Make more string values work as installables

As discussed in #7417, it would be good to make more string values work
as installables. That is to say, if an installable refers to a value,
and the value is a string, it used to not work at all, since #7484, it
works somewhat, and this PR make it work some more.

The new cases that are added for `BuiltPath` contexts:

- Fixed input- or content-addressed derivation:

  ```
  nix-repl> hello.out.outPath
  "/nix/store/jppfl2bp1zhx8sgs2mgifmsx6dv16mv2-hello-2.12"

  nix-repl> :p builtins.getContext hello.out.outPath
  { "/nix/store/c7jrxqjhdda93lhbkanqfs07x2bzazbm-hello-2.12.drv" = { outputs = [ "out" ]; }; }

  The string matches the specified single output of that derivation, so
  it should also be valid.

- Floating content-addressed derivation:

  ```
  nix-repl> (hello.overrideAttrs (_: { __contentAddressed = true; })).out.outPath
  "/1a08j26xqc0zm8agps8anxpjji410yvsx4pcgyn4bfan1ddkx2g0"

  nix-repl> :p builtins.getContext (hello.overrideAttrs (_: { __contentAddressed = true; })).out.outPath
  { "/nix/store/qc645pyf9wl37c6qvqzaqkwsm1gp48al-hello-2.12.drv" = { outputs = [ "out" ]; }; }
  ```

  The string is not a path but a placeholder, however it also matches
  the context, and because it is a CA derivation we have no better
  option. This should also be valid.

We may also want to think about richer attrset based values (also
discussed in that issue and #6507), but this change "completes" our
string-based building blocks, from which the others can be desugared
into or at least described/document/taught in terms of.

Progress towards #7417

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

2 files changed, 55 insertions, 1 deletions

diff --git a/tests/build.sh b/tests/build.sh
index b579fc374..697aff0f9 100644
--- a/tests/build.sh
+++ b/tests/build.sh
@@ -57,6 +57,30 @@ nix build -f multiple-outputs.nix --json 'e^*' --no-link | jq --exit-status '
     (.outputs | keys == ["a_a", "b", "c"]))
 '
 
+# test buidling from non-drv attr path
+
+nix build -f multiple-outputs.nix --json 'e.a_a.outPath' --no-link | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-e.drv")) and
+    (.outputs | keys == ["a_a"]))
+'
+
+# Illegal type of string context
+expectStderr 1 nix build -f multiple-outputs.nix 'e.a_a.drvPath' \
+  | grepQuiet "has a context which refers to a complete source and binary closure."
+
+# No string context
+expectStderr 1 nix build --expr '""' --no-link \
+  | grepQuiet "has 0 entries in its context. It should only have exactly one entry"
+
+# Too much string context
+expectStderr 1 nix build --impure --expr 'with (import ./multiple-outputs.nix).e.a_a; "${drvPath}${outPath}"' --no-link \
+  | grepQuiet "has 2 entries in its context. It should only have exactly one entry"
+
+nix build --impure --json --expr 'builtins.unsafeDiscardOutputDependency (import ./multiple-outputs.nix).e.a_a.drvPath' --no-link | jq --exit-status '
+  (.[0] | .path | match(".*multiple-outputs-e.drv"))
+'
+
 # Test building from raw store path to drv not expression.
 
 drv=$(nix eval -f multiple-outputs.nix --raw a.drvPath)
diff --git a/tests/flakes/build-paths.sh b/tests/flakes/build-paths.sh
index b399a066e..ff012e1b3 100644
--- a/tests/flakes/build-paths.sh
+++ b/tests/flakes/build-paths.sh
@@ -41,10 +41,27 @@ cat > $flake1Dir/flake.nix <<EOF
     a8 = builtins.storePath $dep;
 
     a9 = "$dep";
+
+    drvCall = with import ./config.nix; mkDerivation {
+      name = "simple";
+      builder = ./simple.builder.sh;
+      PATH = "";
+      goodPath = path;
+    };
+
+    a10 = builtins.unsafeDiscardOutputDependency self.drvCall.drvPath;
+
+    a11 = self.drvCall.drvPath;
+
+    a12 = self.drvCall.outPath;
+
+    a13 = "\${self.drvCall.drvPath}\${self.drvCall.outPath}";
   };
 }
 EOF
 
+cp ../simple.nix ../simple.builder.sh ../config.nix $flake1Dir/
+
 echo bar > $flake1Dir/foo
 
 nix build --json --out-link $TEST_ROOT/result $flake1Dir#a1
@@ -63,4 +80,17 @@ nix build --json --out-link $TEST_ROOT/result $flake1Dir#a6
 nix build --impure --json --out-link $TEST_ROOT/result $flake1Dir#a8
 diff common.sh $TEST_ROOT/result
 
-(! nix build --impure --json --out-link $TEST_ROOT/result $flake1Dir#a9)
+expectStderr 1 nix build --impure --json --out-link $TEST_ROOT/result $flake1Dir#a9 \
+  | grepQuiet "has 0 entries in its context. It should only have exactly one entry"
+
+nix build --json --out-link $TEST_ROOT/result $flake1Dir#a10
+[[ $(readlink -e $TEST_ROOT/result) = *simple.drv ]]
+
+expectStderr 1 nix build --json --out-link $TEST_ROOT/result $flake1Dir#a11 \
+  | grepQuiet "has a context which refers to a complete source and binary closure"
+
+nix build --json --out-link $TEST_ROOT/result $flake1Dir#a12
+[[ -e $TEST_ROOT/result/hello ]]
+
+expectStderr 1 nix build --impure --json --out-link $TEST_ROOT/result $flake1Dir#a13 \
+  | grepQuiet "has 2 entries in its context. It should only have exactly one entry"