compiletest: Printing process output panics if the output contains unknown JSON-like text

[Zalathar]

When a process unexpectedly fails/succeeds, compiletest will often print a full dump of its stdout and stderr for diagnostic purposes.

When this happens, ProcRes::print_info calls into extract_rendered, which has special handling for lines starting with {. It tries to parse the line as JSON in one of a few known compiler-specific formats, in order to replace it with a more human-friendly message.

If the line doesn't match any of those formats (or isn't valid JSON), the extractor code will panic, instead of just printing the line as-is. This is awkward in cases where we actually wanted to print the line as-is anyway, because now the error messages are less helpful.

@rustbot label +A-compiletest

[Zalathar]

The obvious solution (just make it not panic) is complicated by the fact that extract_rendered is called in multiple places, so it's harder to know whether the error is load-bearing in one of those cases.