Generate INLINE [1] pragmas for TH-derived instances

This backports changes to GHC's treatment of derived `Generic(1)` instances in
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/2965, which affect GHC 9.2
or later. The Template Haskell-based implementation is here is adapted from
`linear-generics` (see linear-generics/linear-generics#21).

Fixes #89.

CHANGELOG.md

@@ -1,3 +1,8 @@
+# next [????.??.??]
+* Place `INLINE [1]` pragmas on `from` and `to` implementations when types
+  don't have too many constructors or fields, following the heuristics that GHC
+  9.2+ uses for `Generic` deriving.
+
 # 1.14.3 [2023.02.27]
 * Support `th-abstraction-0.5.*`.
 

src/Generics/Deriving/TH.hs

@@ -353,9 +353,44 @@ deriveInstCommon genericName repName gClass fromName toName opts n = do
       fcs = mkBody mkFrom
       tcs = mkBody mkTo
 
+      inline_pragmas
+        | inlining_useful cons
+#if MIN_VERSION_template_haskell(2,7,0)
+        = map (\fun_name ->
+                pragInlD fun_name
+# if MIN_VERSION_template_haskell(2,8,0)
+                         Inline FunLike (FromPhase 1)
+# else
+                         (inlineSpecPhase True False True 1)
+# endif
+              ) [fromName, toName]
+#else
+        = [] -- Sadly, GHC 7.0 and 7.2 appear to suffer from a bug that
+             -- prevents them from attaching INLINE pragmas to class methods
+             -- via Template Haskell, so don't bother generating any pragmas at
+             -- all for these GHC versions.
+#endif
+        | otherwise
+        = []
+
   fmap (:[]) $
     instanceD (cxt []) (conT genericName `appT` return origSigTy)
-                         [return tyIns, funD fromName fcs, funD toName tcs]
+                       (inline_pragmas ++ [return tyIns, funD fromName fcs, funD toName tcs])
+  where
+    -- Adapted from inlining_useful in GHC.Tc.Deriv.Generics.mkBindsRep in the
+    -- GHC source code:
+    --
+    -- https://gitlab.haskell.org/ghc/ghc/-/blob/80729d96e47c99dc38e83612dfcfe01cf565eac0/compiler/GHC/Tc/Deriv/Generics.hs#L368-386
+    inlining_useful cons
+      | ncons <= 1  = True
+      | ncons <= 4  = max_fields <= 5
+      | ncons <= 8  = max_fields <= 2
+      | ncons <= 16 = max_fields <= 1
+      | ncons <= 24 = max_fields == 0
+      | otherwise   = False
+      where
+        ncons      = length cons
+        max_fields = maximum $ map (length . constructorFields) cons
 
 {- $make