1 files changed, 23 insertions, 1 deletions
diff --git a/src/lsp/lsp.zig b/src/lsp/lsp.zig
index 9dd1da5a..7e5595fd 100644
--- a/src/lsp/lsp.zig
+++ b/src/lsp/lsp.zig
@@ -61,7 +61,18 @@ pub fn query(gpa: std.mem.Allocator, arena: std.mem.Allocator, req: Req, out: *s
             return b.query(gpa, arena, req, out);
     }
     if (req.kind == .explain and backends.len > 0)
-        try backends[0].query(gpa, arena, req, out);
+        return backends[0].query(gpa, arena, req, out);
+    // Nothing asked: said why, where an empty answer would read as nothing
+    // found.
+    inline for (backends) |b| if (b.speaks(req.path))
+        return none(out, "the language server for this file does not answer it", .{});
+    try none(out, "no language server for this file", .{});
+}
+
+/// The one line a backend answers with when it cannot answer at all, and
+/// why: the core says it rather than "nothing found".
+pub fn none(out: *std.Io.Writer, comptime fmt: []const u8, args: anytype) std.Io.Writer.Error!void {
+    try out.print("@none " ++ fmt ++ "\n", args);
 }
 
 // Called on server reader threads; the sink must copy text before returning.
@@ -146,3 +157,14 @@ test "LSP encoders report every insufficient output capacity" {
         }
     }
 }
+
+test "a question no backend answers for a file says why rather than find nothing" {
+    if (backends.len == 0) return;
+    var arena: std.heap.ArenaAllocator = .init(std.testing.allocator);
+    defer arena.deinit();
+    var buf: [256]u8 = undefined;
+    var out: std.Io.Writer = .fixed(&buf);
+    // The in-process ZLS answers no call hierarchy for a .zig file.
+    try query(std.testing.allocator, arena.allocator(), .{ .kind = .incoming_calls, .path = "/x/main.zig", .source = "", .offset = 0 }, &out);
+    try std.testing.expectStringStartsWith(out.buffered(), "@none ");
+}