App({ hclOutput: true }) is ignored: the stack writes JSON into cdk.tf #561

Expected Behavior

new App({ hclOutput: true }) synthesizes HCL into stacks/<name>/cdk.tf.

Actual Behavior

The manifest names the file cdk.tf, but the stack writes JSON into it, so the file can't be parsed. Only SYNTH_HCL_OUTPUT=true produces real HCL.

const app = new App({ outdir: "out", hclOutput: true });
const stack = new TerraformStack(app, "s");
new TerraformVariable(stack, "v", { type: VariableType.STRING });
app.synth();
// out/stacks/s/cdk.tf contains `{ "//": ..., "variable": { ... } }`

Reproduced on upstream/main by running the library directly, the same way unit tests and Testing.app() users would.

Cause

Two settings that should agree don't:

• App resolves hclOutput from the config, with SYNTH_HCL_OUTPUT taking precedence (app.ts:102-108). The manifest uses that value for the file extension.
• TerraformStack builds its StackSynthesizer from the env var alone (terraform-stack.ts:124-129), and that decides the file's contents.

Under the CLI, synth-stack.ts always sets SYNTH_HCL_OUTPUT from --hcl (synth-stack.ts:145). The two values match there, but the hclOutput config is always overridden, so it does nothing.

Possible Solutions

Have StackSynthesizer read the resolved App.of(stack).hclOutput at synth time instead of reading the env var in the stack constructor. Then the config, the env var and the manifest all go through the same precedence in App.

Upstream cdktf Issue

hashicorp/terraform-cdk#3875 (open)

References

Blocks HCL-mode unit tests that use Testing.app({ hclOutput: true }). See #563.