Rollup merge of rust-lang#33369 - nikomatsakis:graceful-empty-span, r=jntrnr

degrade gracefully with empty spans

In rust-lang#32756, we solved the final test failure, but digging more into it the handling of that scenario could be better. The error was caused by an empty span supplied by the parser representing EOF. This patch checks that we cope more gracefully with such spans:

r? @jonathandturner

Author: Manishearth

src/libsyntax/errors/snippet/mod.rs

@@ -376,11 +376,21 @@ impl FileInfo {
         // Basically, although this loses information, multi-line spans just
         // never look good.
 
-        let (line, start_col, end_col) = if lines.len() == 1 {
+        let (line, start_col, mut end_col) = if lines.len() == 1 {
             (lines[0].line_index, lines[0].start_col, lines[0].end_col)
         } else {
             (lines[0].line_index, lines[0].start_col, CharPos(lines[0].start_col.0 + 1))
         };
+
+        // Watch out for "empty spans". If we get a span like 6..6, we
+        // want to just display a `^` at 6, so convert that to
+        // 6..7. This is degenerate input, but it's best to degrade
+        // gracefully -- and the parser likes to suply a span like
+        // that for EOF, in particular.
+        if start_col == end_col {
+            end_col.0 += 1;
+        }
+
         let index = self.ensure_source_line(line);
         self.lines[index].push_annotation(start_col,
                                           end_col,

src/libsyntax/errors/snippet/test.rs

@@ -519,3 +519,41 @@ fn span_overlap_label3() {
   |>            ----- bar
 "#[1..]);
 }
+
+#[test]
+fn span_empty() {
+    // In one of the unit tests, we found that the parser sometimes
+    // gives empty spans, and in particular it supplied an EOF span
+    // like this one, which points at the very end. We want to
+    // fallback gracefully in this case.
+
+    let file_text = r#"
+fn main() {
+    struct Foo;
+
+    impl !Sync for Foo {}
+
+    unsafe impl Send for &'static Foo {
+    // error: cross-crate traits with a default impl, like `core::marker::Send`,
+    //        can only be implemented for a struct/enum type, not
+    //        `&'static Foo`
+}"#;
+
+
+    let cm = Rc::new(CodeMap::new());
+    let foo = cm.new_filemap_and_lines("foo.rs", file_text);
+
+    let mut rbrace_span = cm.span_substr(&foo, file_text, "}", 1);
+    rbrace_span.lo = rbrace_span.hi;
+
+    let mut snippet = SnippetData::new(cm.clone(), Some(rbrace_span));
+    snippet.push(rbrace_span, false, None);
+    let lines = snippet.render_lines();
+    let text: String = make_string(&lines);
+    println!("r#\"\n{}\"", text);
+    assert_eq!(text, &r#"
+  --> foo.rs:11:2
+11 |> }
+   |>  -
+"#[1..]);
+}