Document GHC-58656 (#463)

* Document GHC-58656

* Apply suggestion from code review

* Change -fwarn-safe to -Wsafe

Author: BinderDavid

message-index/messages/GHC-58656/index.md

@@ -0,0 +1,12 @@
+---
+title: Module is inferred to be safe
+summary: If a module is inferred as safe, then it should be annotated as safe
+severity: warning
+flag: -Wsafe
+introduced: 9.6.1
+---
+
+GHC implements the `SafeHaskell` extension which allows programmers to restrict modules to a specific subset which is considered safe and does not contain loopholes such as `unsafePerformIO`.
+More details are available in [the GHC User's Guide](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/safe_haskell.html).
+The compiler can automatically infer whether a given module is safe or not.
+GHC can emit a warning if a module is inferred to be safe, but not annotated as safe.

message-index/messages/GHC-58656/inferred-safe/after/InferredSafe.hs

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+{-# LANGUAGE Haskell2010, Safe #-}
+{-# OPTIONS_GHC -Wsafe #-}
+module InferredSafe where
+
+theAnswer :: Int
+theAnswer = 42

message-index/messages/GHC-58656/inferred-safe/before/InferredSafe.hs

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+{-# LANGUAGE Haskell2010 #-}
+{-# OPTIONS_GHC -Wsafe #-}
+module InferredSafe where
+
+theAnswer :: Int
+theAnswer = 42

message-index/messages/GHC-58656/inferred-safe/index.md

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+---
+title: A module is inferred as safe but not annotated as safe
+---
+
+In this example the module has been inferred to be safe, but not annotated as such.
+If the module is annotated, the warning disappears.
+
+```
+messages/GHC-58656/inferred-safe/before/InferredSafe.hs:2:17: warning: [GHC-58656] [-Wsafe]
+    ‘InferredSafe’ has been inferred as safe!
+  |
+2 | {-# OPTIONS_GHC -fwarn-safe #-}
+  |
+```