Add warnings for unused config rules

include/slang/ast/symbols/CompilationUnitSymbols.h

@@ -246,6 +246,9 @@ struct SLANG_EXPORT ConfigRule {
 
     /// The source range where this rule was declared.
     SourceRange sourceRange;
+
+    /// A flag that marks whether this rule has been used during elaboration.
+    mutable bool isUsed = false;
 };
 
 /// Contains information about a resolved configuration rule
@@ -283,6 +286,9 @@ class SLANG_EXPORT ConfigBlockSymbol : public Symbol, public Scope {
         ConfigRule* rule = nullptr;
     };
 
+    /// A flag that marks whether this config block has been used during elaboration.
+    mutable bool isUsed = false;
+
     ConfigBlockSymbol(Compilation& compilation, std::string_view name, SourceLocation loc) :
         Symbol(SymbolKind::ConfigBlock, name, loc), Scope(compilation, this) {}
 
@@ -310,6 +316,8 @@ class SLANG_EXPORT ConfigBlockSymbol : public Symbol, public Scope {
         return instanceOverrides;
     }
 
+    void checkUnusedRules() const;
+
     static ConfigBlockSymbol& fromSyntax(const Scope& scope,
                                          const syntax::ConfigDeclarationSyntax& syntax);
 

scripts/diagnostics.txt

@@ -526,6 +526,8 @@ warning invalid-pulsestyle InvalidPulseStyle "invalid use of {} for '{}' since i
 warning implicit-port-type-mismatch ImplicitNamedPortTypeMismatch "implicit named port '{}' of type {} connects to value of inequivalent type {}"
 warning unknown-library WarnUnknownLibrary "unknown library '{}'"
 warning dup-config-rule DupConfigRule "duplicate config rule for {} -- ignoring this one"
+warning unused-config-cell UnusedConfigCell "unused cell config rule"
+warning unused-config-instance UnusedConfigInstance "unused config rule -- instance path does not exist in the design"
 
 subsystem Expressions
 error BadUnaryExpression "invalid operand type {} to unary expression"
@@ -1066,7 +1068,7 @@ group default = { real-underflow real-overflow vector-overflow int-overflow unco
                   raw-protect-eof protected-envelope specify-param dup-timing-path invalid-pulsestyle
                   negative-timing-limit bad-procedural-force duplicate-defparam implicit-port-type-mismatch
                   split-distweight-op dpi-pure-task multibit-edge unknown-sys-name unknown-library
-                  dup-config-rule }
+                  dup-config-rule unused-config-cell unused-config-instance }
 
 group extra = { empty-member empty-stmt dup-import pointless-void-cast case-gen-none case-gen-dup
                 unused-result format-real ignored-slice task-ignored width-trunc dup-attr event-const
@@ -1082,4 +1084,4 @@ group conversion = { width-trunc width-expand port-width-trunc port-width-expand
 group unused = { unused-def unused-net unused-implicit-net unused-variable undriven-net unassigned-variable
                  unused-but-set-net unused-but-set-variable unused-port undriven-port unused-argument
                  unused-parameter unused-type-parameter unused-typedef unused-genvar unused-assertion-decl
-                 unused-but-set-port }
+                 unused-but-set-port unused-config-cell unused-config-instance }

scripts/warning_docs.txt

@@ -1162,3 +1162,29 @@ config cfg;
     instance d.foo use baz;
 endconfig
 ```
+
+-Wunused-config-cell
+A configuration cell rule is unused, which may mean that there's a typo in the target name.
+@options --top=cfg
+```
+config cfg;
+    design top;
+    cell foo liblist l;
+endconfig
+
+module top;
+endmodule
+```
+
+-Wunused-config-instance
+A configuration instance rule is unused, which means that target path does not exist in the design.
+@options --top=cfg
+```
+config cfg;
+    design top;
+    instance top.foo liblist l;
+endconfig
+
+module top;
+endmodule
+```

source/ast/Compilation.cpp

@@ -471,6 +471,7 @@ const RootSymbol& Compilation::getRoot(bool skipDefParamsAndBinds) {
                 }
 
                 if (foundConf) {
+                    foundConf->isUsed = true;
                     for (auto& cell : foundConf->getTopCells()) {
                         selectedConfigs.emplace(foundConf);
                         topDefs.push_back({&cell.definition, foundConf, cell.rule});
@@ -1350,6 +1351,16 @@ const Diagnostics& Compilation::getSemanticDiagnostics() {
             }
         }
 
+        // Report on unused config rules.
+        if (!configBlocks.empty()) {
+            for (auto& [name, confList] : configBlocks) {
+                for (auto config : confList) {
+                    if (config->isUsed)
+                        config->checkUnusedRules();
+                }
+            }
+        }
+
         if (!hasFlag(CompilationFlags::AllowTopLevelIfacePorts)) {
             // Top level instances cannot have interface or ref ports.
             for (auto inst : getRoot().topInstances) {
@@ -2292,6 +2303,7 @@ auto findDefByLib(TDefList& defList, const SourceLibrary& target)
 std::pair<Compilation::DefinitionLookupResult, bool> Compilation::resolveConfigRule(
     const Scope& scope, const ConfigRule& rule) const {
 
+    rule.isUsed = true;
     auto& id = rule.useCell;
     SLANG_ASSERT(!id.name.empty());
 
@@ -2324,6 +2336,7 @@ std::pair<Compilation::DefinitionLookupResult, bool> Compilation::resolveConfigR
     if (auto configIt = configBlocks.find(id.name); configIt != configBlocks.end()) {
         auto result = findDefByLib(configIt->second, *overrideLib);
         if (result) {
+            result->isUsed = true;
             auto topCells = result->getTopCells();
             if (topCells.size() != 1) {
                 auto syntax = result->getSyntax();
@@ -2373,6 +2386,7 @@ std::pair<Compilation::DefinitionLookupResult, bool> Compilation::resolveConfigR
         if (auto& id = rule->useCell; !id.name.empty())
             return resolveConfigRule(scope, *rule);
 
+        rule->isUsed = true;
         if (rule->liblist)
             liblist = *rule->liblist;
     }

source/ast/symbols/CompilationUnitSymbols.cpp

@@ -373,6 +373,34 @@ ConfigBlockSymbol& ConfigBlockSymbol::fromSyntax(const Scope& scope,
     return *result;
 }
 
+static void checkRuleUnused(const Scope& scope, const ConfigRule& rule, bool isCell) {
+    if (!rule.isUsed) {
+        scope.addDiag(isCell ? diag::UnusedConfigCell : diag::UnusedConfigInstance,
+                      rule.sourceRange);
+    }
+}
+
+static void checkNodeUnused(const Scope& scope, const ConfigBlockSymbol::InstanceOverride& node) {
+    if (node.rule)
+        checkRuleUnused(scope, *node.rule, false);
+
+    for (auto& [name, child] : node.childNodes)
+        checkNodeUnused(scope, child);
+}
+
+void ConfigBlockSymbol::checkUnusedRules() const {
+    for (auto& [_, cellOverride] : getCellOverrides()) {
+        if (cellOverride.defaultRule)
+            checkRuleUnused(*this, *cellOverride.defaultRule, true);
+
+        for (auto& [lib, rule] : cellOverride.specificLibRules)
+            checkRuleUnused(*this, *rule, true);
+    }
+
+    for (auto& [_, instOverride] : getInstanceOverrides())
+        checkNodeUnused(*this, instOverride);
+}
+
 void ConfigBlockSymbol::resolve() const {
     SLANG_ASSERT(!resolved);
     resolved = true;

source/ast/symbols/InstanceSymbols.cpp

@@ -273,8 +273,11 @@ InstanceSymbol& InstanceSymbol::createDefault(Compilation& comp, const Definitio
 
     if (configBlock) {
         auto rc = comp.emplace<ResolvedConfig>(*configBlock, result);
-        if (configRule && configRule->liblist)
-            rc->liblist = *configRule->liblist;
+        if (configRule) {
+            configRule->isUsed = true;
+            if (configRule->liblist)
+                rc->liblist = *configRule->liblist;
+        }
         result.resolvedConfig = rc;
     }
 
@@ -527,6 +530,7 @@ void InstanceSymbol::fromSyntax(Compilation& comp, const HierarchyInstantiationS
         if (confRule) {
             SLANG_ASSERT(resolvedConfig);
             auto rc = comp.emplace<ResolvedConfig>(*resolvedConfig);
+            confRule->isUsed = true;
             if (confRule->liblist)
                 rc->liblist = *confRule->liblist;
             localConfig = rc;

tests/unittests/ast/ConfigTests.cpp

@@ -550,15 +550,17 @@ endmodule
     compilation.addSyntaxTree(tree);
 
     auto& diags = compilation.getAllDiagnostics();
-    REQUIRE(diags.size() == 8);
+    REQUIRE(diags.size() == 10);
     CHECK(diags[0].code == diag::ConfigDupTop);
     CHECK(diags[1].code == diag::WarnUnknownLibrary);
-    CHECK(diags[2].code == diag::DupConfigRule);
+    CHECK(diags[2].code == diag::UnusedConfigCell);
     CHECK(diags[3].code == diag::DupConfigRule);
-    CHECK(diags[4].code == diag::ConfigInstanceWrongTop);
-    CHECK(diags[5].code == diag::ConfigParamsIgnored);
-    CHECK(diags[6].code == diag::WarnUnknownLibrary);
-    CHECK(diags[7].code == diag::NestedConfigMultipleTops);
+    CHECK(diags[4].code == diag::UnusedConfigInstance);
+    CHECK(diags[5].code == diag::DupConfigRule);
+    CHECK(diags[6].code == diag::ConfigInstanceWrongTop);
+    CHECK(diags[7].code == diag::ConfigParamsIgnored);
+    CHECK(diags[8].code == diag::WarnUnknownLibrary);
+    CHECK(diags[9].code == diag::NestedConfigMultipleTops);
 }
 
 TEST_CASE("Config rules with param overrides") {
@@ -786,7 +788,10 @@ endconfig
     Compilation compilation(options);
     compilation.addSyntaxTree(tree2);
     compilation.addSyntaxTree(tree1);
-    NO_COMPILATION_ERRORS;
+
+    auto& diags = compilation.getAllDiagnostics();
+    REQUIRE(diags.size() == 1);
+    CHECK(diags[0].code == diag::UnusedConfigCell);
 
     auto getParam = [&](std::string_view name) {
         auto& param = compilation.getRoot().lookupName<ParameterSymbol>(name);