print type and value in "flake attr is not a derivation" errors

This turns errors like:

error: flake output attribute 'hydraJobs' is not a derivation or path

into errors like:

error: expected flake output attribute 'hydraJobs' to be a derivation or
path but found a set: { binaryTarball = «thunk»; build = «thunk»; etc> }

This change affects all InstallableFlake commands.

Change-Id: I899757af418b6f98201006ec6ee13a448c07077c

2 files changed, 27 insertions, 2 deletions

diff --git a/doc/manual/rl-next/print-value-in-installable-flake-error.md b/doc/manual/rl-next/print-value-in-installable-flake-error.md
new file mode 100644
index 000000000..ae23b4dda
--- /dev/null
+++ b/doc/manual/rl-next/print-value-in-installable-flake-error.md
@@ -0,0 +1,20 @@
+---
+synopsis: New-cli flake commands that expect derivations now print the failing value and its type
+credits: Qyriad
+category: Improvements
+cls: 1177
+---
+
+In errors like `flake output attribute 'legacyPackages.x86_64-linux.lib' is not a derivation or path`, the message now includes the failing value and type.
+
+Before:
+
+```
+    error: flake output attribute 'nixosConfigurations.yuki.config' is not a derivation or path
+````
+
+After:
+
+```
+    error: expected flake output attribute 'nixosConfigurations.yuki.config' to be a derivation or path but found a set: { appstream = «thunk»; assertions = «thunk»; boot = { bcache = «thunk»; binfmt = «thunk»; binfmtMiscRegistrations = «thunk»; blacklistedKernelModules = «thunk»; bootMount = «thunk»; bootspec = «thunk»; cleanTmpDir = «thunk»; consoleLogLevel = «thunk»; «43 attributes elided» }; «48 attributes elided» }
+```
diff --git a/src/libcmd/installable-flake.cc b/src/libcmd/installable-flake.cc
index 615f70945..46bdd411b 100644
--- a/src/libcmd/installable-flake.cc
+++ b/src/libcmd/installable-flake.cc
@@ -105,9 +105,14 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
             fmt("while evaluating the flake output attribute '%s'", attrPath)))
         {
             return { *derivedPathWithInfo };
+        } else {
+            throw Error(
+                "expected flake output attribute '%s' to be a derivation or path but found %s: %s",
+                attrPath,
+                showType(v),
+                ValuePrinter(*this->state, v, errorPrintOptions)
+            );
         }
-        else
-            throw Error("flake output attribute '%s' is not a derivation or path", attrPath);
     }
 
     auto drvPath = attr->forceDerivation();