compiletest's 'SKIPPED 246746 BYTES' logic breaks tests

[m-ou-se]

src/tools/compiletest truncates large outputs by replacing the middle by <<<<<< SKIPPED {} BYTES >>>>>>:

rust/src/tools/compiletest/src/read2.rs

Line 52 in 7ccfe2f

write!(&mut head, "\n\n<<<<<< SKIPPED {} BYTES >>>>>>\n\n", skipped).unwrap();

This can break the test runner which tries to parse the output as json, which will break on the <<< .. >>> message. For example here: https://github.com/rust-lang/rust/runs/4586149338?check_suite_focus=true:

thread '[ui] ui/suggestions/missing-lifetime-specifier.rs' panicked at 'explicit panic', src/tools/compiletest/src/json.rs:121:21

This is blocking #92123 (A workaround for this PR could be to simply increase the threshold, but we should probably have a better solution.)

[Aaron1011]

See also #92211

[Mark-Simulacrum]

#94327 should help with this specific instance, though is not a root cause fix. In general limiting the number of JSON bytes is a pretty poor estimate of the number of bytes in stderr files; it might make sense to avoid the abbreviated read abstraction for that part of compiletest. (Or, at least, set the limit to a few megabytes or so).