trace when the foo part of foo.bar.baz errors

Turns errors like:

let
  errpkg = throw "invalid foobar";
in errpkg.meta

error:
       … while calling the 'throw' builtin
         at «string»:2:12:
            1| let
            2|   errpkg = throw "invalid foobar";
             |            ^
            3| in errpkg.meta

       error: invalid foobar

into errors like:

let
  errpkg = throw "invalid foobar";
in errpkg.meta

error:
       … while evaluating 'errpkg' to select 'meta' on it
         at «string»:3:4:
            2|   errpkg = throw "invalid foobar";
            3| in errpkg.meta
             |    ^

       … while calling the 'throw' builtin
         at «string»:2:12:
            1| let
            2|   errpkg = throw "invalid foobar";
             |            ^
            3| in errpkg.meta

       error: invalid foobar

For the low price of one try/catch, you too can have the incorrect line
of code actually show up in the trace!

Change-Id: If8d6200ec1567706669d405c34adcd7e2d2cd29d

doc/manual/rl-next/better-attrpath-errors.md

@@ -0,0 +1,64 @@
+---
+synopsis: "Trace when the `foo` part of a `foo.bar.baz` expression errors"
+cls: 1505
+credits: Qyriad
+category: Improvements
+---
+
+Previously, if an expression like `linux_4_9.meta.description` errored in the `linux_4_9` part, it wouldn't show you that that's the part of the expression that failed to evaluate, or even that that line of code is what caused evaluation of the failing expression.
+The previous error looks like this:
+
+```
+let
+  inherit (pkgs.linuxKernel.kernels) linux_4_9;
+in linux_4_9.meta.description
+
+error:
+       … while evaluating the attribute 'linux_4_9'
+         at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:5:
+          277|   } // lib.optionalAttrs config.allowAliases {
+          278|     linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
+             |     ^
+          279|     linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
+
+       … while calling the 'throw' builtin
+         at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:17:
+          277|   } // lib.optionalAttrs config.allowAliases {
+          278|     linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
+             |                 ^
+          279|     linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
+
+       error: linux 4.9 was removed because it will reach its end of life within 22.11
+```
+
+Now, the error will look like this:
+
+```
+let
+  inherit (pkgs.linuxKernel.kernels) linux_4_9;
+in linux_4_9.meta.description
+error:
+       … while evaluating 'linux_4_9' to select 'meta.description' on it
+         at «string»:3:4:
+            2|   inherit (pkgs.linuxKernel.kernels) linux_4_9;
+            3| in linux_4_9.meta.description
+             |    ^
+
+       … while evaluating the attribute 'linux_4_9'
+         at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:5:
+          277|   } // lib.optionalAttrs config.allowAliases {
+          278|     linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
+             |     ^
+          279|     linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
+
+       … caused by explicit throw
+         at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:17:
+          277|   } // lib.optionalAttrs config.allowAliases {
+          278|     linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
+             |                 ^
+          279|     linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
+
+       error: linux 4.9 was removed because it will reach its end of life within 22.11
+```
+
+Not only does the line of code that referenced the failing binding show up in the trace, it also tells you that it was specifically the `linux_4_9` part that failed.

src/libexpr/eval.cc

@@ -1432,7 +1432,20 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     Value * vCurrent = &vFirst;
     // Position for the current attrset Value in this select chain.
     PosIdx posCurrent;
-    e->eval(state, env, vFirst);
+
+    try {
+        e->eval(state, env, vFirst);
+    } catch (Error & e) {
+        assert(this->e != nullptr);
+        state.addErrorTrace(
+            e,
+            getPos(),
+            "while evaluating '%s' to select '%s' on it",
+            ExprPrinter(state, *this->e),
+            showAttrPath(state.symbols, this->attrPath)
+        );
+        throw;
+    }
 
     try {
         auto dts = state.debugRepl

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

@@ -1,4 +1,10 @@
 error:
+       … while evaluating 'a' to select 'foo' on it
+         at /pwd/lang/eval-fail-recursion.nix:1:21:
+            1| let a = {} // a; in a.foo
+             |                     ^
+            2|
+
        … in the right operand of the update (//) operator
          at /pwd/lang/eval-fail-recursion.nix:1:12:
             1| let a = {} // a; in a.foo