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At the moment if `-dynamic-too` fails then we rerun the whole pipeline as if we were just in `-dynamic` mode. I argue this is a misfeature and we should remove the so-called `DT_Failed` mode. In what situations do we fall back to `DT_Failed`? 1. If the `dyn_hi` file corresponding to a `hi` file is missing completely. 2. If the interface hash of `dyn_hi` doesn't match the interface hash of `hi`. What happens in `DT_Failed` mode? * The whole compiler pipeline is rerun as if the user had just passed `-dynamic`. * Therefore `dyn_hi/dyn_o` files are used which don't agree with the `hi/o` files. (As evidenced by `dynamicToo001` test). * This is very confusing as now a single compiler invocation has produced further `hi`/`dyn_hi` files which are different to each other. Why should we remove it? * In `--make` mode, which is predominately used `DT_Failed` does not work (#19782), there can't be users relying on this functionality. * In `-c` mode, the recovery doesn't fix the root issue, which is the `dyn_hi` and `hi` files are mismatched. We should instead produce an error and pass responsibility to the build system using `-c` to ensure that the prerequisites for `-dynamic-too` (dyn_hi/hi) files are there before we start compiling. * It is a misfeature to support use cases like `dynamicToo001` which allow you to mix different versions of dynamic/non-dynamic interface files. It's more likely to lead to subtle bugs in your resulting programs where out-dated build products are used rather than a deliberate choice. * In practice, people are usually compiling with `-dynamic-too` rather than separately with `-dynamic` and `-static`, so the build products always match and `DT_Failed` is only entered due to compiler bugs (see !6583) What should we do instead? * In `--make` mode, for home packages check during recompilation checking that `dyn_hi` and `hi` are both present and agree, recompile the modules if they do not. * For package modules, when loading the interface check that `dyn_hi` and `hi` are there and that they agree but fail with an error message if they are not. * In `--oneshot` mode, fail with an error message if the right files aren't already there. Closes #19782 #20446 #9176 #13616
