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>

6 files changed, 110 insertions, 26 deletions

diff --git a/src/libcmd/installable-attr-path.cc b/src/libcmd/installable-attr-path.cc
index cf513126d..b35ca2910 100644
--- a/src/libcmd/installable-attr-path.cc
+++ b/src/libcmd/installable-attr-path.cc
@@ -46,7 +46,15 @@ std::pair<Value *, PosIdx> InstallableAttrPath::toValue(EvalState & state)
 
 DerivedPathsWithInfo InstallableAttrPath::toDerivedPaths()
 {
-    auto v = toValue(*state).first;
+    auto [v, pos] = toValue(*state);
+
+    if (std::optional derivedPathWithInfo = trySinglePathToDerivedPaths(
+        *v,
+        pos,
+        fmt("while evaluating the attribute '%s'", attrPath)))
+    {
+        return { *derivedPathWithInfo };
+    }
 
     Bindings & autoArgs = *cmd.getAutoArgs(*state);
 
diff --git a/src/libcmd/installable-flake.cc b/src/libcmd/installable-flake.cc
index 37e59cfdf..eb944240b 100644
--- a/src/libcmd/installable-flake.cc
+++ b/src/libcmd/installable-flake.cc
@@ -95,31 +95,13 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
         // FIXME: use eval cache?
         auto v = attr->forceValue();
 
-        if (v.type() == nPath) {
-            auto storePath = v.path().fetchToStore(state->store);
-            return {{
-                .path = DerivedPath::Opaque {
-                    .path = std::move(storePath),
-                },
-                .info = make_ref<ExtraPathInfo>(),
-            }};
-        }
-
-        else if (v.type() == nString) {
-            NixStringContext context;
-            auto s = state->forceString(v, context, noPos, fmt("while evaluating the flake output attribute '%s'", attrPath));
-            auto storePath = state->store->maybeParseStorePath(s);
-            if (storePath && context.count(NixStringContextElem::Opaque { .path = *storePath })) {
-                return {{
-                    .path = DerivedPath::Opaque {
-                        .path = std::move(*storePath),
-                    },
-                    .info = make_ref<ExtraPathInfo>(),
-                }};
-            } else
-                throw Error("flake output attribute '%s' evaluates to the string '%s' which is not a store path", attrPath, s);
+        if (std::optional derivedPathWithInfo = trySinglePathToDerivedPaths(
+            v,
+            noPos,
+            fmt("while evaluating the flake output attribute '%s'", attrPath)))
+        {
+            return { *derivedPathWithInfo };
         }
-
         else
             throw Error("flake output attribute '%s' is not a derivation or path", attrPath);
     }
diff --git a/src/libcmd/installable-value.cc b/src/libcmd/installable-value.cc
index 3a7ede4e2..1eff293cc 100644
--- a/src/libcmd/installable-value.cc
+++ b/src/libcmd/installable-value.cc
@@ -41,4 +41,26 @@ ref<InstallableValue> InstallableValue::require(ref<Installable> installable)
     return ref { castedInstallable };
 }
 
+std::optional<DerivedPathWithInfo> InstallableValue::trySinglePathToDerivedPaths(Value & v, const PosIdx pos, std::string_view errorCtx)
+{
+    if (v.type() == nPath) {
+        auto storePath = v.path().fetchToStore(state->store);
+        return {{
+            .path = DerivedPath::Opaque {
+                .path = std::move(storePath),
+            },
+            .info = make_ref<ExtraPathInfo>(),
+        }};
+    }
+
+    else if (v.type() == nString) {
+        return {{
+            .path = state->coerceToDerivedPath(pos, v, errorCtx),
+            .info = make_ref<ExtraPathInfo>(),
+        }};
+    }
+
+    else return std::nullopt;
+}
+
 }
diff --git a/src/libcmd/installable-value.hh b/src/libcmd/installable-value.hh
index 5ab7eee16..3138ce8ec 100644
--- a/src/libcmd/installable-value.hh
+++ b/src/libcmd/installable-value.hh
@@ -98,6 +98,24 @@ struct InstallableValue : Installable
 
     static InstallableValue & require(Installable & installable);
     static ref<InstallableValue> require(ref<Installable> installable);
+
+protected:
+
+    /**
+     * Handles either a plain path, or a string with a single string
+     * context elem in the right format. The latter case is handled by
+     * `EvalState::coerceToDerivedPath()`; see it for details.
+     *
+     * @param v Value that is hopefully a string or path per the above.
+     *
+     * @param pos Position of value to aid with diagnostics.
+     *
+     * @param errorCtx Arbitrary message for use in potential error message when something is wrong with `v`.
+     *
+     * @result A derived path (with empty info, for now) if the value
+     * matched the above criteria.
+     */
+    std::optional<DerivedPathWithInfo> trySinglePathToDerivedPaths(Value & v, const PosIdx pos, std::string_view errorCtx);
 };
 
 }
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"