Update design for PR rust-lang#16198: Update default branch name of rust-lang/rust in subtree sync docs

Author: gauravagerwala

.exp/design-workflow-9-sync-rustc.md

@@ -52,7 +52,7 @@ sequenceDiagram
     Note over D: Updates rustup branch in clippy-local (personal fork)
     D->>CR: git fetch upstream (rust-lang/rust remote)
     D->>CR: git switch rustup
-    D->>CR: git merge upstream/master --no-ff
+    D->>CR: git merge upstream/main --no-ff
     Note over CR: Merge commit syncs rust changes - allowed exception to no-merge policy
     D->>CR: cargo dev sync update_nightly
     Note over CR: Updates rust-toolchain.toml channel=nightly-YYYY-MM-DD<br/>and README.md block
@@ -68,13 +68,13 @@ sequenceDiagram
     participant RR as rust-lang/rust Repo
     participant CR as rust-clippy Repo
     Note over D,RR: Ensure in rust-lang/rust repo dir
-    D->>RR: git switch master (pull latest)
+    D->>RR: git switch main (pull latest)
     D->>RR: git switch -c clippy-subtree-update
     D->>RR: git subtree pull -P src/tools/clippy clippy-upstream master
     Note over RR: Pulls latest from official clippy master into subtree
     D->>RR: git commit changes (if needed)
     D->>RR: git push origin clippy-subtree-update
-    Note over RR: Create and merge PR to rust-lang/rust master
+    Note over RR: Create and merge PR to rust-lang/rust main
 ```
 
 ## Other High-Level Design Aspects

.exp/pr-analysis-16198.md

@@ -0,0 +1,32 @@
+# PR #16198: Workflow Design Impact Analysis
+
+## Affected Workflows
+- sync-rustc: The PR modifies `book/src/development/infrastructure/sync.md`, which is a key documentation file for the sync-rustc workflow as listed in `.exp/workflows.json` under relevant_files. The changes update the default branch name references for the `rust-lang/rust` repository from \"master\" to \"main\", directly impacting the documented steps in this workflow's design.
+
+## sync-rustc Analysis
+### Summary of design changes
+The PR implements a documentation update in the subtree synchronization guide to reflect the current default branch name (\"main\") of the `rust-lang/rust` repository. This affects specific git commands and instructional text used in the bi-directional sync process between rust-clippy and rust-lang/rust.
+
+- **Affected aspects**: Git merge command in the sync from rust to clippy (`upstream/master` → `upstream/main`), and checkout instruction in sync from clippy to rust (`master` → `main`).
+- **Implementation**: Direct string replacements in the markdown documentation file.
+- **Benefits/Implications**: Prevents runtime errors in git operations due to outdated branch names; ensures consistency across project docs. No functional code changes, purely documentary. The design document's Mermaid diagrams have been updated to match via this analysis process.
+
+### Mermaid diagram showing changes to the workflow
+```mermaid
+flowchart TD
+    subgraph "Sync from rust-lang/rust to rust-clippy Changes"
+        OldMerge["Old Step: git merge upstream/master --no-ff<br/>Red: Removal of old ref"] -->|updated to| NewMerge["New Step: git merge upstream/main --no-ff<br/>Green: Addition of new ref"]
+        style OldMerge fill:#ffcccc,stroke:#ff0000,stroke-width:2px
+        style NewMerge fill:#ccffcc,stroke:#00aa00,stroke-width:2px
+    end
+    subgraph "Sync from clippy to rust-lang/rust Changes"
+        OldSwitch["Old Step: git switch master (pull latest)<br/>Red: Removal"] -->|changed to| NewSwitch["New Step: git switch main (pull latest)<br/>Green: Addition"]
+        OldPRNote["Old Note: Create and merge PR to rust-lang/rust master<br/>Red: Removal"] -->|updated| NewPRNote["New Note: Create and merge PR to rust-lang/rust main<br/>Green: Addition"]
+        style OldSwitch fill:#ffcccc,stroke:#ff0000,stroke-width:2px
+        style NewSwitch fill:#ccffcc,stroke:#00aa00,stroke-width:2px
+        style OldPRNote fill:#ffcccc,stroke:#ff0000,stroke-width:2px
+        style NewPRNote fill:#ccffcc,stroke:#00aa00,stroke-width:2px
+    end
+    YellowChange["Yellow: Overall textual changes in branch references throughout diagrams"]:::change
+    classDef change fill:#ffff99,stroke:#cccc00,stroke-width:2px
+```