Improve Hadrian feedback when the boot compiler changes

Motivation

When updating ghc.nix, the boot compiler can be updated (most recently from 8.6 to 8.8), thus producing a build error that is most obscure to the non-initiated:

error logs

hadrian: ghc-pkg dump failed: dieVerbatim: user error (hadrian:
'/nix/store/vysnbfiax8xyanbimqw6ag70z41nk5dd-ghc-8.8.2/bin/ghc-pkg'
exited with an error:
ghc-pkg: _build/stage0/lib/package.conf.d/package.cache:
GHC.PackageDb.readPackageDb: inappropriate type (not enough bytes)
)


interrupted!
Error when running Shake build system:
  at want, called at src/Main.hs:92:30 in main:Main
* Depends on: docs
  at need, called at src/Rules/Documentation.hs:116:9 in main:Rules.Documentation
* Depends on: _build/docs/archives/libraries.html.tar.xz
  at need, called at src/Rules/Documentation.hs:331:9 in main:Rules.Documentation
* Depends on: _build/docs/html/libraries/index.html
  at need, called at src/Rules/Documentation.hs:200:9 in main:Rules.Documentation
* Depends on: _build/docs/html/libraries/ghc-prim/ghc-prim.haddock
  at apply1, called at src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in shake-0.18.5-d08cc570254c2937656e16b0fb9ddd0ad652646c7554b15bb3ec273551ad685b:Development.Shake.Internal.Rules.Oracle
* Depends on: OracleQ (ContextDataKey (Context {stage = Stage1, package = Package {pkgType = Library, pkgName = "ghc-prim", pkgPath = "libraries/ghc-prim"}, way = v}))
  at need, called at src/Hadrian/Oracles/Cabal/Rules.hs:53:9 in main:Hadrian.Oracles.Cabal.Rules
* Depends on: _build/stage1/libraries/ghc-prim/setup-config
  at apply1, called at src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in shake-0.18.5-d08cc570254c2937656e16b0fb9ddd0ad652646c7554b15bb3ec273551ad685b:Development.Shake.Internal.Rules.Oracle
* Depends on: OracleQ (ContextDataKey (Context {stage = Stage1, package = Package {pkgType = Library, pkgName = "rts", pkgPath = "rts"}, way = v}))
  at need, called at src/Hadrian/Oracles/Cabal/Rules.hs:53:9 in main:Hadrian.Oracles.Cabal.Rules
* Depends on: _build/stage1/rts/setup-config
  at need, called at src/Hadrian/Builder.hs:70:5 in main:Hadrian.Builder
* Depends on: _build/stage0/lib/bin/unlit
  at apply1, called at src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in shake-0.18.5-d08cc570254c2937656e16b0fb9ddd0ad652646c7554b15bb3ec273551ad685b:Development.Shake.Internal.Rules.Oracle
* Depends on: OracleQ (ContextDataKey (Context {stage = Stage0, package = Package {pkgType = Program, pkgName = "unlit", pkgPath = "utils/unlit"}, way = v}))
* Depends on: _build/stage0/utils/unlit/setup-config
* Raised the exception:
ExitFailure 1

Indeed, it is not enough to re-run ./boot and ./configure, but _build also needs to be erased.

As advised by @alp, I am writing this ticket to suggest an improvement in that direction.

Proposal

In order to streamline the development process on GHC, Hadrian should be able to remove conflicting build artifacts upon the change of a boot compiler.
Clearly its (current) inability to do so indicates that some dependencies are missing to incorporate this particular kind of knowledge in Hadrian.