Treat empty input as one line still

Otherwise no labels can be printed over an empty input,
not even empty ones!
(`get_source_groups` never returns anything if there are
no source lines.)

src/source.rs

@@ -108,6 +108,22 @@ impl<I: AsRef<str>> From<I> for Source<I> {
     ///
     /// Note that this function can be expensive for long strings. Use an implementor of [`Cache`] where possible.
     fn from(input: I) -> Self {
+        // `input.split_inclusive()` will not iterate at all,
+        // but an empty input still ought to count as a single empty line.
+        if input.as_ref().is_empty() {
+            return Self {
+                text: input,
+                lines: vec![Line {
+                    offset: 0,
+                    char_len: 0,
+                    byte_offset: 0,
+                    byte_len: 0,
+                }],
+                len: 0,
+                byte_len: 0,
+            };
+        }
+
         let mut char_offset = 0;
         let mut byte_offset = 0;
         let mut lines = Vec::new();
@@ -383,7 +399,7 @@ mod tests {
 
     #[test]
     fn source_from_empty() {
-        test_with_lines(vec![]); // Empty string
+        test_with_lines(vec![""]); // Empty string
     }
 
     #[test]

src/write.rs

@@ -1097,6 +1097,102 @@ mod tests {
         "###);
     }
 
+    #[test]
+    fn empty_input() {
+        let source = "";
+        let msg = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
+            .with_config(no_color_and_ascii())
+            .with_message("unexpected end of file")
+            .with_label(Label::new(0..0).with_message("No more fruit!"))
+            .finish()
+            .write_to_string(Source::from(source));
+
+        assert_snapshot!(msg, @r###"
+        Error: unexpected end of file
+           ,-[<unknown>:1:1]
+           |
+         1 | 
+           | | 
+           | `- No more fruit!
+        ---'
+        "###);
+    }
+
+    #[test]
+    fn empty_input_help() {
+        let source = "";
+        let msg = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
+            .with_config(no_color_and_ascii())
+            .with_message("unexpected end of file")
+            .with_label(Label::new(0..0).with_message("No more fruit!"))
+            .with_help("have you tried going to the farmer's market?")
+            .finish()
+            .write_to_string(Source::from(source));
+
+        assert_snapshot!(msg, @r###"
+        Error: unexpected end of file
+           ,-[<unknown>:1:1]
+           |
+         1 | 
+           | | 
+           | `- No more fruit!
+           | 
+           | Help: have you tried going to the farmer's market?
+        ---'
+        "###);
+    }
+
+    #[test]
+    fn empty_input_note() {
+        let source = "";
+        let msg = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
+            .with_config(no_color_and_ascii())
+            .with_message("unexpected end of file")
+            .with_label(Label::new(0..0).with_message("No more fruit!"))
+            .with_note("eat your greens!")
+            .finish()
+            .write_to_string(Source::from(source));
+
+        assert_snapshot!(msg, @r###"
+        Error: unexpected end of file
+           ,-[<unknown>:1:1]
+           |
+         1 | 
+           | | 
+           | `- No more fruit!
+           | 
+           | Note: eat your greens!
+        ---'
+        "###);
+    }
+
+    #[test]
+    fn empty_input_help_note() {
+        let source = "";
+        let msg = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
+            .with_config(no_color_and_ascii())
+            .with_message("unexpected end of file")
+            .with_label(Label::new(0..0).with_message("No more fruit!"))
+            .with_note("eat your greens!")
+            .with_help("have you tried going to the farmer's market?")
+            .finish()
+            .write_to_string(Source::from(source));
+
+        assert_snapshot!(msg, @r###"
+        Error: unexpected end of file
+           ,-[<unknown>:1:1]
+           |
+         1 | 
+           | | 
+           | `- No more fruit!
+           | 
+           | Help: have you tried going to the farmer's market?
+           | 
+           | Note: eat your greens!
+        ---'
+        "###);
+    }
+
     #[test]
     fn byte_spans_never_crash() {
         let source = "apple\np\n\nempty\n";