Add nix formatter build command

`nix formatter build` is sort of like `nix build`: it builds, links, and
prints a path to the formatter program:

    $ nix formatter build
    /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt

Note that unlike `nix build`, this prints the full path to the program,
not just the store path (in the example above that would be
`/nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt`).

Motivation
----------

I maintain a vim plugin that automatically runs `nix fmt` on files on
save. Since `nix fmt` can be quite slow due to nix evaluation, I choose
to cache the `nix fmt `entrypoint. This was very awkward to do, see the
implementation for details:
7864607231/lua/null-ls/builtins/formatting/nix_flake_fmt.lua (L83-L110).

I recently discovered that my implementation was buggy (it didn't handle
flakes that expose a `formatter` package, such as nixpkgs), so I had to
rework the implementation:
https://github.com/nvimtools/none-ls.nvim/pull/272.

With the new `nix formatter build` command, I can delete all this akward
code, and it will be easier for other folks to build performant editor
integrations for `nix fmt`.

src/nix/formatter-build.md

@@ -0,0 +1,23 @@
+R""(
+
+# Description
+
+`nix formatter build` builds the formatter specified in the flake.
+
+Similar to [`nix build`](@docroot@/command-ref/new-cli/nix3-build.md),
+unless `--no-link` is specified, after a successful
+build, it creates a symlink to the store path of the formatter. This symlink is
+named `./result` by default; this can be overridden using the
+`--out-link` option.
+
+It always prints the command to standard output.
+
+# Examples
+
+* Build the formatter:
+
+  ```console
+  # nix formatter build
+  /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt
+  ```
+)""

src/nix/formatter.cc

@@ -1,6 +1,8 @@
 #include "nix/cmd/command.hh"
 #include "nix/cmd/installable-value.hh"
 #include "nix/expr/eval.hh"
+#include "nix/store/local-fs-store.hh"
+#include "nix/cmd/installable-derived-path.hh"
 #include "run.hh"
 
 using namespace nix;
@@ -25,7 +27,7 @@ struct CmdFormatter : NixMultiCommand
 
 static auto rCmdFormatter = registerCommand<CmdFormatter>("formatter");
 
-struct CmdFormatterRun : SourceExprCommand
+struct CmdFormatterRun : SourceExprCommand, MixJSON
 {
     std::vector<std::string> args;
 
@@ -87,6 +89,80 @@ struct CmdFormatterRun : SourceExprCommand
 
 static auto rFormatterRun = registerCommand2<CmdFormatterRun>({"formatter", "run"});
 
+struct CmdFormatterBuild : SourceExprCommand
+{
+    Path outLink = "result";
+
+    CmdFormatterBuild()
+    {
+        addFlag({
+            .longName = "out-link",
+            .shortName = 'o',
+            .description = "Use *path* as prefix for the symlink to the build result. It defaults to `result`.",
+            .labels = {"path"},
+            .handler = {&outLink},
+            .completer = completePath,
+        });
+
+        addFlag({
+            .longName = "no-link",
+            .description = "Do not create symlink to the build results.",
+            .handler = {&outLink, Path("")},
+        });
+    }
+
+    std::string description() override
+    {
+        return "build the current flake's formatter";
+    }
+
+    std::string doc() override
+    {
+        return
+#include "formatter-build.md"
+            ;
+    }
+
+    Category category() override
+    {
+        return catSecondary;
+    }
+
+    Strings getDefaultFlakeAttrPaths() override
+    {
+        return Strings{"formatter." + settings.thisSystem.get()};
+    }
+
+    Strings getDefaultFlakeAttrPathPrefixes() override
+    {
+        return Strings{};
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto evalState = getEvalState();
+        auto evalStore = getEvalStore();
+
+        auto installable_ = parseInstallable(store, ".");
+        auto & installable = InstallableValue::require(*installable_);
+        auto unresolvedApp = installable.toApp(*evalState);
+        auto app = unresolvedApp.resolve(evalStore, store);
+
+        Installables installableContext;
+        for (auto & ctxElt : unresolvedApp.unresolved.context)
+            installableContext.push_back(make_ref<InstallableDerivedPath>(store, DerivedPath{ctxElt}));
+        auto buildables = Installable::build(evalStore, store, Realise::Outputs, installableContext);
+
+        if (outLink != "")
+            if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
+                createOutLinks(outLink, toBuiltPaths(buildables), *store2);
+
+        logger->cout("%s", app.program);
+    };
+};
+
+static auto rFormatterBuild = registerCommand2<CmdFormatterBuild>({"formatter", "build"});
+
 struct CmdFmt : CmdFormatterRun
 {
     void run(ref<Store> store) override

tests/functional/formatter.sh

@@ -7,11 +7,14 @@ TODO_NixOS # Provide a `shell` variable. Try not to `export` it, perhaps.
 clearStoreIfPossible
 rm -rf "$TEST_HOME"/.cache "$TEST_HOME"/.config "$TEST_HOME"/.local
 
-cp ./simple.nix ./simple.builder.sh ./fmt.simple.sh "${config_nix}" "$TEST_HOME"
+cp ./simple.nix ./simple.builder.sh ./formatter.simple.sh "${config_nix}" "$TEST_HOME"
 
 cd "$TEST_HOME"
 
-nix fmt --help | grep "forward"
+nix formatter --help | grep "build or run the formatter"
+nix fmt --help | grep "reformat your code"
+nix fmt run --help | grep "reformat your code"
+nix fmt build --help | grep "build"
 
 cat << EOF > flake.nix
 {
@@ -23,16 +26,37 @@ cat << EOF > flake.nix
         buildCommand = ''
           mkdir -p \$out/bin
           echo "#! ${shell}" > \$out/bin/formatter
-          cat \${./fmt.simple.sh} >> \$out/bin/formatter
+          cat \${./formatter.simple.sh} >> \$out/bin/formatter
           chmod +x \$out/bin/formatter
         '';
       };
   };
 }
 EOF
+
 # No arguments check
 [[ "$(nix fmt)" = "Formatting(0):" ]]
+[[ "$(nix formatter run)" = "Formatting(0):" ]]
+
 # Argument forwarding check
 nix fmt ./file ./folder | grep 'Formatting(2): ./file ./folder'
+nix formatter run ./file ./folder | grep 'Formatting(2): ./file ./folder'
+
+# Build checks
+## Defaults to a ./result.
+nix formatter build | grep ".\+/bin/formatter"
+[[ -L ./result ]]
+rm result
+
+## Can prevent the symlink.
+nix formatter build --no-link
+[[ ! -e ./result ]]
+
+## Can change the symlink name.
+nix formatter build --out-link my-result | grep ".\+/bin/formatter"
+[[ -L ./my-result ]]
+rm ./my-result
+
+# Flake outputs check.
 nix flake check
 nix flake show | grep -P "package 'formatter'"