fluent_macro: fix diagnostics for fluent parse failures

This line number calculation was both wrong and unnecessary.

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/test.stderr

@@ -112,9 +112,9 @@ 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:15:1
+ --> ./many-lines.ftl:8:1
   |
-15 | no_crate_bar =
+8 | no_crate_bar =
   | ^^^^^^^^^^^^^^
   |