Set MIRIFLAGS from .cargo/config.toml

[dtolnay]

You can pass flags to rustc with build.rustflags, and to rustdoc with build.rustdocflags.

[build]
rustflags = ["-C", "target-cpu=native"]

Supporting MIRIFLAGS the same way would be nice. Most of the time I want to run every miri invocation for a project with a particular flag like -Zmiri-disable-isolation, so just setting it globally for the workspace would be ideal.

[miri]
miriflags = ["-Zmiri-disable-isolation"]

Related issue for MIRIFLAGS as command line arguments: #2051

[RalfJung]

Ah yes that makes a lot of sense. Two notes:

• I have no idea how to do that without re-implementing all the logic from cargo, which I'd rather not maintain in our codebase. (It looks up a bunch of files, right?) Can we ask cargo nicely to tell us the value of some option, rather than having to look it up ourselves?
• We probably then also want negated versions of all our flags so you can overwrite your locally adjusted defaults. Like, if you set -Zmiri-disable-isolation, but then want to run with isolation one time to make sure it is deterministic, currently there's no way to do that. So maybe it should be -Zmiri-isolation=true/false instead?

[dtolnay]

It's already exposed by Cargo:

$ cargo -Zunstable-options config get miri.flags --format=json-value
["-Zmiri-disable-isolation"]

It handles the usual cascading/merging of whatever files Cargo normally looks in ($WORKSPACE/.cargo/config, $WORKSPACE/.cargo/config.toml, ~/.cargo/config, ~/.cargo/config.toml, etc(?)).

Tracking issue rust-lang/cargo#9301 but I assume you don't require this to be stable given you're pinned to nightly anyway.

[RalfJung]

Wow, that's cool!

[RalfJung]

#2451 contains a draft implementation of this feature, but needs a bit of work to be finished -- see the PR comments for details.