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

degrade gracefully with empty spans

In https://github.com/rust-lang/rust/pull/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

2 files changed, 49 insertions, 1 deletions

diff --git a/src/libsyntax/errors/snippet/mod.rs b/src/libsyntax/errors/snippet/mod.rs
index e213f623ab8..237e6823e0f 100644
--- a/src/libsyntax/errors/snippet/mod.rs
+++ b/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,
diff --git a/src/libsyntax/errors/snippet/test.rs b/src/libsyntax/errors/snippet/test.rs
index 569d1119919..5a888b48819 100644
--- a/src/libsyntax/errors/snippet/test.rs
+++ b/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..]);
+}