Fix mk_mod_usage_info if the interface file is not already loaded

In #22217 it was observed that the order modules are compiled in affects
the contents of an interface file. This was because a module dependended
on another module indirectly, via a re-export but the interface file for
this module was never loaded because the symbol was never used in the
file.

If we decide that we depend on a module then we jolly well ought to
record this fact in the interface file! Otherwise it could lead to very
subtle recompilation bugs if the dependency is not tracked and the
module is updated.

Therefore the best thing to do is just to make sure the file is loaded
by calling the `loadSysInterface` function.  This first checks the
caches (like we did before) but then actually goes to find the interface
on disk if it wasn't loaded.

Fixes #22217

5 files changed, 36 insertions, 11 deletions

diff --git a/testsuite/tests/backpack/should_fail/T19244a.stderr b/testsuite/tests/backpack/should_fail/T19244a.stderr
index 76f0c86661..5dceaad5f3 100644
--- a/testsuite/tests/backpack/should_fail/T19244a.stderr
+++ b/testsuite/tests/backpack/should_fail/T19244a.stderr
@@ -13,7 +13,17 @@
     Instantiating user[Map=ordmap:Map]
     [1 of 2] Compiling Map[sig]         ( user/Map.hsig, T19244a.out/user/user-GzloW2NeDdA2M0V8qzN4g2/Map.o )
 
-T19244a.bkp:9:9: error:
+T19244a.bkp:22:9: error:
+    • Type constructor ‘Key’ has conflicting definitions in the module
+      and its hsig file
+      Main module: type Key :: * -> Constraint
+                   type Key = GHC.Classes.Ord :: * -> Constraint
+      Hsig file:  type Key :: forall {k}. k -> Constraint
+                  class Key k1
+      The types have different kinds
+    • while checking that ordmap:Map implements signature Map in user[Map=ordmap:Map]
+
+<no location info>: error:
     • Type constructor ‘Map’ has conflicting definitions in the module
       and its hsig file
       Main module: type role Map nominal representational
@@ -31,16 +41,6 @@ T19244a.bkp:9:9: error:
       The types have different kinds
     • while checking that ordmap:Map implements signature Map in user[Map=ordmap:Map]
 
-T19244a.bkp:22:9: error:
-    • Type constructor ‘Key’ has conflicting definitions in the module
-      and its hsig file
-      Main module: type Key :: * -> Constraint
-                   type Key = GHC.Classes.Ord :: * -> Constraint
-      Hsig file:  type Key :: forall {k}. k -> Constraint
-                  class Key k1
-      The types have different kinds
-    • while checking that ordmap:Map implements signature Map in user[Map=ordmap:Map]
-
 <no location info>: error:
     • Identifier ‘lookup’ has conflicting definitions in the module
       and its hsig file
diff --git a/testsuite/tests/determinism/determ024/A.hs b/testsuite/tests/determinism/determ024/A.hs
new file mode 100644
index 0000000000..bd275761e2
--- /dev/null
+++ b/testsuite/tests/determinism/determ024/A.hs
@@ -0,0 +1,6 @@
+module A
+( isExtensionOf
+, stripExtension
+) where
+
+import System.FilePath.Posix
diff --git a/testsuite/tests/determinism/determ024/B.hs b/testsuite/tests/determinism/determ024/B.hs
new file mode 100644
index 0000000000..02c9081858
--- /dev/null
+++ b/testsuite/tests/determinism/determ024/B.hs
@@ -0,0 +1,7 @@
+module B
+( isExtensionOf
+, stripExtension
+) where
+
+import System.FilePath
+
diff --git a/testsuite/tests/determinism/determ024/Makefile b/testsuite/tests/determinism/determ024/Makefile
new file mode 100644
index 0000000000..300a408394
--- /dev/null
+++ b/testsuite/tests/determinism/determ024/Makefile
@@ -0,0 +1,11 @@
+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+determ024:
+	$(RM) A.hi A.o B.hi B.o
+	'$(TEST_HC)' $(TEST_HC_OPTS) -v0 B.hs
+	'$(TEST_HC)' --show-iface B.hi > B_clean_iface
+	'$(TEST_HC)' $(TEST_HC_OPTS) -v0 A.hs B.hs -fforce-recomp
+	'$(TEST_HC)' --show-iface B.hi > B_dirty_iface
+	diff B_clean_iface B_dirty_iface
diff --git a/testsuite/tests/determinism/determ024/all.T b/testsuite/tests/determinism/determ024/all.T
new file mode 100644
index 0000000000..eaa79f6e6c
--- /dev/null
+++ b/testsuite/tests/determinism/determ024/all.T
@@ -0,0 +1 @@
+test('determ024', [extra_files(['A.hs', 'B.hs'])], makefile_test, ['determ024'])