Detect ability of linker to create relocatable objects

Cabal/src/Distribution/Simple/Program/Builtin.hs

@@ -347,7 +347,33 @@ greencardProgram :: Program
 greencardProgram = simpleProgram "greencard"
 
 ldProgram :: Program
-ldProgram = simpleProgram "ld"
+ldProgram =
+  (simpleProgram "ld")
+    { programPostConf = \verbosity ldProg -> do
+        -- The `lld` linker cannot create merge (relocatable) objects so we
+        -- want to detect this.
+        -- If the linker does support relocatable objects, we want to use that
+        -- to create partially pre-linked objects for GHCi, so we get much
+        -- faster loading as we do not have to do the separate loading and
+        -- in-memory linking the static linker in GHC does, but can offload
+        -- parts of this process to a pre-linking step.
+        -- However this requires the linker to support this features. Not all
+        -- linkers do, and notably as of this writing `lld` which is a popular
+        -- choice for windows linking does not support this feature. However
+        -- if using binutils ld or another linker that supports --relocatable,
+        -- we should still be good to generate pre-linked objects.
+        ldHelpOutput <-
+          getProgramInvocationOutput
+            verbosity
+            (programInvocation ldProg ["--help"])
+            -- In case the linker does not support '--help'. Eg the LLVM linker,
+            -- `lld` only accepts `-help`.
+            `catchIO` (\_ -> return "")
+        let k = "Supports relocatable output"
+            v = if "--relocatable" `isInfixOf` ldHelpOutput then "YES" else "NO"
+            m = Map.insert k v (programProperties ldProg)
+        return $ ldProg{programProperties = m}
+    }
 
 tarProgram :: Program
 tarProgram =