Parenthesize let-expressions inside of do-expressions. (#38)

The following syntax is not valid, but can be generated by GHC;

```
do let ...
   in ...
```

It's not always a problem, if GHC manages to wrap it like

```
do let ... in ...
```

But for consistency, and since this ought to be an uncommon case,
always wrap `let`s in parentheses when they're inside of a `do` statement.

src/GHC/SourceGen/Expr.hs

@@ -27,7 +27,7 @@ import HsExpr
 import HsPat (HsRecField'(..), HsRecFields(..))
 import HsTypes (FieldOcc(..), AmbiguousFieldOcc(..))
 import Data.String (fromString)
-import SrcLoc (unLoc, Located)
+import SrcLoc (unLoc, GenLocated(..), Located)
 
 import GHC.SourceGen.Binds.Internal
 import GHC.SourceGen.Binds
@@ -88,7 +88,21 @@ multiIf = noExtOrPlaceHolder HsMultiIf . map builtLoc
 -- > =====
 -- > do' [var "x" <-- var "act", stmt $ var "return" @@ var "x"]
 do' :: [Stmt'] -> HsExpr'
-do' = withPlaceHolder . noExt HsDo DoExpr . builtLoc . map builtLoc
+do' = withPlaceHolder . noExt HsDo DoExpr
+        . builtLoc . map (builtLoc . parenthesizeIfLet)
+  where
+  -- Put parentheses around a "let" in a do-binding, to avoid:
+  --   do let x = ...
+  --      in x
+  -- which is not valid Haskell.
+#if MIN_VERSION_ghc(8,6,0)
+    parenthesizeIfLet (BodyStmt ext e@(L _ HsLet{}) x y)
+        = BodyStmt ext (parExpr e) x y
+#else
+    parenthesizeIfLet (BodyStmt e@(L _ HsLet{}) x y tc)
+        = BodyStmt (parExpr e) x y tc
+#endif
+    parenthesizeIfLet s = s
 
 -- | A type constraint on an expression.
 --

tests/pprint_test.hs

@@ -181,6 +181,10 @@ exprsTest dflags = testGroup "Expr"
         , "let f (A x) = 1 in f" :~
             let' [ funBind "f" $ match [conP "A" [var "x"]] $ int 1] (var "f")
         ]
+    , test "do"
+        -- TODO: add more tests.
+        [ "do (let x = 1 in x)" :~ do' [stmt $ let' [valBind "x" (int 1)] (var "x")]
+        ]
     ]
   where
     test = testExprs dflags