Fix recompilation checking of pure plugins

Previously when switching from using a Plugin with
`RecompMaybe`/`ForceRecompile` in `pluginRecompile` to a Plugin with
`NoForceRecompile` GHC would never even consider recompiling.

However the previously active plugin could have modified the
compilation output so we should recompile.

Test Plan: validate

Reviewers: bgamari, mpickering

Subscribers: mpickering, rwbarton, carter

GHC Trac Issues: #15858

Differential Revision: https://phabricator.haskell.org/D5299

3 files changed, 62 insertions, 0 deletions

diff --git a/testsuite/tests/plugins/T15858.script b/testsuite/tests/plugins/T15858.script
new file mode 100644
index 0000000000..77a1b53557
--- /dev/null
+++ b/testsuite/tests/plugins/T15858.script
@@ -0,0 +1,33 @@
+:set -fobject-code
+-- ^ Without this no recompilation happens at all in ghci, but that's a bug for
+-- another ticket.
+
+:l plugin-recomp-test.hs
+
+-- switching to an impure plugin triggers recomp.
+:! echo ==ImpurePlugin.0 >&2
+:set -fclear-plugins -fplugin ImpurePlugin
+:l plugin-recomp-test.hs
+
+-- ..forever, this also triggers recomp.
+:! echo ==ImpurePlugin.1 >&2
+:l plugin-recomp-test.hs
+
+-- switching from impure to pure plugin triggers recomp.
+:! echo ==PurePlugin.0 >&2
+:set -fclear-plugins -fplugin PurePlugin
+:l plugin-recomp-test.hs
+
+-- switching to a fingerprint plugin triggers recomp.
+:! echo ==FingerprintPlugin.0 >&2
+:set -fclear-plugins -fplugin FingerprintPlugin
+:l plugin-recomp-test.hs
+
+-- same fingerprint plugin doesn't trigger recomp.
+:! echo ==FingerprintPlugin.1 >&2
+:l plugin-recomp-test.hs
+
+-- switching from fingerprint to pure plugin triggers recomp.
+:! echo ==PurePlugin.1 >&2
+:set -fclear-plugins -fplugin PurePlugin
+:l plugin-recomp-test.hs
diff --git a/testsuite/tests/plugins/T15858.stderr b/testsuite/tests/plugins/T15858.stderr
new file mode 100644
index 0000000000..db0610f587
--- /dev/null
+++ b/testsuite/tests/plugins/T15858.stderr
@@ -0,0 +1,21 @@
+==ImpurePlugin.0
+Simple Plugin Passes Queried
+Got options:
+Simple Plugin Pass Run
+==ImpurePlugin.1
+Simple Plugin Passes Queried
+Got options:
+Simple Plugin Pass Run
+==PurePlugin.0
+Simple Plugin Passes Queried
+Got options:
+Simple Plugin Pass Run
+==FingerprintPlugin.0
+Simple Plugin Passes Queried
+Got options:
+Simple Plugin Pass Run
+==FingerprintPlugin.1
+==PurePlugin.1
+Simple Plugin Passes Queried
+Got options:
+Simple Plugin Pass Run
\ No newline at end of file
diff --git a/testsuite/tests/plugins/all.T b/testsuite/tests/plugins/all.T
index 72a42da91a..3b031668bb 100644
--- a/testsuite/tests/plugins/all.T
+++ b/testsuite/tests/plugins/all.T
@@ -199,3 +199,11 @@ test('static-plugins',
       extra_run_opts('"' + config.libdir + '"')],
      compile_and_run,
      ['-package ghc -isimple-plugin/'])
+
+test('T15858',
+     [extra_files(['plugin-recomp/', 'plugin-recomp-test.hs']),
+#      only_ways([config.ghc_plugin_way]),
+      pre_cmd('$MAKE -s --no-print-directory -C plugin-recomp package.plugins01 TOP={top}'),
+      extra_hc_opts("-package-db plugin-recomp/pkg.plugins01/local.package.conf ")
+      ],
+     ghci_script, ['T15858.script'])