Standized code formatting via `rustfmt`

[Gijsreyn]

Summary of the new feature / enhancement

As a contributor to the DSC repository, I want to have a clear and consistent Rust code style across the project, So that:

• Pull request focuses on behavior changes rather than style arguments.
• All code "looks like it came from the same author."
• CI can automatically detect style violations and reject PRs that don't conform
• New contributors have less friction

Currently, formatting style is ad hoc. This causes friction in reviews, occasional style comment debates, and some churn when refactoring.

I propose we adopt rustfmt with a small, curated rustfmt.toml config file and enforce it in the CI (separate or Pester tests) with cargo fmt --check.

Proposed formatting policy

1. Use rustfmt as the canonical formatter:

   • Developers should run cargo fmt locally (or via IDE) before committing.
   • The project includes a rustfmt.toml file with agreed-upon overrides of defaults.

2. Minimal deviations from default style:

   • Keep most defaults. Only override settings that are meaningful
   • Example options to override: max_width, imports_granularity, reorder_imports, wrap_comments.

3. Check in CI and locally

   • Either a CI pipeline or Pester test should run:

   cargo fmt --all -- --check -- --files-with-diff
   

   • If formatting is incorrect, the build should fail and show the diff.

4. One-time bulk reformat

   • Upon acceptance, run cargo fmt over the existing codebase.
   • After that, all future PRs must remain formatted.

5. Developer tooling / integration

   • Configure popular tooling (VS Code) to auto-run formatting on save using rustfmt

6. Versioning and toolchain alignment

   • We pin the rustfmt version implicitly by pinning the Rust toolchain version.

Example rustfmt.toml

Below is a suggestion for a minimal config that tweaks a few settings to get started (whilst retaining most of the default style):

# rustfmt.toml for DSC

# Basic layout
max_width = 100
hard_tabs = false
tab_spaces = 4

# Imports
reorder_imports = true
imports_granularity = "Crate"
group_imports = "StdExternalCrate"

# Comments & docs
wrap_comments = true
format_code_in_doc_comments = true
normalize_doc_attributes = true

# Macro bodies, etc.
format_macro_bodies = true

# Edition (for parsing behavior)
edition = "2021"

Rationale for some choices:

• imports_granularity = "Crate" means use std::{fs, io} rather than separate lines.
• group_imports = "StdExternalCrate" groups standard library imports, external crates, and local modules separately (not sure about compliance here)
• Enabling comment wrapping and doc formatting keeps comments from drifting into odd indent or width shapes.
• format_macro_bodies = true ensures macro invocation bodies are formatted consistently with the rest of the code.