Warning for unused function args

changelog/dmd.warn-unused-params.dd

@@ -0,0 +1,22 @@
+New preview flag ``-preview=warnunusedparams`` to warn on  unused function parameters
+
+A new ``-preview=warnunusedparams`` introduces warnings for named function parameters that are not referenced by the function body.
+
+```d
+void foo(int x, int y) // Warning: function parameter `y` is never used
+{
+    return x * 2;
+}
+```
+
+To suppress the warning, either omit the parameter name or use ``cast(void)``:
+
+```d
+void callback(int x, int) {} // unnamed parameter inhibits warning
+
+void bar(int x, int y)
+{
+    cast(void)y; // explicitly mark as unused, useful in conditional compilation
+    use(x);
+}
+```

compiler/src/dmd/cli.d

@@ -1076,6 +1076,9 @@ dmd -cov -unittest myprog.d
             "https://dlang.org/spec/attribute.html#system-variables"),
         Feature("fastdfa", "useFastDFA",
             "Fast dataflow analysis engine, experimental"),
+        Feature("warnunusedparams", "warnUnused",
+            "enable warnings for unused function parameters",
+            "https://dlang.org/spec/function.html#unused-params"),
     ];
 }
 

compiler/src/dmd/declaration.d

@@ -469,6 +469,7 @@ extern (C++) class VarDeclaration : Declaration
         bool dllExport;         /// __declspec(dllexport)
         mixin VarDeclarationExtra;
         bool systemInferred;    /// @system was inferred from initializer
+        bool wasUsed;           /// variable was referenced
     }
 
     import dmd.common.bitfields : generateBitFields;

compiler/src/dmd/expressionsem.d

@@ -7163,6 +7163,7 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
 
         if (auto v = e.var.isVarDeclaration())
         {
+            v.wasUsed = true;
             if (v.checkNestedReference(sc, e.loc))
                 return setError();
         }
@@ -7212,6 +7213,7 @@ private extern (C++) final class ExpressionSemanticVisitor : Visitor
 
         if (vd)
         {
+            vd.wasUsed = true;
             if (vd.checkNestedReference(sc, e.loc))
                 return setError();
 

compiler/src/dmd/frontend.h

@@ -6639,6 +6639,8 @@ class VarDeclaration : public Declaration
     bool inAlignSection(bool v);
     bool systemInferred() const;
     bool systemInferred(bool v);
+    bool wasUsed() const;
+    bool wasUsed(bool v);
 private:
     uint32_t bitFields;
 public:
@@ -8198,6 +8200,7 @@ struct Param final
     FeatureState dtorFields;
     FeatureState systemVariables;
     bool useFastDFA;
+    bool warnUnused;
     CHECKENABLE useInvariants;
     CHECKENABLE useIn;
     CHECKENABLE useOut;
@@ -8282,6 +8285,7 @@ struct Param final
         fixImmutableConv(),
         fix16997(true),
         useFastDFA(),
+        warnUnused(),
         useInvariants((CHECKENABLE)0u),
         useIn((CHECKENABLE)0u),
         useOut((CHECKENABLE)0u),
@@ -8327,7 +8331,7 @@ struct Param final
         timeTraceFile()
     {
     }
-    Param(bool obj, bool readStdin = false, bool multiobj = false, bool trace = false, bool tracegc = false, bool vcg_ast = false, DiagnosticReporting useDeprecated = (DiagnosticReporting)1u, bool useUnitTests = false, bool useInline = false, bool release = false, bool preservePaths = false, DiagnosticReporting useWarnings = (DiagnosticReporting)2u, bool cov = false, uint8_t covPercent = 0u, bool ctfe_cov = false, bool ignoreUnsupportedPragmas = true, bool useModuleInfo = true, bool useTypeInfo = true, bool useExceptions = true, bool useGC = true, bool betterC = false, bool addMain = false, bool allInst = false, bool bitfields = true, bool rewriteNoExceptionToSeq = false, CppStdRevision cplusplus = (CppStdRevision)201103u, Help help = Help(), Verbose v = Verbose(), Edition edition = (Edition)2023u, void* editionFiles = nullptr, FeatureState useDIP25 = (FeatureState)2u, FeatureState useDIP1000 = (FeatureState)0u, bool ehnogc = false, bool useDIP1021 = false, FeatureState fieldwise = (FeatureState)0u, bool fixAliasThis = false, FeatureState rvalueRefParam = (FeatureState)0u, FeatureState safer = (FeatureState)0u, FeatureState noSharedAccess = (FeatureState)0u, bool previewIn = false, bool inclusiveInContracts = false, bool shortenedMethods = true, bool fixImmutableConv = false, bool fix16997 = true, FeatureState dtorFields = (FeatureState)0u, FeatureState systemVariables = (FeatureState)0u, bool useFastDFA = false, CHECKENABLE useInvariants = (CHECKENABLE)0u, CHECKENABLE useIn = (CHECKENABLE)0u, CHECKENABLE useOut = (CHECKENABLE)0u, CHECKENABLE useArrayBounds = (CHECKENABLE)0u, CHECKENABLE useNullCheck = (CHECKENABLE)0u, CHECKENABLE useAssert = (CHECKENABLE)0u, CHECKENABLE useSwitchError = (CHECKENABLE)0u, CHECKENABLE boundscheck = (CHECKENABLE)0u, CHECKACTION checkAction = (CHECKACTION)0u, CLIIdentifierTable dIdentifierTable = (CLIIdentifierTable)0u, CLIIdentifierTable cIdentifierTable = (CLIIdentifierTable)0u, _d_dynamicArray< const char > argv0 = {}, Array<const char* > modFileAliasStrings = Array<const char* >(), Array<ImportPathInfo > imppath = Array<ImportPathInfo >(), Array<const char* > fileImppath = Array<const char* >(), _d_dynamicArray< const char > objdir = {}, _d_dynamicArray< const char > objname = {}, _d_dynamicArray< const char > libname = {}, Output ddoc = Output(), Output dihdr = Output(), Output cxxhdr = Output(), Output json = Output(), JsonFieldFlags jsonFieldFlags = (JsonFieldFlags)0u, Output makeDeps = Output(), Output mixinOut = Output(), Output moduleDeps = Output(), bool debugEnabled = false, bool run = false, Array<const char* > runargs = Array<const char* >(), Array<const char* > cppswitches = Array<const char* >(), const char* cpp = nullptr, Array<const char* > objfiles = Array<const char* >(), Array<const char* > linkswitches = Array<const char* >(), Array<bool > linkswitchIsForCC = Array<bool >(), Array<const char* > libfiles = Array<const char* >(), Array<const char* > dllfiles = Array<const char* >(), _d_dynamicArray< const char > deffile = {}, _d_dynamicArray< const char > resfile = {}, _d_dynamicArray< const char > exefile = {}, _d_dynamicArray< const char > mapfile = {}, bool fullyQualifiedObjectFiles = false, bool timeTrace = false, uint32_t timeTraceGranularityUs = 500u, const char* timeTraceFile = nullptr) :
+    Param(bool obj, bool readStdin = false, bool multiobj = false, bool trace = false, bool tracegc = false, bool vcg_ast = false, DiagnosticReporting useDeprecated = (DiagnosticReporting)1u, bool useUnitTests = false, bool useInline = false, bool release = false, bool preservePaths = false, DiagnosticReporting useWarnings = (DiagnosticReporting)2u, bool cov = false, uint8_t covPercent = 0u, bool ctfe_cov = false, bool ignoreUnsupportedPragmas = true, bool useModuleInfo = true, bool useTypeInfo = true, bool useExceptions = true, bool useGC = true, bool betterC = false, bool addMain = false, bool allInst = false, bool bitfields = true, bool rewriteNoExceptionToSeq = false, CppStdRevision cplusplus = (CppStdRevision)201103u, Help help = Help(), Verbose v = Verbose(), Edition edition = (Edition)2023u, void* editionFiles = nullptr, FeatureState useDIP25 = (FeatureState)2u, FeatureState useDIP1000 = (FeatureState)0u, bool ehnogc = false, bool useDIP1021 = false, FeatureState fieldwise = (FeatureState)0u, bool fixAliasThis = false, FeatureState rvalueRefParam = (FeatureState)0u, FeatureState safer = (FeatureState)0u, FeatureState noSharedAccess = (FeatureState)0u, bool previewIn = false, bool inclusiveInContracts = false, bool shortenedMethods = true, bool fixImmutableConv = false, bool fix16997 = true, FeatureState dtorFields = (FeatureState)0u, FeatureState systemVariables = (FeatureState)0u, bool useFastDFA = false, bool warnUnused = false, CHECKENABLE useInvariants = (CHECKENABLE)0u, CHECKENABLE useIn = (CHECKENABLE)0u, CHECKENABLE useOut = (CHECKENABLE)0u, CHECKENABLE useArrayBounds = (CHECKENABLE)0u, CHECKENABLE useNullCheck = (CHECKENABLE)0u, CHECKENABLE useAssert = (CHECKENABLE)0u, CHECKENABLE useSwitchError = (CHECKENABLE)0u, CHECKENABLE boundscheck = (CHECKENABLE)0u, CHECKACTION checkAction = (CHECKACTION)0u, CLIIdentifierTable dIdentifierTable = (CLIIdentifierTable)0u, CLIIdentifierTable cIdentifierTable = (CLIIdentifierTable)0u, _d_dynamicArray< const char > argv0 = {}, Array<const char* > modFileAliasStrings = Array<const char* >(), Array<ImportPathInfo > imppath = Array<ImportPathInfo >(), Array<const char* > fileImppath = Array<const char* >(), _d_dynamicArray< const char > objdir = {}, _d_dynamicArray< const char > objname = {}, _d_dynamicArray< const char > libname = {}, Output ddoc = Output(), Output dihdr = Output(), Output cxxhdr = Output(), Output json = Output(), JsonFieldFlags jsonFieldFlags = (JsonFieldFlags)0u, Output makeDeps = Output(), Output mixinOut = Output(), Output moduleDeps = Output(), bool debugEnabled = false, bool run = false, Array<const char* > runargs = Array<const char* >(), Array<const char* > cppswitches = Array<const char* >(), const char* cpp = nullptr, Array<const char* > objfiles = Array<const char* >(), Array<const char* > linkswitches = Array<const char* >(), Array<bool > linkswitchIsForCC = Array<bool >(), Array<const char* > libfiles = Array<const char* >(), Array<const char* > dllfiles = Array<const char* >(), _d_dynamicArray< const char > deffile = {}, _d_dynamicArray< const char > resfile = {}, _d_dynamicArray< const char > exefile = {}, _d_dynamicArray< const char > mapfile = {}, bool fullyQualifiedObjectFiles = false, bool timeTrace = false, uint32_t timeTraceGranularityUs = 500u, const char* timeTraceFile = nullptr) :
         obj(obj),
         readStdin(readStdin),
         multiobj(multiobj),
@@ -8375,6 +8379,7 @@ struct Param final
         dtorFields(dtorFields),
         systemVariables(systemVariables),
         useFastDFA(useFastDFA),
+        warnUnused(warnUnused),
         useInvariants(useInvariants),
         useIn(useIn),
         useOut(useOut),

compiler/src/dmd/globals.d

@@ -219,6 +219,7 @@ extern (C++) struct Param
                                  // https://issues.dlang.org/show_bug.cgi?id=14246
     FeatureState systemVariables; // limit access to variables marked @system from @safe code
     bool useFastDFA;                 // Use fast data flow analysis engine
+    bool warnUnused;                 // warn about unused function parameters
 
     CHECKENABLE useInvariants  = CHECKENABLE._default;  // generate class invariant checks
     CHECKENABLE useIn          = CHECKENABLE._default;  // generate precondition checks

compiler/src/dmd/globals.h

@@ -240,6 +240,7 @@ struct Param
                                  // https://issues.dlang.org/show_bug.cgi?id=14246
     FeatureState systemVariables; // limit access to variables marked @system from @safe code
     d_bool useFastDFA;             // Use fast data flow analysis engine
+    d_bool warnUnused;             // warn about unused function parameters
 
     CHECKENABLE useInvariants;     // generate class invariant checks
     CHECKENABLE useIn;             // generate precondition checks

compiler/src/dmd/semantic3.d

@@ -1441,6 +1441,16 @@ private extern(C++) final class Semantic3Visitor : Visitor
             }
         }
 
+        // Warn about unused named parameters
+        if (global.params.warnUnused && funcdecl.parameters && funcdecl.fbody)
+        {
+            foreach (v; *funcdecl.parameters)
+            {
+                if (!v.wasUsed && !(v.storage_class & STC.temp))
+                    .warning(v.loc, "function parameter `%s` is never used", v.ident.toChars());
+            }
+        }
+
         /* If this function had instantiated with gagging, error reproduction will be
          * done by TemplateInstance::semantic.
          * Otherwise, error gagging should be temporarily ungagged by functionSemantic3.

compiler/test/compilable/previewhelp.d

@@ -20,5 +20,6 @@ Upcoming language changes listed by -preview=name:
   =fixImmutableConv disallow `void[]` data from holding immutable data (https://dlang.org/changelog/2.101.0.html#dmd.fix-immutable-conv, https://issues.dlang.org/show_bug.cgi?id=17148)
   =systemVariables  disable access to variables marked '@system' from @safe code (https://dlang.org/spec/attribute.html#system-variables)
   =fastdfa          Fast dataflow analysis engine, experimental
+  =warnunusedparams enable warnings for unused function parameters (https://dlang.org/spec/function.html#unused-params)
 ----
 */

compiler/test/fail_compilation/unused_params.d

@@ -0,0 +1,30 @@
+// REQUIRED_ARGS: -w -preview=warnunusedparams -o-
+
+/*
+TEST_OUTPUT:
+---
+fail_compilation/unused_params.d(21): Warning: function parameter `x` is never used
+fail_compilation/unused_params.d(21): Warning: function parameter `y` is never used
+fail_compilation/unused_params.d(24): Warning: function parameter `x` is never used
+Error: warnings are treated as errors
+       Use -wi if you wish to treat warnings only as informational.
+---
+*/
+
+// No warning for used parameter
+void used(int x) { auto z = x + 1; }
+
+// No warning for cast(void)suppression
+void suppressed(int x) { cast(void)x; }
+
+// Warn for unused named parameters
+void unused(int x, int y) {}
+
+// No warning for unnamed parameter, warn for named
+void partial(int x, int) {}
+
+// No warning when no body
+interface I
+{
+    void iface(int x);
+}