toSnippet shouldn't crash if the error has no source code

compiler/src/Reporting/Render/Code.hs

@@ -42,13 +42,22 @@ toSource source =
 -- CODE FORMATTING
 
 toSnippet :: Source -> A.Region -> Maybe A.Region -> (D.Doc, D.Doc) -> D.Doc
-toSnippet source region highlight (preHint, postHint) =
-  D.vcat
-    [ preHint,
-      "",
-      render source region highlight,
-      postHint
-    ]
+toSnippet source region@(A.Region (A.Position startLine _) (A.Position _ _)) highlight (preHint, postHint) =
+  if startLine > 0
+    then
+      D.vcat
+        [ preHint,
+          "",
+          render source region highlight,
+          postHint
+        ]
+    else
+      -- The region doesn't point to actual source code. Don't render it.
+      D.vcat
+        [ preHint,
+          "",
+          postHint
+        ]
 
 toPair :: Source -> A.Region -> A.Region -> (D.Doc, D.Doc) -> (D.Doc, D.Doc, D.Doc) -> D.Doc
 toPair source r1 r2 (oneStart, oneEnd) (twoStart, twoMiddle, twoEnd) =

gren.cabal

@@ -254,13 +254,17 @@ Test-Suite gren-tests
         Parse.UnderscorePatternSpec
         Parse.MultilineStringSpec
         Parse.DeclSpec
+        Reporting.Error.ImportSpec
 
     Build-Depends:
         gren:common,
         base >= 4.19 && <5,
+        containers >= 0.6 && < 0.7,
         utf8-string,
         bytestring >= 0.11 && < 0.12,
-        hspec >= 2.7.10 && < 3
+        hspec >= 2.7.10 && < 3,
+        prettyprinter >= 1.7.1 && < 2,
+        text >= 2.1.2 && < 3
 
     Build-Tool-Depends:
         hspec-discover:hspec-discover >= 2.7.10 && < 3

tests/Reporting/Error/ImportSpec.hs

@@ -0,0 +1,49 @@
+module Reporting.Error.ImportSpec (spec) where
+
+import Data.Set qualified as Set
+import Data.Text as T
+import Data.Text.Encoding as T
+import Data.Utf8 qualified as Utf8
+import Prettyprinter qualified as P
+import Prettyprinter.Render.Text (renderStrict)
+import Reporting.Annotation qualified as A
+import Reporting.Error.Import qualified as Import
+import Reporting.Render.Code qualified as Code
+import Reporting.Report qualified as Report
+import Test.Hspec (Spec, describe, it)
+
+spec :: Spec
+spec = do
+  describe "Import error reporting" $ do
+    {-
+        When "gren-lang/core" module is not listed in gren.json, the
+        default imports that the compiler inserts into the code
+        fail, but there is no source code region to report.
+        We want to ensure the reporting code doesn't crash, and
+        actually reports that modules like Basics (and others) are reported
+        as not found.
+    -}
+    it "Give proper error if default package dependencies are not found" $ do
+      let -- Empty source code
+          source = Code.toSource (T.encodeUtf8 (T.pack ""))
+
+          -- The "Module Not Found" Error
+          err =
+            Import.Error
+              { -- IMPORTANT: The region here is zero, not pointing to any
+                -- possible lines of source code. This happens when
+                -- "import Basics" is added by default by the compiler, without
+                -- it appearing in the actual source code.
+                Import._region = A.Region (A.Position 0 0) (A.Position 0 0),
+                Import._import = Utf8.fromChars "foo",
+                Import._unimported = Set.singleton (Utf8.fromChars "Basics"),
+                Import._problem = Import.NotFound
+              }
+
+          -- Create the report
+          report = Import.toReport source err
+
+          -- Convert the Prettyprinter Doc to a Data.Text
+          messageText = renderStrict (P.layoutCompact $ Report._message report)
+       in -- Does it have one of the missing modules reported in it?
+          T.isInfixOf (T.pack "Basics") messageText