rustc source crates do not load any more

[RalfJung]

When working on Miri, RA seems no longer able to load the rustc source crates: none of the symbols defined in rustc crates can be resolved, and the lib.rs file shows errors saying that the extern crate cannot be resolved. This still worked not too long ago; I don't know when exactly it regressed.

Symbols inside Miri resolve just fine. The "r-a language server" logs are empty, and the status bar indicates no error. The "r-a extension" logs don't seem to contain anything unusual, just a lot of this:

2025-09-01 10:13:31.841 [info] Extension version: 0.3.2593
2025-09-01 10:13:31.841 [info] Using configuration {
// lots of JSON here

rust-analyzer version: rust-analyzer version: 0.3.2593-standalone [/home/r/.vscode-oss/extensions/rust-lang.rust-analyzer-0.3.2593-linux-x64/server/rust-analyzer]

rustc version: rustc 1.91.0-nightly (828e45ad1 2025-09-01)

editor or extension: VSCodium, extension version 0.3.2593

relevant settings:

{
    "rust-analyzer.rustc.source": "discover",
    "rust-analyzer.linkedProjects": [
        "Cargo.toml",
        "cargo-miri/Cargo.toml",
        "miri-script/Cargo.toml",
    ],
    "rust-analyzer.check.invocationStrategy": "once",
    "rust-analyzer.check.overrideCommand": [
        "./miri",
        "clippy",
        "--no-default-features",
        "--message-format=json",
    ],
    "rust-analyzer.cargo.extraEnv": {
        "MIRI_AUTO_OPS": "no",
        "MIRI_IN_RA": "1",
    },
    // Contrary to what the name suggests, this also affects proc macros.
    "rust-analyzer.cargo.buildScripts.invocationStrategy": "once",
    "rust-analyzer.cargo.buildScripts.overrideCommand": [
        "./miri",
        "check",
        "--no-default-features",
        "--message-format=json",
    ],
}

repository link (if public, optional): https://github.com/rust-lang/miri/

[RalfJung]

This seems to correlate with an upstream rustc change -- downgrading RA doesn't help, but downgrading rustc does. Will post bisect results once I have them.

[RalfJung]

rust-lang/miri#4548 broke it. That's this rustc diff: rust-lang/rust@269d5b5...e004014.

[RalfJung]

Could this have to do with rust-lang/rust#145740? I don't know if the distributed sources are still inside that workspace. Looking at ~/.rustup/toolchains/miri/lib/rustlib/rustc-src/rust, there's no Cargo.toml there, so this seems like a very probable culprit. Cc @nnethercote

I hope there's some way to get an actual error message from RA, since right now it's pure guesswork.^^

[ChayimFriedman2]

Try setting RA_LOG=project_model=debug.

[RalfJung]

Set that where?

[ChayimFriedman2]

That's an environment variable that need to be set for r-a.

[ShoyuVanilla]

It could be done via "rust-analyzer.server.extraEnv". I'd like to look into this but unfortunately I'm on a vacation by tomorrow. I'll check this if this is not resolved by then.

[RalfJung]

Right, I just wasn't sure whether I can just set that before invoking vscodium or where it has to go.

Setting it on the terminal seems to work, and indeed I do get error messages that way:

2025-09-01T13:36:00.72335433+02:00  WARN `cargo metadata` failed and returning succeeded result with `--no-deps` error=`cargo metadata` exited with an error: error: failed to get `rustc_codegen_ssa` as a dependency of package `rustc-main v0.0.0 (/home/r/.rustup/toolchains/miri/lib/rustlib/rustc-src/rust/compiler/rustc)`

Caused by:
  failed to load source for dependency `rustc_codegen_ssa`

Caused by:
  Unable to update /home/r/.rustup/toolchains/miri/lib/rustlib/rustc-src/rust/compiler/rustc_codegen_ssa

Caused by:
  failed to parse manifest at `/home/r/.rustup/toolchains/miri/lib/rustlib/rustc-src/rust/compiler/rustc_codegen_ssa/Cargo.toml`

Caused by:
  error inheriting `bitflags` from workspace root manifest's `workspace.dependencies.bitflags`

Caused by:
  failed to find a workspace root

(Seems like such errors should really be visible somewhere by default...)

So, it's definitely the dependency change that I already suspected above.

[ShoyuVanilla]

Yes, we really should pop-up error messages on cargo metadata command failures.

[RalfJung]

So, about fixing the actual issue, I only see two ways:

• stop using workspace dependencies :(
• find a way to ship the rustc workspace Cargo.toml file as part of the rustc-dev package. I suspect it can't be shipped unchanged since the rustc-dev package doesn't contain everything listed as members in that manifest, so this requires parsing the toml, adjusting it, and then outputting the result.

[ChayimFriedman2]

Yes, we really should pop-up error messages on cargo metadata command failures.

It's a bit hard to do because you can't tell, by the time you do the discovery, if the project really needs the rustc crates.

[RalfJung]

We have this in the Cargo.toml

[package.metadata.rust-analyzer]
# This crate uses #[feature(rustc_private)].
# See https://github.com/rust-analyzer/rust-analyzer/pull/7891
rustc_private = true

and this in the workspace config

"rust-analyzer.rustc.source": "discover",

I feel like opting-in twice should be a clear enough signal that we do really need the rustc crates?

[ChayimFriedman2]

The setting is not enough because it could be set globally, but yeah I forgot about the rustc_private thing.