Don't panic on span for empty file.

Author: ehuss

src/lib.rs

@@ -122,7 +122,8 @@ fn parse_snippet(span: &DiagnosticSpan) -> Option<Snippet> {
 
     // If we get a DiagnosticSpanLine where highlight_end > text.len(), we prevent an 'out of
     // bounds' access by making sure the index is within the array bounds.
-    let last_tail_index = last.highlight_end.min(last.text.len()) - 1;
+    // `saturating_sub` is used in case of an empty file
+    let last_tail_index = last.highlight_end.min(last.text.len()).saturating_sub(1);
     let last_slice = last.text.chars().collect::<Vec<char>>();
 
     if span.text.len() > 1 {

tests/edge-cases/empty.json

@@ -0,0 +1,42 @@
+{
+  "message": "`main` function not found in crate `empty`",
+  "code": {
+    "code": "E0601",
+    "explanation": "No `main` function was found in a binary crate. To fix this error, add a\n`main` function. For example:\n\n```\nfn main() {\n    // Your program will start here.\n    println!(\"Hello world!\");\n}\n```\n\nIf you don't know the basics of Rust, you can go look to the Rust Book to get\nstarted: https://doc.rust-lang.org/book/\n"
+  },
+  "level": "error",
+  "spans": [
+    {
+      "file_name": "empty.rs",
+      "byte_start": 0,
+      "byte_end": 0,
+      "line_start": 0,
+      "line_end": 0,
+      "column_start": 1,
+      "column_end": 1,
+      "is_primary": true,
+      "text": [
+        {
+          "text": "",
+          "highlight_start": 1,
+          "highlight_end": 1
+        }
+      ],
+      "label": null,
+      "suggested_replacement": null,
+      "suggestion_applicability": null,
+      "expansion": null
+    }
+  ],
+  "children": [
+    {
+      "message": "consider adding a `main` function to `empty.rs`",
+      "code": null,
+      "level": "note",
+      "spans": [],
+      "children": [],
+      "rendered": null
+    }
+  ],
+  "rendered": "error[E0601]: `main` function not found in crate `empty`\n  |\n  = note: consider adding a `main` function to `empty.rs`\n\n"
+}

tests/edge-cases/empty.rs

@@ -21,3 +21,4 @@ macro_rules! expect_empty_json_test {
 expect_empty_json_test! {multiple_fix_options_yield_no_suggestions, "skip-multi-option-lints.json"}
 expect_empty_json_test! {out_of_bounds_test, "out_of_bounds.recorded.json"}
 expect_empty_json_test! {utf8_identifiers_test, "utf8_idents.recorded.json"}
+expect_empty_json_test! {empty, "empty.json"}