Implement a lint to warn about unused macro rules

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -816,6 +816,7 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
     span: Span,
 ) -> Result<&'ll Value, ()> {
     // macros for error handling:
+    #[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
     macro_rules! emit_error {
         ($msg: tt) => {
             emit_error!($msg, )
@@ -1144,6 +1145,7 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
         span: Span,
         args: &[OperandRef<'tcx, &'ll Value>],
     ) -> Result<&'ll Value, ()> {
+        #[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
         macro_rules! emit_error {
             ($msg: tt) => {
                 emit_error!($msg, )

compiler/rustc_expand/src/base.rs

@@ -887,6 +887,8 @@ pub trait ResolverExpand {
         force: bool,
     ) -> Result<Lrc<SyntaxExtension>, Indeterminate>;
 
+    fn record_macro_rule_usage(&mut self, mac_id: NodeId, rule_index: usize);
+
     fn check_unused_macros(&mut self);
 
     // Resolver interfaces for specific built-in macros.

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -156,13 +156,13 @@ impl<'a> ParserAnyMacro<'a> {
 }
 
 struct MacroRulesMacroExpander {
+    node_id: NodeId,
     name: Ident,
     span: Span,
     transparency: Transparency,
     lhses: Vec<Vec<MatcherLoc>>,
     rhses: Vec<mbe::TokenTree>,
     valid: bool,
-    is_local: bool,
 }
 
 impl TTMacroExpander for MacroRulesMacroExpander {
@@ -179,12 +179,12 @@ impl TTMacroExpander for MacroRulesMacroExpander {
             cx,
             sp,
             self.span,
+            self.node_id,
             self.name,
             self.transparency,
             input,
             &self.lhses,
             &self.rhses,
-            self.is_local,
         )
     }
 }
@@ -207,14 +207,17 @@ fn generic_extension<'cx, 'tt>(
     cx: &'cx mut ExtCtxt<'_>,
     sp: Span,
     def_span: Span,
+    node_id: NodeId,
     name: Ident,
     transparency: Transparency,
     arg: TokenStream,
     lhses: &'tt [Vec<MatcherLoc>],
     rhses: &'tt [mbe::TokenTree],
-    is_local: bool,
 ) -> Box<dyn MacResult + 'cx> {
     let sess = &cx.sess.parse_sess;
+    // Macros defined in the current crate have a real node id,
+    // whereas macros from an external crate have a dummy id.
+    let is_local = node_id != DUMMY_NODE_ID;
 
     if cx.trace_macros() {
         let msg = format!("expanding `{}! {{ {} }}`", name, pprust::tts_to_string(&arg));
@@ -296,6 +299,10 @@ fn generic_extension<'cx, 'tt>(
                 let mut p = Parser::new(sess, tts, false, None);
                 p.last_type_ascription = cx.current_expansion.prior_type_ascription;
 
+                if is_local {
+                    cx.resolver.record_macro_rule_usage(node_id, i);
+                }
+
                 // Let the context choose how to interpret the result.
                 // Weird, but useful for X-macros.
                 return Box::new(ParserAnyMacro {
@@ -372,7 +379,7 @@ pub fn compile_declarative_macro(
     features: &Features,
     def: &ast::Item,
     edition: Edition,
-) -> SyntaxExtension {
+) -> (SyntaxExtension, Vec<Span>) {
     debug!("compile_declarative_macro: {:?}", def);
     let mk_syn_ext = |expander| {
         SyntaxExtension::new(
@@ -385,6 +392,7 @@ pub fn compile_declarative_macro(
             &def.attrs,
         )
     };
+    let dummy_syn_ext = || (mk_syn_ext(Box::new(macro_rules_dummy_expander)), Vec::new());
 
     let diag = &sess.parse_sess.span_diagnostic;
     let lhs_nm = Ident::new(sym::lhs, def.span);
@@ -445,17 +453,17 @@ pub fn compile_declarative_macro(
             let s = parse_failure_msg(&token);
             let sp = token.span.substitute_dummy(def.span);
             sess.parse_sess.span_diagnostic.struct_span_err(sp, &s).span_label(sp, msg).emit();
-            return mk_syn_ext(Box::new(macro_rules_dummy_expander));
+            return dummy_syn_ext();
         }
         Error(sp, msg) => {
             sess.parse_sess
                 .span_diagnostic
                 .struct_span_err(sp.substitute_dummy(def.span), &msg)
                 .emit();
-            return mk_syn_ext(Box::new(macro_rules_dummy_expander));
+            return dummy_syn_ext();
         }
         ErrorReported => {
-            return mk_syn_ext(Box::new(macro_rules_dummy_expander));
+            return dummy_syn_ext();
         }
     };
 
@@ -530,6 +538,15 @@ pub fn compile_declarative_macro(
         None => {}
     }
 
+    // Compute the spans of the macro rules
+    // We only take the span of the lhs here,
+    // so that the spans of created warnings are smaller.
+    let rule_spans = if def.id != DUMMY_NODE_ID {
+        lhses.iter().map(|lhs| lhs.span()).collect::<Vec<_>>()
+    } else {
+        Vec::new()
+    };
+
     // Convert the lhses into `MatcherLoc` form, which is better for doing the
     // actual matching. Unless the matcher is invalid.
     let lhses = if valid {
@@ -549,17 +566,16 @@ pub fn compile_declarative_macro(
         vec![]
     };
 
-    mk_syn_ext(Box::new(MacroRulesMacroExpander {
+    let expander = Box::new(MacroRulesMacroExpander {
         name: def.ident,
         span: def.span,
+        node_id: def.id,
         transparency,
         lhses,
         rhses,
         valid,
-        // Macros defined in the current crate have a real node id,
-        // whereas macros from an external crate have a dummy id.
-        is_local: def.id != DUMMY_NODE_ID,
-    }))
+    });
+    (mk_syn_ext(expander), rule_spans)
 }
 
 fn check_lhs_nt_follows(sess: &ParseSess, def: &ast::Item, lhs: &mbe::TokenTree) -> bool {

compiler/rustc_lint/src/lib.rs

@@ -303,6 +303,7 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         PATH_STATEMENTS,
         UNUSED_ATTRIBUTES,
         UNUSED_MACROS,
+        UNUSED_MACRO_RULES,
         UNUSED_ALLOCATION,
         UNUSED_DOC_COMMENTS,
         UNUSED_EXTERN_CRATES,

compiler/rustc_lint_defs/src/builtin.rs

@@ -749,6 +749,10 @@ declare_lint! {
 declare_lint! {
     /// The `unused_macros` lint detects macros that were not used.
     ///
+    /// Note that this lint is distinct from the `unused_macro_rules` lint,
+    /// which checks for single rules that never match of an otherwise used
+    /// macro, and thus never expand.
+    ///
     /// ### Example
     ///
     /// ```rust
@@ -775,6 +779,45 @@ declare_lint! {
     "detects macros that were not used"
 }
 
+declare_lint! {
+    /// The `unused_macro_rules` lint detects macro rules that were not used.
+    ///
+    /// Note that the lint is distinct from the `unused_macros` lint, which
+    /// fires if the entire macro is never called, while this lint fires for
+    /// single unused rules of the macro that is otherwise used.
+    /// `unused_macro_rules` fires only if `unused_macros` wouldn't fire.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// macro_rules! unused_empty {
+    ///     (hello) => { println!("Hello, world!") }; // This rule is unused
+    ///     () => { println!("empty") }; // This rule is used
+    /// }
+    ///
+    /// fn main() {
+    ///     unused_empty!(hello);
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Unused macro rules may signal a mistake or unfinished code. Furthermore,
+    /// they slow down compilation. Right now, silencing the warning is not
+    /// supported on a single rule level, so you have to add an allow to the
+    /// entire macro definition.
+    ///
+    /// If you intended to export the macro to make it
+    /// available outside of the crate, use the [`macro_export` attribute].
+    ///
+    /// [`macro_export` attribute]: https://doc.rust-lang.org/reference/macros-by-example.html#path-based-scope
+    pub UNUSED_MACRO_RULES,
+    Warn,
+    "detects macro rules that were not used"
+}
+
 declare_lint! {
     /// The `warnings` lint allows you to change the level of other
     /// lints which produce warnings.
@@ -3138,6 +3181,7 @@ declare_lint_pass! {
         OVERLAPPING_RANGE_ENDPOINTS,
         BINDINGS_WITH_VARIANT_NAME,
         UNUSED_MACROS,
+        UNUSED_MACRO_RULES,
         WARNINGS,
         UNUSED_FEATURES,
         STABLE_FEATURES,

compiler/rustc_middle/src/mir/visit.rs

@@ -258,6 +258,7 @@ macro_rules! make_mir_visitor {
                 // for best performance, we want to use an iterator rather
                 // than a for-loop, to avoid calling `body::Body::invalidate` for
                 // each basic block.
+                #[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
                 macro_rules! basic_blocks {
                     (mut) => (body.basic_blocks_mut().iter_enumerated_mut());
                     () => (body.basic_blocks().iter_enumerated());
@@ -279,6 +280,7 @@ macro_rules! make_mir_visitor {
                     self.visit_local_decl(local, & $($mutability)? body.local_decls[local]);
                 }
 
+                #[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
                 macro_rules! type_annotations {
                     (mut) => (body.user_type_annotations.iter_enumerated_mut());
                     () => (body.user_type_annotations.iter_enumerated());
@@ -932,6 +934,7 @@ macro_rules! make_mir_visitor {
                 body: &$($mutability)? Body<'tcx>,
                 location: Location
             ) {
+                #[cfg_attr(not(bootstrap), allow(unused_macro_rules))]
                 macro_rules! basic_blocks {
                     (mut) => (body.basic_blocks_mut());
                     () => (body.basic_blocks());

compiler/rustc_resolve/Cargo.toml

@@ -4,7 +4,6 @@ version = "0.0.0"
 edition = "2021"
 
 [lib]
-test = false
 doctest = false
 
 [dependencies]

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -194,7 +194,7 @@ impl<'a> Resolver<'a> {
         }
 
         let ext = Lrc::new(match self.cstore().load_macro_untracked(def_id, &self.session) {
-            LoadedMacro::MacroDef(item, edition) => self.compile_macro(&item, edition),
+            LoadedMacro::MacroDef(item, edition) => self.compile_macro(&item, edition).0,
             LoadedMacro::ProcMacro(ext) => ext,
         });
 
@@ -1218,25 +1218,35 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
     // Mark the given macro as unused unless its name starts with `_`.
     // Macro uses will remove items from this set, and the remaining
     // items will be reported as `unused_macros`.
-    fn insert_unused_macro(&mut self, ident: Ident, def_id: LocalDefId, node_id: NodeId) {
+    fn insert_unused_macro(
+        &mut self,
+        ident: Ident,
+        def_id: LocalDefId,
+        node_id: NodeId,
+        rule_spans: &[Span],
+    ) {
         if !ident.as_str().starts_with('_') {
             self.r.unused_macros.insert(def_id, (node_id, ident));
+            for (rule_i, rule_span) in rule_spans.iter().enumerate() {
+                self.r.unused_macro_rules.insert((def_id, rule_i), (ident, *rule_span));
+            }
         }
     }
 
     fn define_macro(&mut self, item: &ast::Item) -> MacroRulesScopeRef<'a> {
         let parent_scope = self.parent_scope;
         let expansion = parent_scope.expansion;
         let def_id = self.r.local_def_id(item.id);
-        let (ext, ident, span, macro_rules) = match &item.kind {
+        let (ext, ident, span, macro_rules, rule_spans) = match &item.kind {
             ItemKind::MacroDef(def) => {
-                let ext = Lrc::new(self.r.compile_macro(item, self.r.session.edition()));
-                (ext, item.ident, item.span, def.macro_rules)
+                let (ext, rule_spans) = self.r.compile_macro(item, self.r.session.edition());
+                let ext = Lrc::new(ext);
+                (ext, item.ident, item.span, def.macro_rules, rule_spans)
             }
             ItemKind::Fn(..) => match self.proc_macro_stub(item) {
                 Some((macro_kind, ident, span)) => {
                     self.r.proc_macro_stubs.insert(def_id);
-                    (self.r.dummy_ext(macro_kind), ident, span, false)
+                    (self.r.dummy_ext(macro_kind), ident, span, false, Vec::new())
                 }
                 None => return parent_scope.macro_rules,
             },
@@ -1264,7 +1274,7 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                 self.r.define(module, ident, MacroNS, (res, vis, span, expansion, IsMacroExport));
             } else {
                 self.r.check_reserved_macro_name(ident, res);
-                self.insert_unused_macro(ident, def_id, item.id);
+                self.insert_unused_macro(ident, def_id, item.id, &rule_spans);
             }
             self.r.visibilities.insert(def_id, vis);
             let scope = self.r.arenas.alloc_macro_rules_scope(MacroRulesScope::Binding(
@@ -1287,7 +1297,7 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                 _ => self.resolve_visibility(&item.vis),
             };
             if vis != ty::Visibility::Public {
-                self.insert_unused_macro(ident, def_id, item.id);
+                self.insert_unused_macro(ident, def_id, item.id, &rule_spans);
             }
             self.r.define(module, ident, MacroNS, (res, vis, span, expansion));
             self.r.visibilities.insert(def_id, vis);

compiler/rustc_resolve/src/diagnostics.rs

@@ -35,6 +35,9 @@ use crate::{LexicalScopeBinding, NameBinding, NameBindingKind, PrivacyError, Vis
 use crate::{ParentScope, PathResult, ResolutionError, Resolver, Scope, ScopeSet};
 use crate::{Segment, UseError};
 
+#[cfg(test)]
+mod tests;
+
 type Res = def::Res<ast::NodeId>;
 
 /// A vector of spans and replacements, a message and applicability.
@@ -2663,3 +2666,14 @@ fn is_span_suitable_for_use_injection(s: Span) -> bool {
     // import or other generated ones
     !s.from_expansion()
 }
+
+/// Convert the given number into the corresponding ordinal
+crate fn ordinalize(v: usize) -> String {
+    let suffix = match ((11..=13).contains(&(v % 100)), v % 10) {
+        (false, 1) => "st",
+        (false, 2) => "nd",
+        (false, 3) => "rd",
+        _ => "th",
+    };
+    format!("{v}{suffix}")
+}

compiler/rustc_resolve/src/diagnostics/tests.rs

@@ -0,0 +1,40 @@
+use super::ordinalize;
+
+#[test]
+fn test_ordinalize() {
+    assert_eq!(ordinalize(1), "1st");
+    assert_eq!(ordinalize(2), "2nd");
+    assert_eq!(ordinalize(3), "3rd");
+    assert_eq!(ordinalize(4), "4th");
+    assert_eq!(ordinalize(5), "5th");
+    // ...
+    assert_eq!(ordinalize(10), "10th");
+    assert_eq!(ordinalize(11), "11th");
+    assert_eq!(ordinalize(12), "12th");
+    assert_eq!(ordinalize(13), "13th");
+    assert_eq!(ordinalize(14), "14th");
+    // ...
+    assert_eq!(ordinalize(20), "20th");
+    assert_eq!(ordinalize(21), "21st");
+    assert_eq!(ordinalize(22), "22nd");
+    assert_eq!(ordinalize(23), "23rd");
+    assert_eq!(ordinalize(24), "24th");
+    // ...
+    assert_eq!(ordinalize(30), "30th");
+    assert_eq!(ordinalize(31), "31st");
+    assert_eq!(ordinalize(32), "32nd");
+    assert_eq!(ordinalize(33), "33rd");
+    assert_eq!(ordinalize(34), "34th");
+    // ...
+    assert_eq!(ordinalize(7010), "7010th");
+    assert_eq!(ordinalize(7011), "7011th");
+    assert_eq!(ordinalize(7012), "7012th");
+    assert_eq!(ordinalize(7013), "7013th");
+    assert_eq!(ordinalize(7014), "7014th");
+    // ...
+    assert_eq!(ordinalize(7020), "7020th");
+    assert_eq!(ordinalize(7021), "7021st");
+    assert_eq!(ordinalize(7022), "7022nd");
+    assert_eq!(ordinalize(7023), "7023rd");
+    assert_eq!(ordinalize(7024), "7024th");
+}