In frustration, I took a (masked and socially distanced) walk. During my walk I remembered reading in the racer installation instructions that you could set
RUST_SRC_PATH, but racer would find what it needed if you didn't. I realized that the environment variable wasn't set, but I had an entry for it in my settings.json. I removed that entry, restarted VSCode, and racer doesn't crash.
I was so proud of myself. Now @ruabmbua tells me that I don't need racer if I'm using rust-analyzer. I renamed racer in my .cargo/bin, restarted VSCode, and autocomplete works, verifying what he said. Of course, VSCode complains that a Rust tool is missing, but I can ignore that notification. Still, I might want to use racer with another editor, but I can't figure out how to tell VSCode not to even try to use it.
Oh, and by the way, I'm still getting
rust-analyzer failed to load workspace: Failed to find sysroot for Cargo.toml file /Users/alan/Documents/Eclipse/multicell/userspace/cellagent/Cargo.toml. Is rust-src installed?: RUST_SRC_PATH must be absolute:
every time I start VSCode, which is why I started this whole thread in the first place.
I can get my work done, but my Outline says
No symbols found in 'foo.rs' and syntax errors don't show up as squiggly lines in the editor.