Remove DT_Failed state

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

2 files changed, 9 insertions, 3 deletions

diff --git a/compiler/GHC/Unit/Module/Location.hs b/compiler/GHC/Unit/Module/Location.hs
index 866ccf127a..4d70b43b21 100644
--- a/compiler/GHC/Unit/Module/Location.hs
+++ b/compiler/GHC/Unit/Module/Location.hs
@@ -3,6 +3,7 @@ module GHC.Unit.Module.Location
    ( ModLocation(..)
    , addBootSuffix
    , addBootSuffix_maybe
+   , addBootSuffixLocn_maybe
    , addBootSuffixLocn
    , addBootSuffixLocnOut
    , removeBootSuffix
@@ -86,6 +87,11 @@ addBootSuffix_maybe is_boot path = case is_boot of
   IsBoot -> addBootSuffix path
   NotBoot -> path
 
+addBootSuffixLocn_maybe :: IsBootInterface -> ModLocation -> ModLocation
+addBootSuffixLocn_maybe is_boot locn = case is_boot of
+  IsBoot -> addBootSuffixLocn locn
+  _ -> locn
+
 -- | Add the @-boot@ suffix to all file paths associated with the module
 addBootSuffixLocn :: ModLocation -> ModLocation
 addBootSuffixLocn locn
diff --git a/compiler/GHC/Unit/Module/ModSummary.hs b/compiler/GHC/Unit/Module/ModSummary.hs
index 9cf736a37a..20d61ad4f8 100644
--- a/compiler/GHC/Unit/Module/ModSummary.hs
+++ b/compiler/GHC/Unit/Module/ModSummary.hs
@@ -16,6 +16,7 @@ module GHC.Unit.Module.ModSummary
    , ms_home_srcimps
    , ms_home_imps
    , msHiFilePath
+   , msDynHiFilePath
    , msHsFilePath
    , msObjFilePath
    , msDynObjFilePath
@@ -151,12 +152,11 @@ ms_home_imps = home_imps . ms_imps
 -- The ModLocation is stable over successive up-sweeps in GHCi, wheres
 -- the ms_hs_hash and imports can, of course, change
 
-msHsFilePath, msHiFilePath, msObjFilePath :: ModSummary -> FilePath
+msHsFilePath, msDynHiFilePath, msHiFilePath, msObjFilePath, msDynObjFilePath :: ModSummary -> FilePath
 msHsFilePath  ms = expectJust "msHsFilePath" (ml_hs_file  (ms_location ms))
 msHiFilePath  ms = ml_hi_file  (ms_location ms)
+msDynHiFilePath ms = ml_dyn_hi_file (ms_location ms)
 msObjFilePath ms = ml_obj_file (ms_location ms)
-
-msDynObjFilePath :: ModSummary -> FilePath
 msDynObjFilePath ms = ml_dyn_obj_file (ms_location ms)
 
 -- | Did this 'ModSummary' originate from a hs-boot file?