libexpr: Assert: Don't print assertion in error message

The `show` functionality needs to be removed because it is deeply
flawed, and given that we already print position information in the
error message (which probably wasn't always the case in the past) the
assertion printing is redundant anyways.

Change-Id: I1f5e05ab73aaa0ec92994c2211463260fd374898

doc/manual/rl-next/error-message-improvements.md

@@ -0,0 +1,9 @@
+---
+synopsis: Small error message improvements
+issues: []
+cls: [2185]
+category: Improvements
+credits: [piegames]
+---
+
+Failed asserts don't print the failed assertion expression anymore in the error message. That code was buggy and the information was redundant anyways, given that the error position already more accurately shows what exactly failed.

lix/libexpr/eval.cc

@@ -1827,9 +1827,10 @@ void ExprIf::eval(EvalState & state, Env & env, Value & v)
 void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 {
     if (!state.evalBool(env, *cond, pos, "in the condition of the assert statement")) {
-        std::ostringstream out;
-        cond->show(state.ctx.symbols, out);
-        state.ctx.errors.make<AssertionError>("assertion '%1%' failed", out.str()).atPos(pos).withFrame(env, *this).debugThrow();
+        state.ctx.errors.make<AssertionError>("assertion failed")
+            .atPos(pos)
+            .withFrame(env, *this)
+            .debugThrow();
     }
     body->eval(state, env, v);
 }

tests/functional/lang/eval-fail-assert.err.exp

@@ -13,7 +13,7 @@ error:
              |       ^
             3| in
 
-       error: assertion '(arg == "y")' failed
+       error: assertion failed
        at /pwd/lang/eval-fail-assert.nix:2:12:
             1| let
             2|   x = arg: assert arg == "y"; 123;

tests/unit/libexpr/value/print.cc

@@ -514,7 +514,7 @@ TEST_F(ValuePrintingTests, ansiColorsAssert)
 
     ASSERT_EQ(v.type(), nAttrs);
     test(*v.attrs->begin()->value,
-         ANSI_RED "«error: assertion 'false' failed»" ANSI_NORMAL,
+         ANSI_RED "«error: assertion failed»" ANSI_NORMAL,
          PrintOptions {
              .ansiColors = true,
              .force = true