Add Backpack support to Stack

.stan.toml

@@ -140,25 +140,36 @@
 
 # Anti-pattern: Data.ByteString.Char8.pack
 [[ignore]]
-  id = "OBS-STAN-0203-erw24B-1138:3"
+  id = "OBS-STAN-0203-EmXRiW-573:17"
+# ✦ Description:   Usage of 'pack' function that doesn't handle Unicode characters
+# ✦ Category:      #AntiPattern
+# ✦ File:          src/Stack/Build/Backpack.hs
+#
+#   572 ┃
+#   573 ┃         input = S8.pack (L.intercalate "," sorted)
+#   574 ┃                 ^^^^^^^
+
+# Anti-pattern: Data.ByteString.Char8.pack
+[[ignore]]
+  id = "OBS-STAN-0203-erw24B-1150:3"
 # ✦ Description:   Usage of 'pack' function that doesn't handle Unicode characters
 # ✦ Category:      #AntiPattern
 # ✦ File:          src\Stack\Build\ExecuteEnv.hs
 #
-#  1137 ┃
-#  1138 ┃   S8.pack . formatTime defaultTimeLocale "%Y-%m-%dT%H:%M:%S%6Q"
-#  1139 ┃   ^^^^^^^
+#  1149 ┃
+#  1150 ┃   S8.pack . formatTime defaultTimeLocale "%Y-%m-%dT%H:%M:%S%6Q"
+#  1151 ┃   ^^^^^^^
 
 # Anti-pattern: Data.ByteString.Char8.pack
 [[ignore]]
-  id = "OBS-STAN-0203-tuE+RG-252:24"
+  id = "OBS-STAN-0203-tuE+RG-327:24"
 # ✦ Description:   Usage of 'pack' function that doesn't handle Unicode characters
 # ✦ Category:      #AntiPattern
 # ✦ File:          src\Stack\Build\ExecutePackage.hs
 #
-#   251 ┃
-#   252 ┃   newConfigFileRoot <- S8.pack . toFilePath <$> view configFileRootL
-#   253 ┃                        ^^^^^^^
+#   326 ┃
+#   327 ┃   newConfigFileRoot <- S8.pack . toFilePath <$> view configFileRootL
+#   328 ┃                        ^^^^^^^
 
 # Anti-pattern: Data.ByteString.Char8.pack
 [[ignore]]
@@ -193,6 +204,17 @@
 #  351 ┃   hashRepoName = Hash.hash . BS.pack . takeWhile (\c -> c /= ':' && c /= '@')
 #  352 ┃                              ^^^^^^^
 
+# Anti-pattern: Slow 'nub' for lists
+[[ignore]]
+  id = "OBS-STAN-0209-EmXRiW-328:19"
+# ✦ Description:   Usage of 'nub' on lists that runs in quadratic time
+# ✦ Category:      #AntiPattern
+# ✦ File:          src/Stack/Build/Backpack.hs
+#
+#   327 ┃
+#   328 ┃                   L.nub $ collectInheritedSigs sigPkg adrMap' Set.empty
+#   329 ┃                   ^^^^^
+
 # Data types with non-strict fields
 # Defining lazy fields in data types can lead to unexpected space leaks
 # Stack uses lazy fields in many places

ChangeLog.md

@@ -8,12 +8,31 @@ Release notes:
 
 Major changes:
 
+* Stack now supports GHC's Backpack module system for cross-package use. When a
+  package uses `signatures` and `mixins` to depend on an abstract interface
+  provided by another package, Stack automatically creates the extra
+  instantiation build steps that Cabal requires. This includes support for
+  explicit renaming, multiple instantiations of the same indefinite package,
+  transitive signature chains, sub-library signatures, and indefinite packages
+  from Hackage or snapshots. See the
+  [Backpack topic](https://docs.haskellstack.org/en/stable/topics/backpack/) for
+  details. (Private Backpack, where signatures and implementations live in the
+  same package, has always worked without changes.)
+
 Behavior changes:
 
 Other enhancements:
 
 Bug fixes:
 
+* `stack test --no-rerun-tests` correctly skips test suites that have 
+  previously been built and run successfully, including for non-Backpack 
+  packages built by the per-component planner.
+* Stack's per-component build planner correctly handles the cross-package 
+  test/library cycle shape from issue #6905: project package A's library 
+  depending on project package B's library, with B's test suite depending on A, 
+  builds in the right order with the right configure flags.
+
 ##  v3.11.1 - 2026-06-13
 
 **Changes since v3.9.3:**