Document GHC-19244 (#465)

* Document GHC-19244

* Update message-index/messages/GHC-19244/index.md

Co-authored-by: David Thrane Christiansen <[email protected]>

---------

Co-authored-by: David Thrane Christiansen <[email protected]>

Author: BinderDavid

message-index/messages/GHC-19244/index.md

@@ -0,0 +1,13 @@
+---
+title: Module is annotated as trustworthy, but is inferred as safe
+summary: The module is annotated to be trustworthy, but it could be annotated as safe
+severity: warning
+flag: -Wtrustworthy-safe
+introduced: 9.6.1
+---
+
+GHC implements the `SafeHaskell` extension which allows programmers to restrict modules to a specific subset which is considered safe, and restricts access to "escape hatches" such as `unsafePerformIO`.
+Details about Safe Haskell are available in [the GHC User's Guide](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/safe_haskell.html).
+There are two levels of safety: Safe modules are checked by GHC and are guaranteed to be safe, whereas trustworthy modules are checked by
+the programmer who promises that the code cannot violate the guarantees of safe Haskell.
+If GHC sees a module which is annotated as trustworthy, but inferred as safe, it emits a warning that the stricter "safe" annotation should be used instead.

message-index/messages/GHC-19244/trustworthy-safe/after/TrustworthySafe.hs

@@ -0,0 +1,6 @@
+{-# LANGUAGE Haskell2010, Safe #-}
+{-# OPTIONS_GHC -fwarn-trustworthy-safe #-}
+module TrustworthySafe where
+
+theAnswer :: Int
+theAnswer = 42

message-index/messages/GHC-19244/trustworthy-safe/before/TrustworthySafe.hs

@@ -0,0 +1,6 @@
+{-# LANGUAGE Haskell2010, Trustworthy #-}
+{-# OPTIONS_GHC -fwarn-trustworthy-safe #-}
+module TrustworthySafe where
+
+theAnswer :: Int
+theAnswer = 42

message-index/messages/GHC-19244/trustworthy-safe/index.md

@@ -0,0 +1,15 @@
+---
+title: A module was annotated as trustworthy, but inferred as safe
+---
+
+In this example the module was annotated as trustworthy, but GHC inferred that the module is also safe.
+Since "safe" is a stronger guarantee than "trustworthy", GHC warns that the module should be annotated as safe instead.
+The warning can therefore be fixed by changing the annotation from trustworthy to safe.
+
+```
+messages/GHC-19244/trustworthy-safe/before/TrustworthySafe.hs:1:27: warning: [GHC-19244] [-Wtrustworthy-safe]
+    ‘TrustworthySafe’ is marked as Trustworthy but has been inferred as safe!
+  |
+1 | {-# LANGUAGE Haskell2010, Trustworthy #-}
+  |
+```