Improve diagnostic location in nls (#1856)

* Improve diagnostic location

* insta review

lsp/nls/src/diagnostic.rs

@@ -1,7 +1,7 @@
 use std::ops::Range;
 
 use codespan::{FileId, Files};
-use codespan_reporting::diagnostic::{self, Diagnostic};
+use codespan_reporting::diagnostic::{self, Diagnostic, LabelStyle};
 use lsp_types::{DiagnosticRelatedInformation, NumberOrString};
 use nickel_lang_core::{error::UNKNOWN_SOURCE_NAME, position::RawSpan};
 use serde::{Deserialize, Serialize};
@@ -117,12 +117,15 @@ impl DiagnosticCompat for SerializableDiagnostic {
 
         if !diagnostic.message.is_empty() {
             // What location should we use for the "overall" diagnostic? `Diagnostic` doesn't
-            // have an "overall" location, so we arbitrarily take the location of the first label.
-            if let Some(range) = within_file_labels
+            // have an "overall" location, so we arbitrarily take the location of the first primary label
+            // (falling back to the first label if there's no primary one).
+            let maybe_label = within_file_labels
                 .clone()
-                .next()
-                .map(|label| lsp_types::Range::from_codespan(&label.file_id, &label.range, files))
-            {
+                .find(|lab| lab.style == LabelStyle::Primary)
+                .or_else(|| within_file_labels.clone().next());
+
+            if let Some(label) = maybe_label {
+                let range = lsp_types::Range::from_codespan(&label.file_id, &label.range, files);
                 let message = if diagnostic.notes.is_empty() {
                     diagnostic.message.clone()
                 } else {

lsp/nls/tests/snapshots/main__lsp__nls__tests__inputs__diagnostics-array.ncl.snap

@@ -3,11 +3,11 @@ source: lsp/nls/tests/main.rs
 expression: output
 ---
 (file:///diagnostics-array.ncl, 2:4-2:7: applied to this expression)
-(file:///diagnostics-array.ncl, 3:12-3:18: contract broken by a value)
+(file:///diagnostics-array.ncl, 2:4-2:7: contract broken by a value)
 (file:///diagnostics-array.ncl, 3:12-3:18: expected array element type)
 (file:///diagnostics-array.ncl, 5:12-5:17: applied to this expression)
-(file:///diagnostics-array.ncl, 6:20-6:26: contract broken by the value of `num`)
+(file:///diagnostics-array.ncl, 5:12-5:17: contract broken by the value of `num`)
 (file:///diagnostics-array.ncl, 6:20-6:26: expected type)
 (file:///diagnostics-array.ncl, 8:6-8:9: applied to this expression)
-(file:///diagnostics-array.ncl, 9:19-9:25: contract broken by a value)
+(file:///diagnostics-array.ncl, 8:6-8:9: contract broken by a value)
 (file:///diagnostics-array.ncl, 9:19-9:25: expected array element type)

lsp/nls/tests/snapshots/main__lsp__nls__tests__inputs__diagnostics-basic.ncl.snap

@@ -2,10 +2,9 @@
 source: lsp/nls/tests/main.rs
 expression: output
 ---
-(file:///diagnostics-basic.ncl, 1:8-1:14: contract broken by the value of `num`)
 (file:///diagnostics-basic.ncl, 1:8-1:14: expected type)
 (file:///diagnostics-basic.ncl, 1:17-1:20: applied to this expression)
+(file:///diagnostics-basic.ncl, 1:17-1:20: contract broken by the value of `num`)
 (file:///diagnostics-basic.ncl, 2:9-2:12: applied to this expression)
-(file:///diagnostics-basic.ncl, 3:13-3:19: contract broken by the value of `num2`)
+(file:///diagnostics-basic.ncl, 2:9-2:12: contract broken by the value of `num2`)
 (file:///diagnostics-basic.ncl, 3:13-3:19: expected type)
-