I can shed some light here! This is going to be longish comment, but hopefully by the end of it you should understand _why_ we decided to avoid using the disk initially, even if you don't agree with that decision.

Historically, the decision to not use disk traces back to this comment https://github.com/rust-lang/rfcs/pull/1317#issuecomment-150..., which is perhaps the single GitHub comment that influenced my life most. Very high impact, thanks dgrunwald! Specifically,

>Don't store anything to disk. It's likely the oracle can be fast enough without doing this; and unnecessary complexity creates bugs. "Have you tried deleting the .ncb file?" (I remember having to do this a couple times per day when using VS, ca. 2005)

>Use lazy evaluation. The IDE is only interested in very specific bits of information, almost always restricted to a couple of lines around the cursor. Avoid calculating stuff that might never get used before it gets invalidated by the next code change.

>At least for C#, laziness saves so much time that incremental compilation is unnecessary for IDE purposes

The other part of historical context was that the motivation for creating rust-analyzer was that I didn't want to write a second Rust compiler (having been doing that for a couple of years at JetBrains). So it was explicitly an experimental project to prototype the right architecture for an IDE, to ultimately change how rustc works internally, so that, down the line, an IDE and a command-line compiler could use the same core. Given that rust-analyzer is now effectively a separate rust compiler, it's safe to say I am not good at achieving my life's goals!

In that context, I believe that avoiding disk was the _right_ decision:

* It's not really germane to the problem space, if all you need is literally a cache, it can always be added later.

* Disk is a can of worms of data consistency problems. They can be overcome with engineering effort to ultimately give better user experience, but user experience wasn't the primary goal. And using disk wouldn't actually illuminate the interesting aspects of the architecture, the intended primary goal.

* Finally, _not_ using disk would be a forcing function to keep analysis fast enough, to not make startup prohibitive.

The last one was a particularly big argument in my mind --- I didn't want to reach out for "easy" solutions prematurely, to avoid avoiding hard problems. And, again, my recollection is probably not 100% correct, but, until we added support for proc macros and build scripts, it was fine-ish from the perspective of startup time (RAM usage is a different story). The problem with proc_macros and build.rs is that they need to run the rust code, so they have to run the real rustc compiler, so all our usual IDE tricks ("information ... restricted to a couple of lines around the cursor") just don't apply.

The reason why we didn't add it later was that it seemed a relatively lower priority task than the work to share the parser between rust-analyzer and rustc. So that's what I was focusing on, though, I didn't deliver that. I still think we should do it! There's no _insurmountable_ technical reasons why the parsers can't be shared! It's just (a lot of) engineering work. And, while the parser is the boring part of compiler, it's the interesting part of an IDE.

Anyway, that explains how we ended up where we are.

That being said, I don't think that "just adding disk cache" is the right approach --- the salsa in-memory data structure is very sparse and pointy. Dumping that to disk would help somewhat, but wouldn't be a great long term solution. What is needed (I also explain this in https://matklad.github.io/2026/08/21/rust-glancer.html) is to design a compact, first class data format for representing analysis information about the crate, and than teaching rust-analyzer to be polymorphic in the source of data. For current workspace, you want to use a lazy incremental in-memory data structure (I do think we sadly need incrementally for Rust, given its compilation unit structure). For dependencies, you want to work off a compact on disk index. And, if the user "goes to definition" and mutates its file in place, we want to transparently switch between the two. The _pre requisite_ for that was to define a backend agnostic analysis API, and that work was always slowly progressing in the background (https://hackmd.io/ytd82QNiT_Ku2XFr1EAtiQ), but it generally took the backseat, while sharing the parser was the main focus.

Rust Glancer: Rust LSP using 100x less RAM