nix/doctor: test whether the current profile generation points somewhere

This helps users to debug whether their current profile symlinks are
correctly set.

Expected outputs look like this:
```
❯ sudo ./outputs/out/bin/nix doctor
[snip]

[PASS] All profiles are gcroots.
[PASS] Client protocol matches store protocol.
[INFO] You are trusted by store uri: local
[FAIL] Error: current generation cannot be discovered for profile:
'/nix/var/nix/profiles/default'
```

```
❯ ./outputs/out/bin/nix doctor
[snip]

[PASS] All profiles are gcroots.
[PASS] Client protocol matches store protocol.
[INFO] You are trusted by store uri: daemon
[PASS] You have 28 generations for profile
'/nix/var/nix/profiles/per-user/raito/profile'
The current generation number is '290'
```

Change-Id: I50c69cbeac3291d668f4c2332803411579adc944
Signed-off-by: Raito Bezarius <raito@lix.systems>
Co-authored-by: Qyriad <qyriad@qyriad.me>

doc/manual/rl-next/assess-profile-generations.md

@@ -0,0 +1,12 @@
+---
+synopsis: Assess current profile generations pointers in `nix doctor`
+cls: [3108]
+category: Improvements
+credits: [raito]
+---
+
+Added a new check to `nix doctor` that verifies whether the current generation of
+a Nix profile can be resolved. This helps users diagnose issues with broken or
+misconfigured profile symlinks.
+
+This helps determining if you have broken symlinks or misconfigured packaging.

lix/nix/doctor.cc

@@ -8,6 +8,7 @@
 #include "lix/libstore/local-fs-store.hh"
 #include "lix/libstore/worker-protocol.hh"
 #include "lix/libutil/exit.hh"
+#include "lix/libstore/profiles.hh"
 #include "doctor.hh"
 
 namespace nix {
@@ -69,6 +70,10 @@ struct CmdDoctor : StoreCommand
         }
         success &= checkStoreProtocol(aio().blockOn(store->getProtocol()));
         checkTrustedUser(store);
+        {
+            Path profile = getEnv("NIX_PROFILE").value_or(getDefaultProfile());
+            checkValidCurrentProfileGeneration(profile);
+        }
 
         if (!success)
             throw Exit(2);
@@ -152,6 +157,31 @@ struct CmdDoctor : StoreCommand
         std::string_view trustedness = trustedMay ? (*trustedMay ? "trusted" : "not trusted") : "unknown trust";
         checkInfo(fmt("You are %s by store uri: %s", trustedness, store->getUri()));
     }
+
+    void checkValidCurrentProfileGeneration(const Path & profile)
+    {
+        Generations generations;
+        std::optional<GenerationNumber> currentGeneration;
+        std::string errStr;
+
+        try {
+            std::tie(generations, currentGeneration) = findGenerations(profile);
+        } catch (SysError const & e) {
+            errStr = fmt(": %s", e.msg());
+        }
+
+        if (!currentGeneration) {
+            std::stringstream ss;
+            ss << "Error: current generation cannot be discovered for profile: '" << profile << "'";
+            ss << errStr << "\n";
+            checkFail(ss.str());
+        } else {
+            std::stringstream ss;
+            ss << "You have " << generations.size() << " generations for profile '" << profile << "'\n";
+            ss << "The current generation number is '" << *currentGeneration << "'\n";
+            checkPass(ss.str());
+        }
+    }
 };
 
 void registerNixDoctor()