distinguish between throws & errors during throw

Turns errors like this:

let
  throwMsg = a: throw (a + " invalid bar");
in throwMsg "bullshit"

error:
       … from call site
         at «string»:3:4:
            2|   throwMsg = a: throw (a + " invalid bar");
            3| in throwMsg "bullshit"
             |    ^

       … while calling 'throwMsg'
         at «string»:2:14:
            1| let
            2|   throwMsg = a: throw (a + " invalid bar");
             |              ^
            3| in throwMsg "bullshit"

       … while calling the 'throw' builtin
         at «string»:2:17:
            1| let
            2|   throwMsg = a: throw (a + " invalid bar");
             |                 ^
            3| in throwMsg "bullshit"

       error: bullshit invalid bar

into errors like this:

let
  throwMsg = a: throw (a + " invalid bar");
in throwMsg "bullshit"

error:
       … from call site
         at «string»:3:4:
            2|   throwMsg = a: throw (a + " invalid bar");
            3| in throwMsg "bullshit"
             |    ^

       … while calling 'throwMsg'
         at «string»:2:14:
            1| let
            2|   throwMsg = a: throw (a + " invalid bar");
             |              ^
            3| in throwMsg "bullshit"

       … caused by explicit throw
         at «string»:2:17:
            1| let
            2|   throwMsg = a: throw (a + " invalid bar");
             |                 ^
            3| in throwMsg "bullshit"

       error: bullshit invalid bar

Change-Id: I593688928ece20f97999d1bf03b2b46d9ac338cb

doc/manual/rl-next/distinguish-throw-errors.md

@@ -0,0 +1,70 @@
+---
+synopsis: "Distinguish between explicit throws and errors that happened while evaluating a throw"
+cls: 1511
+credits: Qyriad
+category: Improvements
+---
+
+Previously, errors caused by an expression like `throw "invalid argument"` were treated like an error that happened simply while some builtin function was being called:
+
+```
+let
+  throwMsg = p: throw "${p} isn't the right package";
+in throwMsg "linuz"
+
+error:
+       … while calling the 'throw' builtin
+         at «string»:2:17:
+            1| let
+            2|   throwMsg = p: throw "${p} isn't the right package";
+             |                 ^
+            3| in throwMsg "linuz"
+
+       error: linuz isn't the right package
+```
+
+But the error didn't just happen "while" calling the `throw` builtin — it's a throw error!
+Now it looks like this:
+
+```
+let
+  throwMsg = p: throw "${p} isn't the right package";
+in throwMsg "linuz"
+
+error:
+       … caused by explicit throw
+         at «string»:2:17:
+            1| let
+            2|   throwMsg = p: throw "${p} isn't the right package";
+             |                 ^
+            3| in throwMsg "linuz"
+
+       error: linuz isn't the right package
+```
+
+This also means that incorrect usage of `throw` or errors evaluating its arguments are easily distinguishable from explicit throws:
+
+```
+let
+  throwMsg = p: throw "${p} isn't the right package";
+in throwMsg { attrs = "error when coerced in string interpolation"; }
+
+error:
+       … while calling the 'throw' builtin
+         at «string»:2:17:
+            1| let
+            2|   throwMsg = p: throw "${p} isn't the right package";
+             |                 ^
+            3| in throwMsg { attrs = "error when coerced in string interpolation"; }
+
+       … while evaluating a path segment
+         at «string»:2:24:
+            1| let
+            2|   throwMsg = p: throw "${p} isn't the right package";
+             |                        ^
+            3| in throwMsg { attrs = "error when coerced in string interpolation"; }
+
+       error: cannot coerce a set to a string: { attrs = "error when coerced in string interpolation"; }
+```
+
+Here, instead of an actual thrown error, a type error happens first (trying to coerce an attribute set to a string), but that type error happened *while* calling `throw`.

src/libexpr/eval.cc

@@ -1807,6 +1807,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
 
                 try {
                     fn->fun(*this, vCur.determinePos(noPos), args, vCur);
+                } catch (ThrownError & e) {
+                    // Distinguish between an error that simply happened while "throw"
+                    // was being evaluated and an explicit thrown error.
+                    if (fn->name == "throw") {
+                        addErrorTrace(e, pos, "caused by explicit %s", "throw");
+                    } else {
+                        addErrorTrace(e, pos, "while calling the '%s' builtin", fn->name);
+                    }
+                    throw;
                 } catch (Error & e) {
                     addErrorTrace(e, pos, "while calling the '%1%' builtin", fn->name);
                     throw;

tests/functional/eval.sh

@@ -25,7 +25,7 @@ nix eval -E 'assert 1 + 2 == 3; true'
 # Top-level eval errors should be printed to stderr with a traceback.
 topLevelThrow="$(expectStderr 1 nix eval --expr 'throw "a sample throw message"')"
 [[ "$topLevelThrow" =~ "a sample throw message" ]]
-[[ "$topLevelThrow" =~ "while calling the 'throw' builtin" ]]
+[[ "$topLevelThrow" =~ "caused by explicit throw" ]]
 
 # But errors inside something should print an elided version, and exit with 0.
 outputOfNestedThrow="$(nix eval --expr '{ throws = throw "a sample throw message"; }')"

tests/functional/lang/eval-fail-duplicate-traces.err.exp

@@ -41,7 +41,7 @@ error:
              |                ^
             5|     if n > 0
 
-       … while calling the 'throw' builtin
+       … caused by explicit throw
          at /pwd/lang/eval-fail-duplicate-traces.nix:7:10:
             6|     then throwAfter (n - 1)
             7|     else throw "Uh oh!";

tests/functional/lang/eval-fail-foldlStrict-strict-op-application.err.exp

@@ -27,7 +27,7 @@ error:
              |      ^
             6|
 
-       … while calling the 'throw' builtin
+       … caused by explicit throw
          at /pwd/lang/eval-fail-foldlStrict-strict-op-application.nix:5:9:
             4|   null
             5|   [ (_: throw "Not the final value, but is still forced!") (_: 23) ]

tests/functional/lang/eval-fail-mutual-recursion.err.exp

@@ -54,7 +54,7 @@ error:
 
        (21 duplicate frames omitted)
 
-       … while calling the 'throw' builtin
+       … caused by explicit throw
          at /pwd/lang/eval-fail-mutual-recursion.nix:34:10:
            33|     then throwAfterB true 10
            34|     else throw "Uh oh!";

tests/functional/lang/eval-fail-not-throws.err.exp

@@ -5,7 +5,7 @@ error:
              |    ^
             2|
 
-       … while calling the 'throw' builtin
+       … caused by explicit throw
          at /pwd/lang/eval-fail-not-throws.nix:1:4:
             1| ! (throw "uh oh!")
              |    ^

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

@@ -40,7 +40,7 @@ error:
              |       ^
             8|     }
 
-       … while calling the 'throw' builtin
+       … caused by explicit throw
          at /pwd/lang/eval-fail-toJSON.nix:7:13:
             6|     {
             7|       c.d = throw "hah no";