builtins.warn: Use new EvalBaseError + "evaluation warning"

doc/manual/rl-next/builtins-warn.md

@@ -0,0 +1,10 @@
+---
+synopsis: "New builtin: `builtins.warn`"
+issues: 306026
+prs: 10592
+---
+
+`builtins.warn` behaves like `builtins.trace "warning: ${msg}`, has an accurate log level, and is controlled by the options
+[`debugger-on-trace`](@docroot@/command-ref/conf-file.md#conf-debugger-on-trace),
+[`debugger-on-warn`](@docroot@/command-ref/conf-file.md#conf-debugger-on-warn) and
+[`abort-on-warn`](@docroot@/command-ref/conf-file.md#conf-abort-on-warn).

src/libexpr/eval-error.cc

@@ -70,6 +70,13 @@ EvalErrorBuilder<T>::addTrace(PosIdx pos, std::string_view formatString, const A
     return *this;
 }
 
+template<class T>
+EvalErrorBuilder<T> & EvalErrorBuilder<T>::setIsFromExpr()
+{
+    error.err.isFromExpr = true;
+    return *this;
+}
+
 template<class T>
 void EvalErrorBuilder<T>::debugThrow()
 {
@@ -85,6 +92,7 @@ void EvalErrorBuilder<T>::debugThrow()
     throw error;
 }
 
+template class EvalErrorBuilder<EvalBaseError>;
 template class EvalErrorBuilder<EvalError>;
 template class EvalErrorBuilder<AssertionError>;
 template class EvalErrorBuilder<ThrownError>;

src/libexpr/eval-error.hh

@@ -15,27 +15,39 @@ class EvalState;
 template<class T>
 class EvalErrorBuilder;
 
-class EvalError : public Error
+/**
+ * Base class for all errors that occur during evaluation.
+ *
+ * Most subclasses should inherit from `EvalError` instead of this class.
+ */
+class EvalBaseError : public Error
 {
     template<class T>
     friend class EvalErrorBuilder;
 public:
     EvalState & state;
 
-    EvalError(EvalState & state, ErrorInfo && errorInfo)
+    EvalBaseError(EvalState & state, ErrorInfo && errorInfo)
         : Error(errorInfo)
         , state(state)
     {
     }
 
     template<typename... Args>
-    explicit EvalError(EvalState & state, const std::string & formatString, const Args &... formatArgs)
+    explicit EvalBaseError(EvalState & state, const std::string & formatString, const Args &... formatArgs)
         : Error(formatString, formatArgs...)
         , state(state)
     {
     }
 };
 
+/**
+ * `EvalError` is the base class for almost all errors that occur during evaluation.
+ *
+ * All instances of `EvalError` should show a degree of purity that allows them to be
+ * cached in pure mode. This means that they should not depend on the configuration or the overall environment.
+ */
+MakeError(EvalError, EvalBaseError);
 MakeError(ParseError, Error);
 MakeError(AssertionError, EvalError);
 MakeError(ThrownError, AssertionError);
@@ -90,6 +102,8 @@ public:
 
     [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & addTrace(PosIdx pos, HintFmt hint);
 
+    [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> & setIsFromExpr();
+
     template<typename... Args>
     [[nodiscard, gnu::noinline]] EvalErrorBuilder<T> &
     addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs);

src/libexpr/primops.cc

@@ -806,7 +806,7 @@ static RegisterPrimOp primop_abort({
         NixStringContext context;
         auto s = state.coerceToString(pos, *args[0], context,
                 "while evaluating the error message passed to builtins.abort").toOwned();
-        state.error<Abort>("evaluation aborted with the following error message: '%1%'", s).debugThrow();
+        state.error<Abort>("evaluation aborted with the following error message: '%1%'", s).setIsFromExpr().debugThrow();
     }
 });
 
@@ -825,7 +825,7 @@ static RegisterPrimOp primop_throw({
       NixStringContext context;
       auto s = state.coerceToString(pos, *args[0], context,
               "while evaluating the error message passed to builtin.throw").toOwned();
-      state.error<ThrownError>(s).debugThrow();
+      state.error<ThrownError>(s).setIsFromExpr().debugThrow();
     }
 });
 
@@ -1052,12 +1052,13 @@ static void prim_warn(EvalState & state, const PosIdx pos, Value * * args, Value
         msg.atPos(state.positions[pos]);
         auto info = msg.info();
         info.level = lvlWarn;
+        info.isFromExpr = true;
         logWarning(info);
     }
 
     if (evalSettings.builtinsAbortOnWarn) {
         // Not an EvalError or subclass, which would cause the error to be stored in the eval cache.
-        state.error<Error>("aborting to reveal stack trace of warning, as abort-on-warn is set").debugThrow();
+        state.error<EvalBaseError>("aborting to reveal stack trace of warning, as abort-on-warn is set").setIsFromExpr().debugThrow();
     }
     if (evalSettings.builtinsTraceDebugger || evalSettings.builtinsDebuggerOnWarn) {
         state.runDebugRepl(nullptr);
@@ -1080,6 +1081,11 @@ static RegisterPrimOp primop_warn({
       option is set to `true` and the `--debugger` flag is given, the
       interactive debugger will be started when `warn` is called (like
       [`break`](@docroot@/language/builtins.md#builtins-break)).
+
+      If the
+      [`abort-on-warn`](@docroot@/command-ref/conf-file.md#conf-abort-on-warn)
+      option is set, the evaluation will be aborted after the warning is printed.
+      This is useful to reveal the stack trace of the warning, when the context is non-interactive and a debugger can not be launched.
     )",
     .fun = prim_warn,
 });

src/libutil/error.cc

@@ -240,6 +240,9 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
             break;
         }
         case Verbosity::lvlWarn: {
+            if (einfo.isFromExpr)
+                prefix = ANSI_WARNING "evaluation warning";
+            else
                 prefix = ANSI_WARNING "warning";
             break;
         }

src/libutil/error.hh

@@ -89,6 +89,11 @@ struct ErrorInfo {
     HintFmt msg;
     std::shared_ptr<Pos> pos;
     std::list<Trace> traces;
+    /**
+     * Some messages are generated directly by expressions; notably `builtins.warn`, `abort`, `throw`.
+     * These may be rendered differently, so that users can distinguish them.
+     */
+    bool isFromExpr = false;
 
     /**
      * Exit status.