I also had an issue where Rust was recompiling dependencies unnecessarily. It turned out to be because rust-analyzer has one more level of `Bash` than VSCode's integrated terminal. My `.bashrc` was loading something unconditionally which meant if you nested bash sessions the PKG_CONFIG_PATH changed (duplicate entries).
I had OpenSSL as a dependency, and if `PKG_CONFIG_PATH` changes it rebuilds it (this is correct), which means if you make an edit to a file and save it, rust-analyzer would blow away the cache, then you build it on the command line and it blows it away again.
To test:
1. If you quit VSCode and do an incremental build is it still slow?
2. Try `CARGO_LOG=cargo::core::compiler::fingerprint=info cargo build` (took me a while to find that; there is a bug open to make it less stupidly hard to find).
That will print a load of info about why Cargo is rebuilding stuff. Note that it isn't really in a sensible order - the first message isn't necessarily the cause. The message about the environment variable changing was somewhere in the middle for me, so read the whole log.
Thanks for that very helpful tip! According to the output of that, no unnecessary recompilation is taking place.
I actually figured out the issue! It turns out, I had unquestioningly transplanted the following from the blog post in discussion, and that is the cause of all my issues:
I had OpenSSL as a dependency, and if `PKG_CONFIG_PATH` changes it rebuilds it (this is correct), which means if you make an edit to a file and save it, rust-analyzer would blow away the cache, then you build it on the command line and it blows it away again.
To test:
1. If you quit VSCode and do an incremental build is it still slow? 2. Try `CARGO_LOG=cargo::core::compiler::fingerprint=info cargo build` (took me a while to find that; there is a bug open to make it less stupidly hard to find).
That will print a load of info about why Cargo is rebuilding stuff. Note that it isn't really in a sensible order - the first message isn't necessarily the cause. The message about the environment variable changing was somewhere in the middle for me, so read the whole log.