libexpr: add builtins.warn

Added builtins.warn` which takes two arguments: a message that is
displayed as a warning during evaluation which must be a string and a
value that is returned from the expression.

The next commits add new settings to control the behavior of the new
builtin: `debugger-on-warn` allows the user to start the debugger and
`abort-on-warn` aborts evaluation with an error.

Unlike upstream, I chose not to mark evaluation warnings from
`builtins.warn` as distinct from other warnings because that breaks the
commonly expected logging format `level: message`.

Co-authored-by: Qyriad <qyriad@qyriad.me>
Upstream-PR: https://github.com/NixOS/nix/pull/10592
Fixes: https://git.lix.systems/lix-project/lix/issues/579
Change-Id: I8658c88e5c27952b65e8b9f5525a572e0680cc1f

doc/manual/change-authors.yml

@@ -176,6 +176,11 @@ midnightveil:
   forgejo: midnightveil
   github: midnightveil
 
+milibopp:
+  display_name: Emilia Bopp
+  forgejo: milibopp
+  github: milibopp
+
 nan-git:
   display_name: NaN-git
   github: NaN-git

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

@@ -0,0 +1,11 @@
+---
+synopsis: "Add `builtins.warn` for emitting warnings from Nix code"
+cls: [2248]
+category: "Features"
+credits: [milibopp, Qyriad]
+---
+
+Lix now has a builtin function for emitting warnings.
+Like `builtins.trace`, it takes two arguments: the message to emit, and the expression to return.
+_Unlike_ `builtins.trace`, `builtins.warn` requires the first argument — the message — to be a string.
+In the future we may extend `builtins.warn` to accept a more structured API.

lix/libexpr/builtins/warn.md

@@ -0,0 +1,6 @@
+---
+name: warn
+args: [msg, e2]
+---
+Evaluate string *msg* and print it on standard error. Then return *e2*.
+This function is useful for warning about unexpected conditions without aborting evaluation.

lix/libexpr/meson.build

@@ -148,6 +148,7 @@ builtin_definitions = files(
   'builtins/typeOf.md',
   'builtins/unsafeDiscardOutputDependency.md',
   'builtins/unsafeGetAttrPos.md',
+  'builtins/warn.md',
   'builtins/zipAttrsWith.md',
   # keep-sorted end
 )

lix/libexpr/primops.cc

@@ -768,6 +768,19 @@ static void prim_trace(EvalState & state, Value * * args, Value & v)
     v = *args[1];
 }
 
+static void prim_warn(EvalState & state, Value ** args, Value & v)
+{
+    // 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 const msg =
+        state.forceString(*args[0], noPos, "while evaluating message for builtins.warn");
+
+    printTaggedWarning("%s", Uncolored(msg));
+
+    state.forceValue(*args[1], noPos);
+    v = *args[1];
+}
 
 /* Takes two arguments and evaluates to the second one. Used as the
  * builtins.traceVerbose implementation when --trace-verbose is not enabled

tests/functional2/lang/builtins.warn/eval-fail-reject-non-string.err.exp

@@ -0,0 +1,10 @@
+error:
+       … while calling the 'warn' builtin
+         at /pwd/in.nix:1:1:
+            1| builtins.warn { cat = "girl"; } 1337
+             | ^
+            2|
+
+       … while evaluating message for builtins.warn
+
+       error: expected a string but found a set: { cat = "girl"; }

tests/functional2/lang/builtins.warn/eval-okay.err.exp

@@ -0,0 +1 @@
+warning: meow

tests/functional2/lang/builtins.warn/eval-okay.out.exp

@@ -0,0 +1 @@
+1337

tests/functional2/lang/builtins.warn/in-reject-non-string.nix

@@ -0,0 +1 @@
+builtins.warn { cat = "girl"; } 1337

tests/functional2/lang/builtins.warn/in.nix

@@ -0,0 +1 @@
+builtins.warn "meow" 1337

tests/functional2/lang/builtins.warn/test.toml

@@ -0,0 +1,6 @@
+[[test]]
+runner = "eval-okay"
+
+[[test]]
+runner = "eval-fail"
+in = "in-reject-non-string.nix"

tests/unit/libexpr/primops.cc

@@ -121,6 +121,14 @@ namespace nix {
         ASSERT_NE(text.find("test string 123"), std::string::npos);
     }
 
+    TEST_F(PrimOpTest, warn) {
+        CaptureLogging l;
+        auto const v = eval("builtins.warn \"test string 123\" 123");
+        ASSERT_THAT(v, IsIntEq(123));
+        auto const text = l.get();
+        ASSERT_NE(text.find("test string 123"), std::string::npos);
+    }
+
     TEST_F(PrimOpTest, placeholder) {
         auto v = eval("builtins.placeholder \"out\"");
         ASSERT_THAT(v, IsStringEq("/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9"));