resolve: Implement a lint for out-of-scope use of macro_rules

compiler/rustc_lint/messages.ftl

@@ -604,6 +604,9 @@ lint_opaque_hidden_inferred_bound_sugg = add this bound
 lint_or_patterns_back_compat = the meaning of the `pat` fragment specifier is changing in Rust 2021, which may affect this macro
     .suggestion = use pat_param to preserve semantics
 
+lint_out_of_scope_macro_calls = cannot find macro `{$path}` in this scope
+    .help = import `macro_rules` with `use` to make it callable above its definition
+
 lint_overflowing_bin_hex = literal out of range for `{$ty}`
     .negative_note = the literal `{$lit}` (decimal `{$dec}`) does not fit into the type `{$ty}`
     .negative_becomes_note = and the value `-{$lit}` will become `{$actually}{$ty}`

compiler/rustc_lint/src/context/diagnostics.rs

@@ -434,5 +434,8 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
             lints::InnerAttributeUnstable::CustomInnerAttribute
         }
         .decorate_lint(diag),
+        BuiltinLintDiag::OutOfScopeMacroCalls { path } => {
+            lints::OutOfScopeMacroCalls { path }.decorate_lint(diag)
+        }
     }
 }

compiler/rustc_lint/src/lints.rs

@@ -2911,3 +2911,10 @@ pub struct UnsafeAttrOutsideUnsafeSuggestion {
     #[suggestion_part(code = ")")]
     pub right: Span,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_out_of_scope_macro_calls)]
+#[help]
+pub struct OutOfScopeMacroCalls {
+    pub path: String,
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -4945,3 +4945,42 @@ declare_lint! {
         reference: "issue #123757 <https://github.com/rust-lang/rust/issues/123757>",
     };
 }
+
+declare_lint! {
+    /// The `out_of_scope_macro_calls` lint detects `macro_rules` called when they are not in scope,
+    /// above their definition, which may happen in key-value attributes.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #![doc = in_root!()]
+    ///
+    /// macro_rules! in_root { () => { "" } }
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The scope in which a `macro_rules` item is visible starts at that item and continues
+    /// below it. This is more similar to `let` than to other items, which are in scope both above
+    /// and below their definition.
+    /// Due to a bug `macro_rules` were accidentally in scope inside some key-value attributes
+    /// above their definition. The lint catches such cases.
+    /// To address the issue turn the `macro_rules` into a regularly scoped item by importing it
+    /// with `use`.
+    ///
+    /// This is a [future-incompatible] lint to transition this to a
+    /// hard error in the future.
+    ///
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub OUT_OF_SCOPE_MACRO_CALLS,
+    Warn,
+    "detects out of scope calls to `macro_rules` in key-value attributes",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
+        reference: "issue #124535 <https://github.com/rust-lang/rust/issues/124535>",
+    };
+}

compiler/rustc_lint_defs/src/lib.rs

@@ -744,6 +744,9 @@ pub enum BuiltinLintDiag {
     InnerAttributeUnstable {
         is_macro: bool,
     },
+    OutOfScopeMacroCalls {
+        path: String,
+    },
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -14,7 +14,7 @@ use crate::{Determinacy, ExternPreludeEntry, Finalize, Module, ModuleKind, Modul
 use crate::{NameBinding, NameBindingKind, ParentScope, PathResult, ResolutionError};
 use crate::{Resolver, ResolverArenas, Segment, ToNameBinding, Used, VisResolutionError};
 
-use rustc_ast::visit::{self, AssocCtxt, Visitor};
+use rustc_ast::visit::{self, AssocCtxt, Visitor, WalkItemKind};
 use rustc_ast::{self as ast, AssocItem, AssocItemKind, MetaItemKind, StmtKind};
 use rustc_ast::{Block, ForeignItem, ForeignItemKind, Impl, Item, ItemKind, NodeId};
 use rustc_attr as attr;
@@ -1313,7 +1313,17 @@ impl<'a, 'b, 'tcx> Visitor<'b> for BuildReducedGraphVisitor<'a, 'b, 'tcx> {
             _ => {
                 let orig_macro_rules_scope = self.parent_scope.macro_rules;
                 self.build_reduced_graph_for_item(item);
-                visit::walk_item(self, item);
+                match item.kind {
+                    ItemKind::Mod(..) => {
+                        // Visit attributes after items for backward compatibility.
+                        // This way they can use `macro_rules` defined later.
+                        self.visit_vis(&item.vis);
+                        self.visit_ident(item.ident);
+                        item.kind.walk(item, AssocCtxt::Trait, self);
+                        visit::walk_list!(self, visit_attribute, &item.attrs);
+                    }
+                    _ => visit::walk_item(self, item),
+                }
                 match item.kind {
                     ItemKind::Mod(..) if self.contains_macro_use(&item.attrs) => {
                         self.parent_scope.macro_rules
@@ -1514,7 +1524,10 @@ impl<'a, 'b, 'tcx> Visitor<'b> for BuildReducedGraphVisitor<'a, 'b, 'tcx> {
         if krate.is_placeholder {
             self.visit_invoc_in_module(krate.id);
         } else {
-            visit::walk_crate(self, krate);
+            // Visit attributes after items for backward compatibility.
+            // This way they can use `macro_rules` defined later.
+            visit::walk_list!(self, visit_item, &krate.items);
+            visit::walk_list!(self, visit_attribute, &krate.attrs);
             self.contains_macro_use(&krate.attrs);
         }
     }

compiler/rustc_resolve/src/macros.rs

@@ -5,7 +5,7 @@ use crate::errors::CannotDetermineMacroResolution;
 use crate::errors::{self, AddAsNonDerive, CannotFindIdentInThisScope};
 use crate::errors::{MacroExpectedFound, RemoveSurroundingDerive};
 use crate::Namespace::*;
-use crate::{BindingKey, BuiltinMacroState, Determinacy, MacroData, Used};
+use crate::{BindingKey, BuiltinMacroState, Determinacy, MacroData, NameBindingKind, Used};
 use crate::{DeriveData, Finalize, ParentScope, ResolutionError, Resolver, ScopeSet};
 use crate::{ModuleKind, ModuleOrUniformRoot, NameBinding, PathResult, Segment, ToNameBinding};
 use rustc_ast::expand::StrippedCfgItem;
@@ -18,15 +18,18 @@ use rustc_errors::{Applicability, StashKey};
 use rustc_expand::base::{Annotatable, DeriveResolution, Indeterminate, ResolverExpand};
 use rustc_expand::base::{SyntaxExtension, SyntaxExtensionKind};
 use rustc_expand::compile_declarative_macro;
-use rustc_expand::expand::{AstFragment, Invocation, InvocationKind, SupportsMacroExpansion};
+use rustc_expand::expand::{
+    AstFragment, AstFragmentKind, Invocation, InvocationKind, SupportsMacroExpansion,
+};
 use rustc_hir::def::{self, DefKind, Namespace, NonMacroAttrKind};
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
 use rustc_middle::middle::stability;
 use rustc_middle::ty::RegisteredTools;
 use rustc_middle::ty::{TyCtxt, Visibility};
-use rustc_session::lint::builtin::UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES;
-use rustc_session::lint::builtin::{LEGACY_DERIVE_HELPERS, SOFT_UNSTABLE};
-use rustc_session::lint::builtin::{UNUSED_MACROS, UNUSED_MACRO_RULES};
+use rustc_session::lint::builtin::{
+    LEGACY_DERIVE_HELPERS, OUT_OF_SCOPE_MACRO_CALLS, SOFT_UNSTABLE,
+    UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES, UNUSED_MACROS, UNUSED_MACRO_RULES,
+};
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::parse::feature_err;
 use rustc_span::edit_distance::edit_distance;
@@ -288,6 +291,16 @@ impl<'a, 'tcx> ResolverExpand for Resolver<'a, 'tcx> {
         let parent_scope = &ParentScope { derives, ..parent_scope };
         let supports_macro_expansion = invoc.fragment_kind.supports_macro_expansion();
         let node_id = invoc.expansion_data.lint_node_id;
+        // This is a heuristic, but it's good enough for the lint.
+        let looks_like_invoc_in_mod_inert_attr = self
+            .invocation_parents
+            .get(&invoc_id)
+            .or_else(|| self.invocation_parents.get(&eager_expansion_root))
+            .map(|&(mod_def_id, _)| mod_def_id)
+            .filter(|&mod_def_id| {
+                invoc.fragment_kind == AstFragmentKind::Expr
+                    && self.tcx.def_kind(mod_def_id) == DefKind::Mod
+            });
         let (ext, res) = self.smart_resolve_macro_path(
             path,
             kind,
@@ -298,6 +311,7 @@ impl<'a, 'tcx> ResolverExpand for Resolver<'a, 'tcx> {
             force,
             soft_custom_inner_attributes_gate(path, invoc),
             deleg_impl,
+            looks_like_invoc_in_mod_inert_attr,
         )?;
 
         let span = invoc.span();
@@ -520,6 +534,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         force: bool,
         soft_custom_inner_attributes_gate: bool,
         deleg_impl: Option<LocalDefId>,
+        invoc_in_mod_inert_attr: Option<LocalDefId>,
     ) -> Result<(Lrc<SyntaxExtension>, Res), Indeterminate> {
         let (ext, res) = match self.resolve_macro_or_delegation_path(
             path,
@@ -528,6 +543,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             true,
             force,
             deleg_impl,
+            invoc_in_mod_inert_attr.map(|def_id| (def_id, node_id)),
         ) {
             Ok((Some(ext), res)) => (ext, res),
             Ok((None, res)) => (self.dummy_ext(kind), res),
@@ -682,20 +698,21 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         trace: bool,
         force: bool,
     ) -> Result<(Option<Lrc<SyntaxExtension>>, Res), Determinacy> {
-        self.resolve_macro_or_delegation_path(path, kind, parent_scope, trace, force, None)
+        self.resolve_macro_or_delegation_path(path, kind, parent_scope, trace, force, None, None)
     }
 
     fn resolve_macro_or_delegation_path(
         &mut self,
-        path: &ast::Path,
+        ast_path: &ast::Path,
         kind: Option<MacroKind>,
         parent_scope: &ParentScope<'a>,
         trace: bool,
         force: bool,
         deleg_impl: Option<LocalDefId>,
+        invoc_in_mod_inert_attr: Option<(LocalDefId, NodeId)>,
     ) -> Result<(Option<Lrc<SyntaxExtension>>, Res), Determinacy> {
-        let path_span = path.span;
-        let mut path = Segment::from_path(path);
+        let path_span = ast_path.span;
+        let mut path = Segment::from_path(ast_path);
 
         // Possibly apply the macro helper hack
         if deleg_impl.is_none()
@@ -761,6 +778,12 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
 
             let res = binding.map(|binding| binding.res());
             self.prohibit_imported_non_macro_attrs(binding.ok(), res.ok(), path_span);
+            self.report_out_of_scope_macro_calls(
+                ast_path,
+                parent_scope,
+                invoc_in_mod_inert_attr,
+                binding.ok(),
+            );
             res
         };
 
@@ -1013,6 +1036,45 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
     }
 
+    fn report_out_of_scope_macro_calls(
+        &mut self,
+        path: &ast::Path,
+        parent_scope: &ParentScope<'a>,
+        invoc_in_mod_inert_attr: Option<(LocalDefId, NodeId)>,
+        binding: Option<NameBinding<'a>>,
+    ) {
+        if let Some((mod_def_id, node_id)) = invoc_in_mod_inert_attr
+            && let Some(binding) = binding
+            // This is a `macro_rules` itself, not some import.
+            && let NameBindingKind::Res(res) = binding.kind
+            && let Res::Def(DefKind::Macro(MacroKind::Bang), def_id) = res
+            // And the `macro_rules` is defined inside the attribute's module,
+            // so it cannot be in scope unless imported.
+            && self.tcx.is_descendant_of(def_id, mod_def_id.to_def_id())
+        {
+            // Try to resolve our ident ignoring `macro_rules` scopes.
+            // If such resolution is successful and gives the same result
+            // (e.g. if the macro is re-imported), then silence the lint.
+            let no_macro_rules = self.arenas.alloc_macro_rules_scope(MacroRulesScope::Empty);
+            let fallback_binding = self.early_resolve_ident_in_lexical_scope(
+                path.segments[0].ident,
+                ScopeSet::Macro(MacroKind::Bang),
+                &ParentScope { macro_rules: no_macro_rules, ..*parent_scope },
+                None,
+                false,
+                None,
+            );
+            if fallback_binding.ok().and_then(|b| b.res().opt_def_id()) != Some(def_id) {
+                self.tcx.sess.psess.buffer_lint(
+                    OUT_OF_SCOPE_MACRO_CALLS,
+                    path.span,
+                    node_id,
+                    BuiltinLintDiag::OutOfScopeMacroCalls { path: pprust::path_to_string(path) },
+                );
+            }
+        }
+    }
+
     pub(crate) fn check_reserved_macro_name(&mut self, ident: Ident, res: Res) {
         // Reserve some names that are not quite covered by the general check
         // performed on `Resolver::builtin_attrs`.

tests/ui/attributes/key-value-expansion-scope-pass.rs

@@ -0,0 +1,18 @@
+// Imports suppress the `out_of_scope_macro_calls` lint.
+
+//@ check-pass
+//@ edition:2018
+
+#![doc = in_root!()]
+
+macro_rules! in_root { () => { "" } }
+use in_root;
+
+mod macros_stay {
+    #![doc = in_mod!()]
+
+    macro_rules! in_mod { () => { "" } }
+    use in_mod;
+}
+
+fn main() {}

tests/ui/attributes/key-value-expansion-scope.rs

@@ -1,6 +1,8 @@
-#![doc = in_root!()] //~ ERROR cannot find macro `in_root` in this scope
+#![doc = in_root!()] //~ WARN cannot find macro `in_root` in this scope
+                     //~| WARN this was previously accepted by the compiler
 #![doc = in_mod!()] //~ ERROR cannot find macro `in_mod` in this scope
-#![doc = in_mod_escape!()] //~ ERROR cannot find macro `in_mod_escape` in this scope
+#![doc = in_mod_escape!()] //~ WARN cannot find macro `in_mod_escape` in this scope
+                           //~| WARN this was previously accepted by the compiler
 #![doc = in_block!()] //~ ERROR cannot find macro `in_block` in this scope
 
 #[doc = in_root!()] //~ ERROR cannot find macro `in_root` in this scope
@@ -16,8 +18,11 @@ fn before() {
 
 macro_rules! in_root { () => { "" } }
 
+#[doc = in_mod!()] //~ WARN cannot find macro `in_mod` in this scope
+                   //~| WARN this was previously accepted by the compiler
 mod macros_stay {
-    #![doc = in_mod!()] //~ ERROR cannot find macro `in_mod` in this scope
+    #![doc = in_mod!()] //~ WARN cannot find macro `in_mod` in this scope
+                        //~| WARN this was previously accepted by the compiler
 
     macro_rules! in_mod { () => { "" } }
 
@@ -28,8 +33,11 @@ mod macros_stay {
 }
 
 #[macro_use]
+#[doc = in_mod_escape!()] //~ WARN cannot find macro `in_mod_escape` in this scope
+                          //~| WARN this was previously accepted by the compiler
 mod macros_escape {
-    #![doc = in_mod_escape!()] //~ ERROR cannot find macro `in_mod_escape` in this scope
+    #![doc = in_mod_escape!()] //~ WARN cannot find macro `in_mod_escape` in this scope
+                               //~| WARN this was previously accepted by the compiler
 
     macro_rules! in_mod_escape { () => { "" } }
 
@@ -39,8 +47,9 @@ mod macros_escape {
     }
 }
 
+#[doc = in_block!()] //~ ERROR cannot find macro `in_block` in this scope
 fn block() {
-    #![doc = in_block!()]  //~ ERROR cannot find macro `in_block` in this scope
+    #![doc = in_block!()] //~ ERROR cannot find macro `in_block` in this scope
 
     macro_rules! in_block { () => { "" } }