nix-profile-install: show helpful error upon package conflict

Whenever a file conflict happens during "nix profile install" an error
is shown that was previously thrown inside builtins.buildEnv.

We catch BuildProfileConflictError here so that we can provide the user
with more useful instructions on what to do next.

Most notably, we give the user concrete commands to use with all
parameters  already filled in. This avoids the need for the user to look
up these commands in manual pages.

src/nix/profile.cc

@@ -329,7 +329,66 @@ struct CmdProfileInstall : InstallablesCommand, MixDefaultProfile
             manifest.elements.push_back(std::move(element));
         }
 
-        updateProfile(manifest.build(store));
+        try {
+            updateProfile(manifest.build(store));
+        } catch (BuildProfileConflictError & conflictError) {
+            auto findInstallableOfPkgPath = [&](const Path & pkgPath) {
+                std::string fallbackInstallable = "INSTALLABLE";
+                auto maybeStorePath = store->maybeParseStorePath(pkgPath);
+                if (!maybeStorePath) {
+                    printError("warning: unable to determine installable from package path (%1%)", pkgPath);
+                    return fallbackInstallable;
+                }
+                auto storePath = *maybeStorePath;
+                auto manifestElement = find_if(manifest.elements.begin(), manifest.elements.end(), [&](const ProfileElement & currentProfileElement) {
+                    return currentProfileElement.storePaths.contains(storePath);
+                });
+                if (manifestElement == manifest.elements.end()) {
+                    printError("warning: unable to find profile element for store path (%1%)", storePath.to_string());
+                    return fallbackInstallable;
+                }
+                if (!manifestElement->source) {
+                    printError("warning: store path does not have a source (%1%)", storePath.to_string());
+                    return fallbackInstallable;
+                }
+                return manifestElement->source->originalRef.to_string();
+            };
+            std::string originalInstallable = findInstallableOfPkgPath(conflictError.originalPkgPath);
+            std::string conflictingInstallable = findInstallableOfPkgPath(conflictError.conflictingPkgPath);
+
+            throw Error(
+                "An existing package already provides the following file:\n"
+                "\n"
+                "  %1%\n"
+                "\n"
+                "This is the conflicting file from the new package:\n"
+                "\n"
+                "  %3%\n"
+                "\n"
+                "To remove the existing package:\n"
+                "\n"
+                "  nix profile remove %5%\n"
+                "\n"
+                "The new package can also be installed next to the existing one by assigning a different priority.\n"
+                "The conflicting packages have a priority of %7%.\n"
+                "To prioritise the new package:\n"
+                "\n"
+                "  nix profile install %6% --priority %8%\n"
+                "\n"
+                "To prioritise the existing package:\n"
+                "\n"
+                "  nix profile install %6% --priority %9%\n",
+                conflictError.originalFile,
+                conflictError.originalPkgPath,
+                conflictError.conflictingFile,
+                conflictError.conflictingPkgPath,
+                originalInstallable,
+                conflictingInstallable,
+                conflictError.priority,
+                conflictError.priority - 1,
+                conflictError.priority + 1
+            );
+        }
     }
 };