Making the most of exception stacks

[c42f]

Now that #28878 has been merged, let's consider how to make the most of it. Largely, that PR added the exception stack runtime and addressed several bugs in the previous system, but left aside some other possible benefits such as improved error reporting. Here's a list of follow-on features and improvements which have crossed my mind at one time or another.

Base / stdlib

• Add some convenient way of displaying exception stacks; showerror integration, perhaps (Display exception stacks #30900)
• Consider whether catch_stack() API types could be improved. For example, returning a custom type rather than a bare Vector would allow normal show to give something useful here rather than needing a special showerror and display_error? (Refine exception stack API #29901)
• Improved error reporting in the REPL with catch_stack and Base.display_error - figure out how to display all the errors in the stack in a way which emphasizes the information most likely to help fix the problem. (Display exception stacks #30900)
• rethrow(exc) cleanup: this can become rethrow() in nearly all cases now (Replace trivial uses of rethrow(exc) with rethrow() #29794 Replace trivial uses of rethrow(exc) with rethrow() Pkg.jl#873)
• Consider whether the exception nesting behavior of LoadError is the right thing. This looks like it might have quite some overlap with the exception stack system for root cause analysis, being in some ways more convenient and some ways less. (Display exception stacks #30900)

Documentation

• Consider whether the manual should discourage rethrow(exc), except in special circumstances. (If the stacks print nicely enough, are there any remaining reasons to use rethrow(exc) at all?)

Runtime

• Task cleanup: Remove task.backtrace which now duplicates the storage in task->excstack (is this considered a public API? If so, consider getproperty?)
• Replace panic error dumping (eg, jl_static_show(JL_STDERR, jl_current_exception())) with a dump of the full exception stack.
• jl_current_exception() — Asses other uses; consider whether they should be rewritten.
• Expose full exception stack to embedding API rather than just previous_exception?
• Teach static analyzer about rooting of the return value from jl_current_exception() if possible (and add jl_excstack_t as a tracked type). Would be helpful to have Import ClangSA.jl source tree #28838 merged first.

@vtjnash

[vtjnash]

AFAIK, the necessary and useful bits here are now done, and the issue is no longer useful