`./x test mir-opt --bless` fails due to missing -Zjson-target-spec

[RalfJung]

When I run ./x test mir-opt --bless, I get a bunch of output that ends in

Testing stage1 with compiletest suite=mir-opt mode=mir-opt (i686-pc-windows-msvc)

running 383 tests
i.......................................................................................  88/383
......................................................................................i. 176/383
.......i..........................i..................................................... 264/383
.......................................................i................................ 352/383
...............................

test result: ok. 378 passed; 0 failed; 5 ignored; 0 measured; 0 filtered out; finished in 1.68s

Building stage1 library artifacts (stage1:x86_64-unknown-linux-gnu -> stage1:x86_64-apple-darwin-synthetic-miropt-abort(/home/r/src/rust/rustc/build/synthetic-target-specs/x86_64-apple-darwin-synthetic-miropt-abort.json))
error: `.json` target specs require -Zjson-target-spec
Build completed unsuccessfully in 0:00:50

Seems like yet another bit of fallout from #150151; Cc @davidtwco

[davidtwco]

Huh, I thought this was supposed to be fixed by #152236

[jieyouxu]

Might need to double-check if that is "outsourced" to not compiletest, but also src/tools/mir-opt-test-tool (or whatever that was called) which compiletest calls for mir-opt tests

[RalfJung]

That PR added "-Zunstable-options". But what's missing is apparently -Zjson-target-spec.

[davidtwco]

Ah, the issue is that since #152236, Cargo added the -Zjson-target-spec flag that it needs to allow .json targets, so when bootstrap builds the standard library for a synthetic target, it now needs to pass -Zjson-target-spec, afterwards compiletest works fine.

Fixed in #154428