: Option<Namespace>, // `None` indicates a module path
+        invoc_id: Mark,
+        record_used: bool,
+        path_span: Span,
+        crate_lint: CrateLint,
+    ) -> PathResult<'a> {
         let mut module = base_module;
         let mut allow_super = true;
         let mut second_binding = None;
@@ -3572,8 +3599,9 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
                 self.resolve_ident_in_module(module, ident, ns, record_used, path_span)
             } else if opt_ns == Some(MacroNS) {
                 assert!(ns == TypeNS);
-                self.resolve_lexical_macro_path_segment(ident, ns, record_used, record_used,
-                                                        false, path_span).map(|(b, _)| b)
+                self.resolve_lexical_macro_path_segment(ident, ns, invoc_id, record_used,
+                                                        record_used, false, path_span)
+                                                        .map(|(binding, _)| binding)
             } else {
                 let record_used_id =
                     if record_used { crate_lint.node_id().or(Some(CRATE_NODE_ID)) } else { None };
@@ -4514,35 +4542,33 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         vis.is_accessible_from(module.normal_ancestor_id, self)
     }
 
-    fn report_ambiguity_error(
-        &self, name: Name, span: Span, _lexical: bool,
-        def1: Def, is_import1: bool, is_glob1: bool, from_expansion1: bool, span1: Span,
-        def2: Def, is_import2: bool, _is_glob2: bool, _from_expansion2: bool, span2: Span,
-    ) {
+    fn report_ambiguity_error(&self, ident: Ident, b1: &NameBinding, b2: &NameBinding) {
         let participle = |is_import: bool| if is_import { "imported" } else { "defined" };
-        let msg1 = format!("`{}` could refer to the name {} here", name, participle(is_import1));
+        let msg1 =
+            format!("`{}` could refer to the name {} here", ident, participle(b1.is_import()));
         let msg2 =
-            format!("`{}` could also refer to the name {} here", name, participle(is_import2));
-        let note = if from_expansion1 {
-            Some(if let Def::Macro(..) = def1 {
+            format!("`{}` could also refer to the name {} here", ident, participle(b2.is_import()));
+        let note = if b1.expansion != Mark::root() {
+            Some(if let Def::Macro(..) = b1.def() {
                 format!("macro-expanded {} do not shadow",
-                        if is_import1 { "macro imports" } else { "macros" })
+                        if b1.is_import() { "macro imports" } else { "macros" })
             } else {
                 format!("macro-expanded {} do not shadow when used in a macro invocation path",
-                        if is_import1 { "imports" } else { "items" })
+                        if b1.is_import() { "imports" } else { "items" })
             })
-        } else if is_glob1 {
-            Some(format!("consider adding an explicit import of `{}` to disambiguate", name))
+        } else if b1.is_glob_import() {
+            Some(format!("consider adding an explicit import of `{}` to disambiguate", ident))
         } else {
             None
         };
 
-        let mut err = struct_span_err!(self.session, span, E0659, "`{}` is ambiguous", name);
-        err.span_note(span1, &msg1);
-        match def2 {
-            Def::Macro(..) if span2.is_dummy() =>
-                err.note(&format!("`{}` is also a builtin macro", name)),
-            _ => err.span_note(span2, &msg2),
+        let mut err = struct_span_err!(self.session, ident.span, E0659, "`{}` is ambiguous", ident);
+        err.span_label(ident.span, "ambiguous name");
+        err.span_note(b1.span, &msg1);
+        match b2.def() {
+            Def::Macro(..) if b2.span.is_dummy() =>
+                err.note(&format!("`{}` is also a builtin macro", ident)),
+            _ => err.span_note(b2.span, &msg2),
         };
         if let Some(note) = note {
             err.note(&note);
@@ -4551,7 +4577,6 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
     }
 
     fn report_errors(&mut self, krate: &Crate) {
-        self.report_shadowing_errors();
         self.report_with_use_injections(krate);
         self.report_proc_macro_import(krate);
         let mut reported_spans = FxHashSet();
@@ -4567,15 +4592,9 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
             );
         }
 
-        for &AmbiguityError { span, name, b1, b2, lexical } in &self.ambiguity_errors {
-            if reported_spans.insert(span) {
-                self.report_ambiguity_error(
-                    name, span, lexical,
-                    b1.def(), b1.is_import(), b1.is_glob_import(),
-                    b1.expansion != Mark::root(), b1.span,
-                    b2.def(), b2.is_import(), b2.is_glob_import(),
-                    b2.expansion != Mark::root(), b2.span,
-                );
+        for &AmbiguityError { ident, b1, b2 } in &self.ambiguity_errors {
+            if reported_spans.insert(ident.span) {
+                self.report_ambiguity_error(ident, b1, b2);
             }
         }
 
@@ -4595,20 +4614,6 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         }
     }
 
-    fn report_shadowing_errors(&mut self) {
-        let mut reported_errors = FxHashSet();
-        for binding in replace(&mut self.disallowed_shadowing, Vec::new()) {
-            if self.resolve_legacy_scope(&binding.parent, binding.ident, false).is_some() &&
-               reported_errors.insert((binding.ident, binding.span)) {
-                let msg = format!("`{}` is already in scope", binding.ident);
-                self.session.struct_span_err(binding.span, &msg)
-                    .note("macro-expanded `macro_rules!`s may not shadow \
-                           existing macros (see RFC 1560)")
-                    .emit();
-            }
-        }
-    }
-
     fn report_conflict<'b>(&mut self,
                        parent: Module,
                        ident: Ident,
diff --git a/src/librustc_resolve/macros.rs b/src/librustc_resolve/macros.rs
index fc6e0eb3b7f..93874ee0e85 100644
--- a/src/librustc_resolve/macros.rs
+++ b/src/librustc_resolve/macros.rs
@@ -45,18 +45,21 @@ use std::mem;
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::small_vec::ExpectOne;
 
+#[derive(Clone, Copy)]
 crate struct FromPrelude(bool);
-crate struct FromExpansion(bool);
 
 #[derive(Clone)]
 pub struct InvocationData<'a> {
-    pub module: Cell<Module<'a>>,
-    pub def_index: DefIndex,
-    // The scope in which the invocation path is resolved.
-    pub legacy_scope: Cell<LegacyScope<'a>>,
-    // The smallest scope that includes this invocation's expansion,
-    // or `Empty` if this invocation has not been expanded yet.
-    pub expansion: Cell<LegacyScope<'a>>,
+    def_index: DefIndex,
+    /// Module in which the macro was invoked.
+    crate module: Cell<Module<'a>>,
+    /// Legacy scope in which the macro was invoked.
+    /// The invocation path is resolved in this scope.
+    crate parent_legacy_scope: Cell<LegacyScope<'a>>,
+    /// Legacy scope *produced* by expanding this macro invocation,
+    /// includes all the macro_rules items, other invocations, etc generated by it.
+    /// Set to the parent scope if the macro is not expanded yet (as if the macro produced nothing).
+    crate output_legacy_scope: Cell<LegacyScope<'a>>,
 }
 
 impl<'a> InvocationData<'a> {
@@ -64,31 +67,38 @@ impl<'a> InvocationData<'a> {
         InvocationData {
             module: Cell::new(graph_root),
             def_index: CRATE_DEF_INDEX,
-            legacy_scope: Cell::new(LegacyScope::Empty),
-            expansion: Cell::new(LegacyScope::Empty),
+            parent_legacy_scope: Cell::new(LegacyScope::Empty),
+            output_legacy_scope: Cell::new(LegacyScope::Empty),
         }
     }
 }
 
+/// Binding produced by a `macro_rules` item.
+/// Not modularized, can shadow previous legacy bindings, etc.
+pub struct LegacyBinding<'a> {
+    binding: &'a NameBinding<'a>,
+    /// Legacy scope into which the `macro_rules` item was planted.
+    parent_legacy_scope: LegacyScope<'a>,
+    ident: Ident,
+}
+
+/// Scope introduced by a `macro_rules!` macro.
+/// Starts at the macro's definition and ends at the end of the macro's parent module
+/// (named or unnamed), or even further if it escapes with `#[macro_use]`.
+/// Some macro invocations need to introduce legacy scopes too because they
+/// potentially can expand into macro definitions.
 #[derive(Copy, Clone)]
 pub enum LegacyScope<'a> {
+    /// Created when invocation data is allocated in the arena,
+    /// must be replaced with a proper scope later.
+    Uninitialized,
+    /// Empty "root" scope at the crate start containing no names.
     Empty,
-    Invocation(&'a InvocationData<'a>), // The scope of the invocation, not including its expansion
-    Expansion(&'a InvocationData<'a>), // The scope of the invocation, including its expansion
+    /// Scope introduced by a `macro_rules!` macro definition.
     Binding(&'a LegacyBinding<'a>),
-}
-
-pub struct LegacyBinding<'a> {
-    pub parent: Cell<LegacyScope<'a>>,
-    pub ident: Ident,
-    def_id: DefId,
-    pub span: Span,
-}
-
-impl<'a> LegacyBinding<'a> {
-    fn def(&self) -> Def {
-        Def::Macro(self.def_id, MacroKind::Bang)
-    }
+    /// Scope introduced by a macro invocation that can potentially
+    /// create a `macro_rules!` macro definition.
+    Invocation(&'a InvocationData<'a>),
 }
 
 pub struct ProcMacError {
@@ -110,8 +120,8 @@ impl<'a, 'crateloader: 'a> base::Resolver for Resolver<'a, 'crateloader> {
         self.invocations.insert(mark, self.arenas.alloc_invocation_data(InvocationData {
             module: Cell::new(module),
             def_index: module.def_id().unwrap().index,
-            legacy_scope: Cell::new(LegacyScope::Empty),
-            expansion: Cell::new(LegacyScope::Empty),
+            parent_legacy_scope: Cell::new(LegacyScope::Empty),
+            output_legacy_scope: Cell::new(LegacyScope::Empty),
         }));
         mark
     }
@@ -183,11 +193,11 @@ impl<'a, 'crateloader: 'a> base::Resolver for Resolver<'a, 'crateloader> {
         }
         let mut visitor = BuildReducedGraphVisitor {
             resolver: self,
-            legacy_scope: LegacyScope::Invocation(invocation),
+            current_legacy_scope: invocation.parent_legacy_scope.get(),
             expansion: mark,
         };
         fragment.visit_with(&mut visitor);
-        invocation.expansion.set(visitor.legacy_scope);
+        invocation.output_legacy_scope.set(visitor.current_legacy_scope);
     }
 
     fn add_builtin(&mut self, ident: ast::Ident, ext: Lrc<SyntaxExtension>) {
@@ -437,12 +447,12 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         Ok((def, self.get_macro(def)))
     }
 
-    pub fn resolve_macro_to_def_inner(&mut self, path: &ast::Path, kind: MacroKind, scope: Mark,
+    pub fn resolve_macro_to_def_inner(&mut self, path: &ast::Path, kind: MacroKind, invoc_id: Mark,
                                       derives_in_scope: &[ast::Path], force: bool)
                                       -> Result<Def, Determinacy> {
         let ast::Path { ref segments, span } = *path;
         let mut path: Vec<_> = segments.iter().map(|seg| seg.ident).collect();
-        let invocation = self.invocations[&scope];
+        let invocation = self.invocations[&invoc_id];
         let module = invocation.module.get();
         self.current_module = if module.is_trait() { module.parent.unwrap() } else { module };
 
@@ -454,8 +464,8 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         }
 
         if path.len() > 1 {
-            let res = self.resolve_path(None, &path, Some(MacroNS), false, span, CrateLint::No);
-            let def = match res {
+            let def = match self.resolve_path_with_invoc_id(None, &path, Some(MacroNS), invoc_id,
+                                                            false, span, CrateLint::No) {
                 PathResult::NonModule(path_res) => match path_res.base_def() {
                     Def::Err => Err(Determinacy::Determined),
                     def @ _ => {
@@ -486,11 +496,13 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
             }
         }
 
-        let legacy_resolution = self.resolve_legacy_scope(&invocation.legacy_scope, path[0], false);
-        let result = if let Some((legacy_binding, _)) = legacy_resolution {
+        let legacy_resolution = self.resolve_legacy_scope(
+            path[0], invoc_id, invocation.parent_legacy_scope.get(), false
+        );
+        let result = if let Some(legacy_binding) = legacy_resolution {
             Ok(legacy_binding.def())
         } else {
-            match self.resolve_lexical_macro_path_segment(path[0], MacroNS, false, force,
+            match self.resolve_lexical_macro_path_segment(path[0], MacroNS, invoc_id, false, force,
                                                           kind == MacroKind::Attr, span) {
                 Ok((binding, _)) => Ok(binding.def_ignoring_ambiguity()),
                 Err(Determinacy::Undetermined) => return Err(Determinacy::Undetermined),
@@ -502,7 +514,7 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         };
 
         self.current_module.nearest_item_scope().legacy_macro_resolutions.borrow_mut()
-            .push((scope, path[0], kind, result.ok()));
+            .push((invoc_id, path[0], kind, result.ok()));
 
         if let Ok(Def::NonMacroAttr(NonMacroAttrKind::Custom)) = result {} else {
             return result;
@@ -521,7 +533,7 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         enum ConvertToDeriveHelper { Yes, No, DontKnow }
         let mut convert_to_derive_helper = ConvertToDeriveHelper::No;
         for derive in derives_in_scope {
-            match self.resolve_macro_path(derive, MacroKind::Derive, scope, &[], force) {
+            match self.resolve_macro_path(derive, MacroKind::Derive, invoc_id, &[], force) {
                 Ok(ext) => if let SyntaxExtension::ProcMacroDerive(_, ref inert_attrs, _) = *ext {
                     if inert_attrs.contains(&path[0].name) {
                         convert_to_derive_helper = ConvertToDeriveHelper::Yes;
@@ -549,10 +561,11 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         &mut self,
         mut ident: Ident,
         ns: Namespace,
+        invoc_id: Mark,
         record_used: bool,
         force: bool,
         is_attr: bool,
-        path_span: Span
+        path_span: Span,
     ) -> Result<(&'a NameBinding<'a>, FromPrelude), Determinacy> {
         // General principles:
         // 1. Not controlled (user-defined) names should have higher priority than controlled names
@@ -585,15 +598,18 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         assert!(force || !record_used); // `record_used` implies `force`
         ident = ident.modern();
 
-        // Names from inner scope that can't shadow names from outer scopes, e.g.
-        // mod m { ... }
+        // This is *the* result, resolution from the scope closest to the resolved identifier.
+        // However, sometimes this result is "weak" because it comes from a glob import or
+        // a macro expansion, and in this case it cannot shadow names from outer scopes, e.g.
+        // mod m { ... } // solution in outer scope
         // {
-        //     use prefix::*; // if this imports another `m`, then it can't shadow the outer `m`
-        //                    // and we have and ambiguity error
+        //     use prefix::*; // imports another `m` - innermost solution
+        //                    // weak, cannot shadow the outer `m`, need to report ambiguity error
         //     m::mac!();
         // }
-        // This includes names from globs and from macro expansions.
-        let mut potentially_ambiguous_result: Option<(&NameBinding, FromPrelude)> = None;
+        // So we have to save the innermost solution and continue searching in outer scopes
+        // to detect potential ambiguities.
+        let mut innermost_result: Option<(&NameBinding, FromPrelude)> = None;
 
         enum WhereToResolve<'a> {
             Module(Module<'a>),
@@ -736,33 +752,24 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                         return Ok(result);
                     }
 
-                    // Found a solution that is ambiguous with a previously found solution.
-                    // Push an ambiguity error for later reporting and
-                    // return something for better recovery.
-                    if let Some(previous_result) = potentially_ambiguous_result {
-                        if result.0.def() != previous_result.0.def() {
+                    if let Some(innermost_result) = innermost_result {
+                        // Found another solution, if the first one was "weak", report an error.
+                        if result.0.def() != innermost_result.0.def() &&
+                           (innermost_result.0.is_glob_import() ||
+                            innermost_result.0.may_appear_after(invoc_id, result.0)) {
                             self.ambiguity_errors.push(AmbiguityError {
-                                span: path_span,
-                                name: ident.name,
-                                b1: previous_result.0,
+                                ident,
+                                b1: innermost_result.0,
                                 b2: result.0,
-                                lexical: true,
                             });
-                            return Ok(previous_result);
+                            return Ok(innermost_result);
                         }
+                    } else {
+                        // Found the first solution.
+                        innermost_result = Some(result);
                     }
 
-                    // Found a solution that's not an ambiguity yet, but is "suspicious" and
-                    // can participate in ambiguities later on.
-                    // Remember it and go search for other solutions in outer scopes.
-                    if result.0.is_glob_import() || result.0.expansion != Mark::root() {
-                        potentially_ambiguous_result = Some(result);
-
-                        continue_search!();
-                    }
-
-                    // Found a solution that can't be ambiguous, great success.
-                    return Ok(result);
+                    continue_search!();
                 },
                 Err(Determinacy::Determined) => {
                     continue_search!();
@@ -771,9 +778,9 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
             }
         }
 
-        // Previously found potentially ambiguous result turned out to not be ambiguous after all.
-        if let Some(previous_result) = potentially_ambiguous_result {
-            return Ok(previous_result);
+        // The first found solution was the only one, return it.
+        if let Some(innermost_result) = innermost_result {
+            return Ok(innermost_result);
         }
 
         let determinacy = Determinacy::determined(force);
@@ -791,45 +798,79 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         }
     }
 
-    crate fn resolve_legacy_scope(&mut self,
-                                  mut scope: &'a Cell<LegacyScope<'a>>,
-                                  ident: Ident,
-                                  record_used: bool)
-                                  -> Option<(&'a LegacyBinding<'a>, FromExpansion)> {
+    fn resolve_legacy_scope(&mut self,
+                            ident: Ident,
+                            invoc_id: Mark,
+                            invoc_parent_legacy_scope: LegacyScope<'a>,
+                            record_used: bool)
+                            -> Option<&'a NameBinding<'a>> {
         let ident = ident.modern();
-        let mut relative_depth: u32 = 0;
+
+        // This is *the* result, resolution from the scope closest to the resolved identifier.
+        // However, sometimes this result is "weak" because it comes from a macro expansion,
+        // and in this case it cannot shadow names from outer scopes, e.g.
+        // macro_rules! m { ... } // solution in outer scope
+        // {
+        //     define_m!(); // generates another `macro_rules! m` - innermost solution
+        //                  // weak, cannot shadow the outer `m`, need to report ambiguity error
+        //     m!();
+        // }
+        // So we have to save the innermost solution and continue searching in outer scopes
+        // to detect potential ambiguities.
+        let mut innermost_result: Option<&NameBinding> = None;
+
+        // Go through all the scopes and try to resolve the name.
+        let mut where_to_resolve = invoc_parent_legacy_scope;
         loop {
-            match scope.get() {
-                LegacyScope::Empty => break,
-                LegacyScope::Expansion(invocation) => {
-                    match invocation.expansion.get() {
-                        LegacyScope::Invocation(_) => scope.set(invocation.legacy_scope.get()),
-                        LegacyScope::Empty => {
-                            scope = &invocation.legacy_scope;
-                        }
-                        _ => {
-                            relative_depth += 1;
-                            scope = &invocation.expansion;
-                        }
+            let result = match where_to_resolve {
+                LegacyScope::Binding(legacy_binding) if ident == legacy_binding.ident =>
+                    Some(legacy_binding.binding),
+                _ => None,
+            };
+
+            macro_rules! continue_search { () => {
+                where_to_resolve = match where_to_resolve {
+                    LegacyScope::Empty => break, // nowhere else to search
+                    LegacyScope::Binding(binding) => binding.parent_legacy_scope,
+                    LegacyScope::Invocation(invocation) => invocation.output_legacy_scope.get(),
+                    LegacyScope::Uninitialized => unreachable!(),
+                };
+
+                continue;
+            }}
+
+            match result {
+                Some(result) => {
+                    if !record_used {
+                        return Some(result);
                     }
-                }
-                LegacyScope::Invocation(invocation) => {
-                    relative_depth = relative_depth.saturating_sub(1);
-                    scope = &invocation.legacy_scope;
-                }
-                LegacyScope::Binding(potential_binding) => {
-                    if potential_binding.ident == ident {
-                        if record_used && relative_depth > 0 {
-                            self.disallowed_shadowing.push(potential_binding);
+
+                    if let Some(innermost_result) = innermost_result {
+                        // Found another solution, if the first one was "weak", report an error.
+                        if result.def() != innermost_result.def() &&
+                           innermost_result.may_appear_after(invoc_id, result) {
+                            self.ambiguity_errors.push(AmbiguityError {
+                                ident,
+                                b1: innermost_result,
+                                b2: result,
+                            });
+                            return Some(innermost_result);
                         }
-                        return Some((potential_binding, FromExpansion(relative_depth > 0)));
+                    } else {
+                        // Found the first solution.
+                        innermost_result = Some(result);
                     }
-                    scope = &potential_binding.parent;
+
+                    continue_search!();
                 }
-            };
+                None => {
+                    continue_search!();
+                }
+            }
         }
 
-        None
+        // The first found solution was the only one (or there was no solution at all), return it.
+        innermost_result
     }
 
     pub fn finalize_current_module_macro_resolutions(&mut self) {
@@ -844,17 +885,19 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
             }
         }
 
-        for &(mark, ident, kind, def) in module.legacy_macro_resolutions.borrow().iter() {
+        for &(invoc_id, ident, kind, def) in module.legacy_macro_resolutions.borrow().iter() {
             let span = ident.span;
-            let legacy_scope = &self.invocations[&mark].legacy_scope;
-            let legacy_resolution = self.resolve_legacy_scope(legacy_scope, ident, true);
-            let resolution = self.resolve_lexical_macro_path_segment(ident, MacroNS, true, true,
-                                                                     kind == MacroKind::Attr, span);
+            let invocation = self.invocations[&invoc_id];
+            let legacy_resolution = self.resolve_legacy_scope(
+                ident, invoc_id, invocation.parent_legacy_scope.get(), true
+            );
+            let resolution = self.resolve_lexical_macro_path_segment(
+                ident, MacroNS, invoc_id, true, true, kind == MacroKind::Attr, span
+            );
 
             let check_consistency = |this: &Self, new_def: Def| {
                 if let Some(def) = def {
-                    if this.ambiguity_errors.is_empty() && this.disallowed_shadowing.is_empty() &&
-                       new_def != def && new_def != Def::Err {
+                    if this.ambiguity_errors.is_empty() && new_def != def && new_def != Def::Err {
                         // Make sure compilation does not succeed if preferred macro resolution
                         // has changed after the macro had been expanded. In theory all such
                         // situations should be reported as ambiguity errors, so this is span-bug.
@@ -882,31 +925,23 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                     self.suggest_macro_name(&ident.as_str(), kind, &mut err, span);
                     err.emit();
                 },
-                (Some((legacy_binding, FromExpansion(from_expansion))),
-                 Ok((binding, FromPrelude(false)))) |
-                (Some((legacy_binding, FromExpansion(from_expansion @ true))),
-                 Ok((binding, FromPrelude(true)))) => {
-                    if legacy_binding.def() != binding.def_ignoring_ambiguity() {
-                        self.report_ambiguity_error(
-                            ident.name, span, true,
-                            legacy_binding.def(), false, false,
-                            from_expansion, legacy_binding.span,
-                            binding.def(), binding.is_import(), binding.is_glob_import(),
-                            binding.expansion != Mark::root(), binding.span,
-                        );
+                (Some(legacy_binding), Ok((binding, FromPrelude(from_prelude))))
+                        if !from_prelude || legacy_binding.may_appear_after(invoc_id, binding) => {
+                    if legacy_binding.def_ignoring_ambiguity() != binding.def_ignoring_ambiguity() {
+                        self.report_ambiguity_error(ident, legacy_binding, binding);
                     }
                 },
-                // OK, non-macro-expanded legacy wins over macro prelude even if defs are different
-                (Some((legacy_binding, FromExpansion(false))), Ok((_, FromPrelude(true)))) |
+                // OK, non-macro-expanded legacy wins over prelude even if defs are different
+                (Some(legacy_binding), Ok(_)) |
                 // OK, unambiguous resolution
-                (Some((legacy_binding, _)), Err(_)) => {
+                (Some(legacy_binding), Err(_)) => {
                     check_consistency(self, legacy_binding.def());
                 }
                 // OK, unambiguous resolution
                 (None, Ok((binding, FromPrelude(from_prelude)))) => {
                     check_consistency(self, binding.def_ignoring_ambiguity());
                     if from_prelude {
-                        self.record_use(ident, MacroNS, binding, span);
+                        self.record_use(ident, MacroNS, binding);
                         self.err_if_macro_use_proc_macro(ident.name, span, binding);
                     }
                 }
@@ -981,8 +1016,8 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                 arenas.alloc_invocation_data(InvocationData {
                     def_index: invoc.def_index,
                     module: Cell::new(graph_root),
-                    expansion: Cell::new(LegacyScope::Empty),
-                    legacy_scope: Cell::new(LegacyScope::Empty),
+                    parent_legacy_scope: Cell::new(LegacyScope::Uninitialized),
+                    output_legacy_scope: Cell::new(LegacyScope::Uninitialized),
                 })
             });
         };
@@ -997,7 +1032,7 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
     pub fn define_macro(&mut self,
                         item: &ast::Item,
                         expansion: Mark,
-                        legacy_scope: &mut LegacyScope<'a>) {
+                        current_legacy_scope: &mut LegacyScope<'a>) {
         self.local_macro_def_scopes.insert(item.id, self.current_module);
         let ident = item.ident;
         if ident.name == "macro_rules" {
@@ -1014,10 +1049,13 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         if def.legacy {
             let ident = ident.modern();
             self.macro_names.insert(ident);
-            *legacy_scope = LegacyScope::Binding(self.arenas.alloc_legacy_binding(LegacyBinding {
-                parent: Cell::new(*legacy_scope), ident: ident, def_id: def_id, span: item.span,
-            }));
             let def = Def::Macro(def_id, MacroKind::Bang);
+            let vis = ty::Visibility::Invisible; // Doesn't matter for legacy bindings
+            let binding = (def, vis, item.span, expansion).to_name_binding(self.arenas);
+            let legacy_binding = self.arenas.alloc_legacy_binding(LegacyBinding {
+                parent_legacy_scope: *current_legacy_scope, binding, ident
+            });
+            *current_legacy_scope = LegacyScope::Binding(legacy_binding);
             self.all_macros.insert(ident.name, def);
             if attr::contains_name(&item.attrs, "macro_export") {
                 let module = self.graph_root;
diff --git a/src/librustc_resolve/resolve_imports.rs b/src/librustc_resolve/resolve_imports.rs
index a72ee7ae379..f8767fa553f 100644
--- a/src/librustc_resolve/resolve_imports.rs
+++ b/src/librustc_resolve/resolve_imports.rs
@@ -242,22 +242,19 @@ impl<'a, 'crateloader> Resolver<'a, 'crateloader> {
         if record_used {
             if let Some(binding) = resolution.binding {
                 if let Some(shadowed_glob) = resolution.shadowed_glob {
-                    let name = ident.name;
                     // Forbid expanded shadowing to avoid time travel.
                     if restricted_shadowing &&
                        binding.expansion != Mark::root() &&
                        ns != MacroNS && // In MacroNS, `try_define` always forbids this shadowing
                        binding.def() != shadowed_glob.def() {
                         self.ambiguity_errors.push(AmbiguityError {
-                            span: path_span,
-                            name,
-                            lexical: false,
+                            ident,
                             b1: binding,
                             b2: shadowed_glob,
                         });
                     }
                 }
-                if self.record_use(ident, ns, binding, path_span) {
+                if self.record_use(ident, ns, binding) {
                     return Ok(self.dummy_binding);
                 }
                 if !self.is_accessible(binding.vis) {
@@ -937,7 +934,7 @@ impl<'a, 'b:'a, 'c: 'b> ImportResolver<'a, 'b, 'c> {
         self.per_ns(|this, ns| if !type_ns_only || ns == TypeNS {
             if let Ok(binding) = result[ns].get() {
                 all_ns_err = false;
-                if this.record_use(ident, ns, binding, directive.span) {
+                if this.record_use(ident, ns, binding) {
                     if let ModuleOrUniformRoot::Module(module) = module {
                         this.resolution(module, ident, ns).borrow_mut().binding =
                             Some(this.dummy_binding);