Write generated files to output-dir

[pepone]

fix #760

[Copilot]

Pull request overview

Fixes #760 by ensuring generated code files emitted by generator plugins are written under the --output-dir specified to slicec, rather than always using the generator-provided paths relative to the current working directory.

Changes:

• Thread output_dir through generator response handling to the file-writing routine.
• Compute the on-disk output path by prefixing generated file paths with --output-dir when provided.
• Use the computed output path for read/overwrite checks and file creation.

Comments suppressed due to low confidence (1)

slicec/src/main.rs:131

• The IO diagnostic reports path: generated_file.path, which no longer matches the actual file location when --output-dir is used. Update the diagnostic to use the computed output path (including output_dir) so users can see where the write failed.

                let diagnostic = slicec::diagnostics::Error::IO {
                    action: "write generated file",
                    path: generated_file.path.to_owned(),
                    error: io_error,
                };

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[Copilot]

When output_dir is set, PathBuf::join will ignore it if generated_file.path is absolute, and it will also allow .. path traversal to escape the output directory. Since generated_file.path comes from an external generator process, validate it’s a relative path without parent/root/prefix components (or explicitly enforce that the final resolved path stays within output_dir) and return an error if it doesn’t.

[Copilot]

File::create(&generated_file_path) will fail if the parent directories don’t exist (e.g., when --output-dir points to a new directory or the generator emits paths like foo/bar.rs). Consider creating generated_file_path.parent() (and/or the output dir) with std::fs::create_dir_all before creating the file, and surface a clear IO diagnostic if that fails.

[InsertCreativityHere]

I’d just add a “use std::path::PathBuf” to this file so you don’t need to qualify it here.