Ramblings

so we wrote 2026-11-03-rust-worktrees-sccache-build-perf.md a few days ago
its basic research about worktree workflows in rust/cargo development
then we tried to impelment it in one of our prohejcts loom-datawarehouse and it didnt go accrodingly to plan
- espcailly the sccache which theroetical seems to amke ense but fo rosme reson didnt work
  - Linking PR for reference
    - Implemting scache https://github.com/ran-codes/loom-warehouse/pull/30
    - realizing it doesnt wont then adding another pore-warm mitigaiton workfow - https://github.com/ran-codes/loom-warehouse/pull/39
and thne we tried anotehr method kache prust pacakge. which also didnt work
endedupw itht he conflusiona sof now
- there isn’t a great tooling for windwos rust worktree work
here we jsut not try to solve ht problme but jsut try to undresatnd why sscache didn’t work for dbt-oss fork

Reference/Vocab

cargo asks sccache to compile a crate
│
├─ Own crate (dbt-schemas, dbt-adapter…)?
│   └─ yes → not cached, always compiles
│
└─ Dependency (parquet 50.0) → make its ID tag:
      parquet 50.0 + compiler flags + folder path
      │
      ├─ main:     ...parquet + C:\repo\loom-warehouse → tag 7f3a
      ├─ linked-1: ...parquet + C:\repo\wt-1          → tag 91c2
      └─ linked-2: ...parquet + C:\repo\wt-2          → tag c48e
            │
            Is tag c48e on the shelf?
            ├─ yes → cache hit: reuse saved copy (fast)
            └─ no → cache miss: compile, save under c48e
                    ← every new worktree ends up here

Lookup keys (parquet only):
  main:      C:\repo\loom-warehouse\target\debug\deps\libparquet  → 7f3a
  linked-1:  C:\repo\wt-1\target\debug\deps\libparquet            → 91c2
  linked-2:  C:\repo\wt-2\target\debug\deps\libparquet            → c48e

Shelf after all three build:
  7f3a → parquet compiled in main
  91c2 → parquet compiled in linked-1
  c48e → parquet compiled in linked-2   ← same code, saved 3 times

Vocab

Cache: a shelf of saved compiled dependencies
Lookup key (ID tag): points to one saved copy of one dependency
- Computed from crate version, flags, and folder path
- Same inputs → same tag, every time
- New folder → new tag → no match
Compiler flags: the build settings passed to the compiler
Folder path: where the worktree lives on disk
Cache hit / miss: tag found → reuse; not found → compile

Understanding

Core Question answered

A deicison tree is printed above along wiht new covabulary needed to understnaind the symptom
So sscache works by using a ID-tag caleld -Lookup Key that is linked to each depencny-folder-path
the reason why scache doesnt work for worktrees is because the dpeenceis folder path is dffernt across worktrees
So even tohught paruqet on main worktree is cache parquet on new worktree diff path is not ocnsidere cache ebcuase the fodler apth si part of the cache lookup
Thats why sccache doesnt work for worktrees

Ideation

Pitch idea
- This being the case
- Sccache is still incredimble valuable as the depencie compaitlion is abig chunk fo rht eocmpaitlion time.
- Is there a way we can use a just script or a sscacahe config to just tell scchace to not include path in the lookup key? or can we manipulat eth oloopup key withinna proejct like loom-datawarehouse to only unitilzie on path across allw orktrees?
  - would this solve the problem
  - is this feasible
  - shoudl we do it for loomwarhouse
CC intiial evalution
- for dbt-loom sccahce onyl saved about 60 seconds forma 300 sec build. so not a huge lever
- also try shared path for wotkree but the conflict becuae overwiting dbt.exe and contaiinaing dbt.exe across worktrees
- so dont impelment for now!