Rollup merge of rust-lang#96150 - est31:unused_macro_rules, r=fee1-dead

Implement a lint to warn about unused macro rules

This implements a new lint to warn about unused macro rules (arms/matchers), similar to the `unused_macros` lint added by rust-lang#41907 that warns about entire macros.

```rust
macro_rules! unused_empty {
    (hello) => { println!("Hello, world!") };
    () => { println!("empty") }; //~ ERROR: 1st rule of macro `unused_empty` is never used
}

fn main() {
    unused_empty!(hello);
}
```

Builds upon ~~(and currently integrates)~~ rust-lang#96149 and rust-lang#96156.

Fixes rust-lang#73576

Author: Dylan-DPC

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

@@ -711,6 +711,9 @@ pub struct SyntaxExtension {
     /// Built-in macros have a couple of special properties like availability
     /// in `#[no_implicit_prelude]` modules, so we have to keep this flag.
     pub builtin_name: Option<Symbol>,
+    /// Rule spans. Used for linting. If the macro is not made up of rules,
+    /// it's empty.
+    pub rule_spans: Vec<Span>,
 }
 
 impl SyntaxExtension {
@@ -740,6 +743,7 @@ impl SyntaxExtension {
             edition,
             builtin_name: None,
             kind,
+            rule_spans: Vec::new(),
         }
     }
 
@@ -753,6 +757,7 @@ impl SyntaxExtension {
         edition: Edition,
         name: Symbol,
         attrs: &[ast::Attribute],
+        rule_spans: Vec<Span>,
     ) -> SyntaxExtension {
         let allow_internal_unstable =
             attr::allow_internal_unstable(sess, &attrs).collect::<Vec<Symbol>>();
@@ -800,6 +805,7 @@ impl SyntaxExtension {
             helper_attrs,
             edition,
             builtin_name,
+            rule_spans,
         }
     }
 
@@ -887,6 +893,8 @@ 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

@@ -156,6 +156,7 @@ impl<'a> ParserAnyMacro<'a> {
 }
 
 struct MacroRulesMacroExpander {
+    id: NodeId,
     name: Ident,
     span: Span,
     transparency: Transparency,
@@ -179,6 +180,7 @@ impl TTMacroExpander for MacroRulesMacroExpander {
             cx,
             sp,
             self.span,
+            self.id,
             self.name,
             self.transparency,
             input,
@@ -207,6 +209,7 @@ fn generic_extension<'cx, 'tt>(
     cx: &'cx mut ExtCtxt<'_>,
     sp: Span,
     def_span: Span,
+    id: NodeId,
     name: Ident,
     transparency: Transparency,
     arg: TokenStream,
@@ -297,6 +300,8 @@ fn generic_extension<'cx, 'tt>(
                 let mut p = Parser::new(sess, tts, false, None);
                 p.last_type_ascription = cx.current_expansion.prior_type_ascription;
 
+                cx.resolver.record_macro_rule_usage(id, i);
+
                 // Let the context choose how to interpret the result.
                 // Weird, but useful for X-macros.
                 return Box::new(ParserAnyMacro {
@@ -375,7 +380,7 @@ pub fn compile_declarative_macro(
     edition: Edition,
 ) -> SyntaxExtension {
     debug!("compile_declarative_macro: {:?}", def);
-    let mk_syn_ext = |expander| {
+    let mk_syn_ext = |expander, rule_spans| {
         SyntaxExtension::new(
             sess,
             SyntaxExtensionKind::LegacyBang(expander),
@@ -384,8 +389,10 @@ pub fn compile_declarative_macro(
             edition,
             def.ident.name,
             &def.attrs,
+            rule_spans,
         )
     };
+    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);
@@ -446,17 +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();
         }
     };
 
@@ -531,6 +538,11 @@ 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 = lhses.iter().map(|lhs| lhs.span()).collect::<Vec<_>>();
+
     // Convert the lhses into `MatcherLoc` form, which is better for doing the
     // actual matching. Unless the matcher is invalid.
     let lhses = if valid {
@@ -550,17 +562,21 @@ pub fn compile_declarative_macro(
         vec![]
     };
 
-    mk_syn_ext(Box::new(MacroRulesMacroExpander {
-        name: def.ident,
-        span: def.span,
-        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(
+        Box::new(MacroRulesMacroExpander {
+            name: def.ident,
+            span: def.span,
+            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,
+        }),
+        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

@@ -775,6 +775,35 @@ declare_lint! {
     "detects macros that were not used"
 }
 
+declare_lint! {
+    /// The `unused_macro_rules` lint detects macro rules that were not used.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// macro_rules! unused_empty {
+    ///     (hello) => { println!("Hello, world!") };
+    ///     () => { println!("empty") };
+    /// }
+    ///
+    /// 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.
+    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 +3167,7 @@ declare_lint_pass! {
         OVERLAPPING_RANGE_ENDPOINTS,
         BINDINGS_WITH_VARIANT_NAME,
         UNUSED_MACROS,
+        UNUSED_MACRO_RULES,
         WARNINGS,
         UNUSED_FEATURES,
         STABLE_FEATURES,

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -932,6 +932,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             self.root.edition,
             Symbol::intern(name),
             &attrs,
+            Vec::new(),
         )
     }
 

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

@@ -1218,9 +1218,18 @@ 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_count: usize,
+    ) {
         if !ident.as_str().starts_with('_') {
             self.r.unused_macros.insert(def_id, (node_id, ident));
+            for rule_i in 0..rule_count {
+                self.r.unused_macro_rules.insert((def_id, rule_i), (node_id, ident));
+            }
         }
     }
 
@@ -1244,6 +1253,7 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
         };
 
         let res = Res::Def(DefKind::Macro(ext.macro_kind()), def_id.to_def_id());
+        let rule_count = ext.rule_spans.len();
         self.r.macro_map.insert(def_id.to_def_id(), ext);
         self.r.local_macro_def_scopes.insert(def_id, parent_scope.module);
 
@@ -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_count);
             }
             self.r.visibilities.insert(def_id, vis);
             self.r.arenas.alloc_macro_rules_scope(MacroRulesScope::Binding(
@@ -1285,7 +1295,7 @@ 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_count);
             }
             self.r.define(module, ident, MacroNS, (res, vis, span, expansion));
             self.r.visibilities.insert(def_id, vis);

compiler/rustc_resolve/src/lib.rs

@@ -969,6 +969,7 @@ pub struct Resolver<'a> {
     local_macro_def_scopes: FxHashMap<LocalDefId, Module<'a>>,
     ast_transform_scopes: FxHashMap<LocalExpnId, Module<'a>>,
     unused_macros: FxHashMap<LocalDefId, (NodeId, Ident)>,
+    unused_macro_rules: FxHashMap<(LocalDefId, usize), (NodeId, Ident)>,
     proc_macro_stubs: FxHashSet<LocalDefId>,
     /// Traces collected during macro resolution and validated when it's complete.
     single_segment_macro_resolutions:
@@ -1354,6 +1355,7 @@ impl<'a> Resolver<'a> {
             potentially_unused_imports: Vec::new(),
             struct_constructors: Default::default(),
             unused_macros: Default::default(),
+            unused_macro_rules: Default::default(),
             proc_macro_stubs: Default::default(),
             single_segment_macro_resolutions: Default::default(),
             multi_segment_macro_resolutions: Default::default(),