Fix fluent diagnostics

compiler/rustc_fluent_macro/src/fluent.rs

@@ -138,25 +138,8 @@ pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::Tok
                 // with a lowercase as rustc errors do.
                 err.replace_range(0..1, &err.chars().next().unwrap().to_lowercase().to_string());
 
-                let line_starts: Vec<usize> = std::iter::once(0)
-                    .chain(
-                        this.source()
-                            .char_indices()
-                            .filter_map(|(i, c)| Some(i + 1).filter(|_| c == '\n')),
-                    )
-                    .collect();
-                let line_start = line_starts
-                    .iter()
-                    .enumerate()
-                    .map(|(line, idx)| (line + 1, idx))
-                    .filter(|(_, idx)| **idx <= pos.start)
-                    .last()
-                    .unwrap()
-                    .0;
-
                 let message = annotate_snippets::Level::Error.title(&err).snippet(
                     Snippet::source(this.source())
-                        .line_start(line_start)
                         .origin(&relative_ftl_path)
                         .fold(true)
                         .annotation(annotate_snippets::Level::Error.span(pos.start..pos.end - 1)),

tests/ui-fulldeps/fluent-messages/many-lines.ftl

@@ -0,0 +1,11 @@
+no_crate_foo = foo
+
+# This file tests error reporting for
+# fluent files with many lines.
+# The error message should point to the correct line number
+# and include no more context than necessary.
+
+no_crate_bar =
+
+no_crate_baz =
+    baz

tests/ui-fulldeps/fluent-messages/test.rs

@@ -80,3 +80,8 @@ mod bad_escape {
     //~| ERROR invalid escape `\"`
     //~| ERROR invalid escape `\'`
 }
+
+mod many_lines {
+    rustc_fluent_macro::fluent_messages! { "./many-lines.ftl" }
+    //~^ ERROR could not parse Fluent resource
+}

tests/ui-fulldeps/fluent-messages/test.stderr

@@ -103,5 +103,20 @@ LL |     rustc_fluent_macro::fluent_messages! { "./invalid-escape.ftl" }
    |
    = note: Fluent does not interpret these escape sequences (<https://projectfluent.org/fluent/guide/special.html>)
 
-error: aborting due to 13 previous errors
+error: could not parse Fluent resource
+  --> $DIR/test.rs:85:44
+   |
+LL |     rustc_fluent_macro::fluent_messages! { "./many-lines.ftl" }
+   |                                            ^^^^^^^^^^^^^^^^^^
+   |
+   = help: see additional errors emitted
+
+error: expected a message field for "no_crate_bar"
+ --> ./many-lines.ftl:8:1
+  |
+8 | no_crate_bar =
+  | ^^^^^^^^^^^^^^
+  |
+
+error: aborting due to 14 previous errors