diagnostics: fix crash on completely empty included file

Author: notriddle

compiler/rustc_errors/src/emitter.rs

@@ -1513,7 +1513,7 @@ impl HumanEmitter {
                 for line_idx in 0..annotated_file.lines.len() {
                     let file = annotated_file.file.clone();
                     let line = &annotated_file.lines[line_idx];
-                    if let Some(source_string) = file.get_line(line.line_index - 1) {
+                    if let Some(source_string) = line.line_index.checked_sub(1).and_then(|l| file.get_line(l)) {
                         let leading_whitespace = source_string
                             .chars()
                             .take_while(|c| c.is_whitespace())

tests/ui/include-macros/mismatched-types.stderr

@@ -1,8 +1,11 @@
 error[E0308]: mismatched types
-  --> $DIR/mismatched-types.rs:2:20
+  --> $DIR/file.txt:0:1
+   |
+   |
+  ::: $DIR/mismatched-types.rs:2:12
    |
 LL |     let b: &[u8] = include_str!("file.txt");
-   |            -----   ^^^^^^^^^^^^^^^^^^^^^^^^ expected `&[u8]`, found `&str`
+   |            -----   ------------------------ in this macro invocation
    |            |
    |            expected due to this
    |