fluent_macro: fix diagnostics for fluent parse failures

Author: Xiretza

compiler/rustc_fluent_macro/src/fluent.rs

@@ -132,7 +132,7 @@ pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::Tok
             Diagnostic::spanned(resource_span, Level::Error, "could not parse Fluent resource")
                 .help("see additional errors emitted")
                 .emit();
-            for ParserError { pos, slice: _, kind } in errs {
+            for ParserError { pos, slice, kind } in errs {
                 let mut err = kind.to_string();
                 // Entirely unnecessary string modification so that the error message starts
                 // with a lowercase as rustc errors do.
@@ -142,17 +142,16 @@ pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::Tok
                     .chain(
                         this.source()
                             .char_indices()
-                            .filter_map(|(i, c)| Some(i + 1).filter(|_| c == '\n')),
+                            .filter_map(|(i, c)| (c == '\n').then_some(i + 1)),
                     )
                     .collect();
-                let line_start = line_starts
-                    .iter()
-                    .enumerate()
-                    .map(|(line, idx)| (line + 1, idx))
-                    .filter(|(_, idx)| **idx <= pos.start)
-                    .last()
-                    .unwrap()
-                    .0;
+                let get_line = |pos| match line_starts.binary_search(&pos) {
+                    Ok(n) => n,
+                    Err(n) => n.saturating_sub(1),
+                };
+
+                let start_line = get_line(slice.as_ref().unwrap_or(&pos).start);
+                let start_line_pos = line_starts[start_line];
 
                 let snippet = Snippet {
                     title: Some(Annotation {
@@ -162,14 +161,15 @@ pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::Tok
                     }),
                     footer: vec![],
                     slices: vec![Slice {
-                        source: this.source(),
-                        line_start,
+                        source: &this.source()[start_line_pos..],
+                        // lines are one-indexed
+                        line_start: start_line + 1,
                         origin: Some(&relative_ftl_path),
                         fold: true,
                         annotations: vec![SourceAnnotation {
                             label: "",
                             annotation_type: AnnotationType::Error,
-                            range: (pos.start, pos.end - 1),
+                            range: (pos.start - start_line_pos, pos.end - start_line_pos),
                         }],
                     }],
                 };