builtins.warn: Require string argument

... so that we may perhaps later extend the interface.
Note that Nixpkgs' lib.warn already requires a string coercible
argument, so this is reasonable. Also note that string coercible
values aren't all strings, but in practice, for warn, they are.

src/libexpr/primops.cc

@@ -1045,16 +1045,12 @@ static RegisterPrimOp primop_trace({
 
 static void prim_warn(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceValue(*args[0], pos);
+    // We only accept a string argument for now. The use case for pretty printing a value is covered by `trace`.
+    // By rejecting non-strings we allow future versions to add more features without breaking existing code.
+    auto msgStr = state.forceString(*args[0], pos, "while evaluating the first argument; the message passed to builtins.warn");
 
     {
-        BaseError msg(args[0]->type() == nString
-                ? std::string(args[0]->string_view())
-                : ({
-                    std::stringstream s;
-                    s << ValuePrinter(state, *args[0]);
-                    s.str();
-                }));
+        BaseError msg(std::string{msgStr});
         msg.atPos(state.positions[pos]);
         auto info = msg.info();
         info.level = lvlWarn;

tests/functional/lang.sh

@@ -33,8 +33,10 @@ nix-instantiate --eval -E 'let x = { repeating = x; tracing = builtins.trace x t
 
 nix-instantiate --eval -E 'builtins.warn "Hello" 123' 2>&1 | grepQuiet 'warning: Hello'
 nix-instantiate --eval -E 'builtins.addErrorContext "while doing ${"something"} interesting" (builtins.warn "Hello" 123)' 2>/dev/null | grepQuiet 123
-nix-instantiate --eval -E 'let x = builtins.warn { x = x; } true; in x' \
-  2>&1 | grepQuiet -E 'warning: { x = «potential infinite recursion»; }'
+
+# warn does not accept non-strings for now
+expectStderr 1 nix-instantiate --eval -E 'let x = builtins.warn { x = x; } true; in x' \
+  | grepQuiet "expected a string but found a set"
 expectStderr 1 nix-instantiate --eval --abort-on-warn -E 'builtins.warn "Hello" 123' | grepQuiet Hello
 NIX_ABORT_ON_WARN=1 expectStderr 1 nix-instantiate --eval -E 'builtins.addErrorContext "while doing ${"something"} interesting" (builtins.warn "Hello" 123)' | grepQuiet "while doing something interesting"