views/json: Fix diag crash with invalid highlight

Some diagnostic sources (I'm looking at you, HCL) fail to set the end of
the subject range. This is a bug in those code paths, but we can ensure
that we generate valid JSON diagnostics by checking for it here.

By doing so before the range normalization, we ensure that we generate a
unit width highlight whenever possible, so that at least something
useful is displayed.

command/views/json/diagnostic.go

@@ -133,6 +133,12 @@ func NewDiagnostic(diag tfdiags.Diagnostic, sources map[string][]byte) *Diagnost
 		// We'll borrow HCL's range implementation here, because it has some
 		// handy features to help us produce a nice source code snippet.
 		highlightRange := sourceRefs.Subject.ToHCL()
+
+		// Some diagnostic sources fail to set the end of the subject range.
+		if highlightRange.End == (hcl.Pos{}) {
+			highlightRange.End = highlightRange.Start
+		}
+
 		snippetRange := highlightRange
 		if sourceRefs.Context != nil {
 			snippetRange = sourceRefs.Context.ToHCL()

command/views/json/diagnostic_test.go

@@ -246,6 +246,44 @@ func TestNewDiagnostic(t *testing.T) {
 				},
 			},
 		},
+		"error with unset highlight end position": {
+			&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "There is no end",
+				Detail:   "But there is a beginning",
+				Subject: &hcl.Range{
+					Filename: "test.tf",
+					Start:    hcl.Pos{Line: 1, Column: 16, Byte: 15},
+					End:      hcl.Pos{Line: 0, Column: 0, Byte: 0},
+				},
+			},
+			&Diagnostic{
+				Severity: "error",
+				Summary:  "There is no end",
+				Detail:   "But there is a beginning",
+				Range: &DiagnosticRange{
+					Filename: "test.tf",
+					Start: Pos{
+						Line:   1,
+						Column: 16,
+						Byte:   15,
+					},
+					End: Pos{
+						Line:   1,
+						Column: 17,
+						Byte:   16,
+					},
+				},
+				Snippet: &DiagnosticSnippet{
+					Context:              strPtr(`resource "test_resource" "test"`),
+					Code:                 `resource "test_resource" "test" {`,
+					StartLine:            1,
+					HighlightStartOffset: 15,
+					HighlightEndOffset:   16,
+					Values:               []DiagnosticExpressionValue{},
+				},
+			},
+		},
 		"error with source code subject and known expression": {
 			&hcl.Diagnostic{
 				Severity: hcl.DiagError,
@@ -698,6 +736,11 @@ func TestNewDiagnostic(t *testing.T) {
 				"diagnostic",
 				fmt.Sprintf("%s.json", strings.ReplaceAll(name, " ", "-")),
 			)
+
+			// Generate golden reference by uncommenting the next two lines:
+			// gotBytes = append(gotBytes, '\n')
+			// os.WriteFile(filename, gotBytes, 0644)
+
 			wantFile, err := os.Open(filename)
 			if err != nil {
 				t.Fatalf("failed to open golden file: %s", err)

command/views/json/testdata/diagnostic/error-with-unset-highlight-end-position.json

@@ -0,0 +1,26 @@
+{
+  "severity": "error",
+  "summary": "There is no end",
+  "detail": "But there is a beginning",
+  "range": {
+    "filename": "test.tf",
+    "start": {
+      "line": 1,
+      "column": 16,
+      "byte": 15
+    },
+    "end": {
+      "line": 1,
+      "column": 17,
+      "byte": 16
+    }
+  },
+  "snippet": {
+    "context": "resource \"test_resource\" \"test\"",
+    "code": "resource \"test_resource\" \"test\" {",
+    "start_line": 1,
+    "highlight_start_offset": 15,
+    "highlight_end_offset": 16,
+    "values": []
+  }
+}