nix build: add --print-out-paths flag

has the same functionality as default nix-build

$ nix-build . -A "bash" -A "bash.dev" -A "tinycc"
/nix/store/4nmqxajzaf60yjribkgvj5j54x9yvr1r-bash-5.1-p12
/nix/store/c49i1ggnr5cc8gxmk9xm0cn961z104dn-bash-5.1-p12-dev
/nix/store/dbapb08862ajgaax3621fz8hly9fdah3-tcc-0.9.27+date=2022-01-11

$ nix-build . -A "bash"
/nix/store/4nmqxajzaf60yjribkgvj5j54x9yvr1r-bash-5.1-p12

$ $HOME/nixgits/nix/result/bin/nix build "nixpkgs#bash" "nixpkgs#bash.dev" "nixpkgs#tinycc" --print-out-paths
/nix/store/4nmqxajzaf60yjribkgvj5j54x9yvr1r-bash-5.1-p12
/nix/store/c49i1ggnr5cc8gxmk9xm0cn961z104dn-bash-5.1-p12-dev
/nix/store/dbapb08862ajgaax3621fz8hly9fdah3-tcc-0.9.27+date=2022-01-11

$ $HOME/nixgits/nix/result/bin/nix build "nixpkgs#bash" --print-out-paths
/nix/store/4nmqxajzaf60yjribkgvj5j54x9yvr1r-bash-5.1-p12

doc/manual/src/release-notes/rl-next.md

@@ -2,3 +2,6 @@
 
 * `nix repl` has a new build-'n-link (`:bl`) command that builds a derivation
   while creating GC root symlinks.
+
+* `nix build` has a new `--print-out-paths` flag to print the resulting output paths.
+  This matches the default behaviour of `nix-build`.

src/nix/build.cc

@@ -4,6 +4,7 @@
 #include "shared.hh"
 #include "store-api.hh"
 #include "local-fs-store.hh"
+#include "progress-bar.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -12,6 +13,7 @@ using namespace nix;
 struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
 {
     Path outLink = "result";
+    bool printOutputPaths = false;
     BuildMode buildMode = bmNormal;
 
     CmdBuild()
@@ -31,6 +33,12 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
             .handler = {&outLink, Path("")},
         });
 
+        addFlag({
+            .longName = "print-out-paths",
+            .description = "Print the resulting output paths",
+            .handler = {&printOutputPaths, true},
+        });
+
         addFlag({
             .longName = "rebuild",
             .description = "Rebuild an already built package and compare the result to the existing store paths.",
@@ -93,6 +101,22 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
                     }, buildable.raw());
                 }
 
+        if (printOutputPaths) {
+            stopProgressBar();
+            for (auto & buildable : buildables) {
+                std::visit(overloaded {
+                    [&](const BuiltPath::Opaque & bo) {
+                        std::cout << store->printStorePath(bo.path) << std::endl;
+                    },
+                    [&](const BuiltPath::Built & bfd) {
+                        for (auto & output : bfd.outputs) {
+                            std::cout << store->printStorePath(output.second) << std::endl;
+                        }
+                    },
+                }, buildable.raw());
+            }
+        }
+
         updateProfile(buildables);
     }
 };

src/nix/build.md

@@ -25,6 +25,13 @@ R""(
   lrwxrwxrwx 1 … result-1 -> /nix/store/rkfrm0z6x6jmi7d3gsmma4j53h15mg33-cowsay-3.03+dfsg2
   ```
 
+* Build GNU Hello and print the resulting store path.
+
+  ```console
+  # nix build nixpkgs#hello --print-out-paths
+  /nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10
+  ```
+
 * Build a specific output:
 
   ```console

tests/build-remote.sh

@@ -34,6 +34,14 @@ outPath=$(readlink -f $TEST_ROOT/result)
 
 grep 'FOO BAR BAZ' $TEST_ROOT/machine0/$outPath
 
+testPrintOutPath=$(nix build -L -v -f $file --print-out-paths --max-jobs 0 \
+  --arg busybox $busybox \
+  --store $TEST_ROOT/machine0 \
+  --builders "$(join_by '; ' "${builders[@]}")"
+)
+
+[[ $testPrintOutPath =~ store.*build-remote ]]
+
 set -o pipefail
 
 # Ensure that input1 was built on store1 due to the required feature.