Replace SYSROOT env var with --sysroot flag

[daivinhtran]

rust_toolchain generates a rust sysroot from collection of toolchain components. The sysroot path is then assigned to RUST_SYSROOT make variable and ToolchainInfo.sysroot. When constructing rustc compile action, we set ToolchainInfo.sysroot to SYSROOT env var.

Apparently, SYSROOT env var isn't read by rustc and I suspect it is currently a no-op. To reproduce the no-op of SYSROOT, simply change it to any random string and expect the builds to still succeed.

Per rustc doc, we can set --sysroot to override the system sysroot (i.e. `rustc --print sysroot). I believe this is needed to prevent rustc from managing to escape the Bazel sandbox and seeing unspecified inputs.

As a concrete example, say we have a toolchain defined as

rust_toolchain(
  name = "foo",
  rustc = "prebuilt/bin/rustc",
  rustc_lib = "sysroot_with_stdlibs", # outside of $WORKSPACE/prebuilt
  rust_std = "stdlibs_built_from_source", # outside of $WORKSPACE/prebuilt
)

The system sysroot is

$ ~/prebuilt/bin/rustc --print sysroot
~/prebuilt

Under ~/prebuilt, there are prebuilt stdlibs

<target-tripple>/
  lib/
    libstd-<hash>.rlib|so

So in addition to the system sysroot which always points to prebuilt stdlibs, rustc compile action sets -L to the path of stdlibs built from source. Hence, it fails with

error[E0464]: multiple candidates for `rlib` dependency `std` found
 = note: candidate #1 bazel-built libstd-<hash>.rlib
 = note: candidate #1 prebuilt libstd-<hash>.so.  <------- Unspecified in rust_toolchain foo

Fortunately, we can fix this by setting rustc_flags = ["--sysroot=$(RUST_SYSROOT)"] to the rust_toolchain. However, the error with multiple std candidates can be confusing because we don't specify the prebuilt libstd in rust_toolchain at all. rustc should not be able to escape the Bazel sandbox.

To sort-of fix this issue, one solution is to add --sysroot rustc flag to point to the generated sysroot to avoid this issue.

[UebelAndre]

I think this would be a great change! Would you be willing to make a pull-request for it where a new build flag is added (experimental_rustc_sysroot_flag or something) that can set this flag instead of setting the environment variable?

[daivinhtran]

@UebelAndre I'll be more than happy to contribute a PR. I'm not 100% sure but the fix might also remove the need for the custom handling in #1500. I'll verify that.

[daivinhtran]

Hi @UebelAndre . I finally got some time for this issue. Please take a look #2223.