No useful output when dependency build fails

[RalfJung]

A dependency build seems to have failed during ./x test miri, but the output doesn't contain much useful information:

Testing stage1 miri (x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized + debuginfo] target(s) in 0.09s
     Running unittests src/lib.rs (build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/deps/miri-4cf924b5898b79a3)

running 3 tests
...
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 34 filtered out; finished in 0.00s

     Running tests/ui.rs (build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/deps/ui-30e8267c59f953d1)
## Running ui tests in tests/pass for x86_64-unknown-linux-gnu
   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-ugFzJd" "RUST_BACKTRACE"="1" /home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/miri "--error-format=json" "--sysroot=/home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--out-dir" OUT_DIR
█████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████ 15/15

test result: ok. 15 passed; 342 filtered out

## Running ui tests in tests/pass-dep for x86_64-unknown-linux-gnu
   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-ugFzJd" "RUST_BACKTRACE"="1" /home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/miri "--error-format=json" "--sysroot=/home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--out-dir" OUT_DIR
███████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████ 4/4

FAILED TEST: tests/pass-dep/libc/close-std-streams.rs (revision `default`)
command: env -u MIRIFLAGS -u RUSTFLAGS MIRI_BUILD_TEST_DEPS="1" "/home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/cargo-miri" "miri" "build" "-Zbinary-dep-depinfo" "--target-dir" "/home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0" "--manifest-path" "tests/deps/Cargo.toml" "--target=x86_64-unknown-linux-gnu" "--message-format=json"


FAILED TEST: tests/pass-dep/libc/close-std-streams.rs (revision `null`)
command: "Building dependencies"

FAILURES:
    tests/pass-dep/libc/close-std-streams.rs (revision default)
    tests/pass-dep/libc/close-std-streams.rs (revision null)

test result: FAIL. 2 failed; 58 filtered out

Error: 
   0: ui tests in tests/pass-dep for x86_64-unknown-linux-gnu failed
   1: tests failed

Location:
   /home/r/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.30.4/src/lib.rs:365

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`

Caused by:
  process didn't exit successfully: `/home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/deps/ui-30e8267c59f953d1 fun clo --quiet` (exit status: 1)
Command `/home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage0/bin/cargo test -Zwarnings --target x86_64-unknown-linux-gnu -Zbinary-dep-depinfo -j 16 -Zroot-dir=/home/r/src/rust/rustc.2 --profile=release --manifest-path /home/r/src/rust/rustc.2/src/tools/miri/Cargo.toml -- fun clo --quiet [workdir=/home/r/src/rust/rustc.2]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:191:21
Executed at: src/bootstrap/src/core/build_steps/test.rs:740:19

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:00:20

[RalfJung]

FWIW the underlying problem was that I used --keep-stage-std too aggressively. But even then ui_test should show the error from the failed cargo invocation.