Callers and Callees on a .zig file say the language server does not answer them

The in-process ZLS answers no call hierarchy, and no other backend speaks .zig, so the question went nowhere and came back empty, as if nothing called the function. A question no backend answers, or a server that advertises no call or type hierarchy, now answers @none with why, which the core says.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

2 files changed, 26 insertions, 3 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 ");
+}
diff --git a/src/lsp/lsp_client.zig b/src/lsp/lsp_client.zig
index c3762f08..55df1f91 100644
--- a/src/lsp/lsp_client.zig
+++ b/src/lsp/lsp_client.zig
@@ -408,6 +408,7 @@ pub fn query(gpa: std.mem.Allocator, arena: std.mem.Allocator, req: lsp.Req, out
         tr.note("STOP: no language server spec matches {s}", .{
             if (req.path.len == 0) "a pane with no file" else req.path,
         });
+        if (req.kind != .explain) return lsp.none(out, "no language server for {s}", .{if (req.path.len == 0) "a pane with no file" else std.fs.path.basename(req.path)});
         return traceOut(&tr, req, out, 0, 0);
     };
     tr.note("file {s} -> {s} (languageId {s})", .{ std.fs.path.basename(req.path), specs[si].name, specs[si].lang });
@@ -807,11 +808,11 @@ fn hierarchy(c: *Conn, si: usize, arena: std.mem.Allocator, cx: *Cx, uri: []cons
     const call_side = h == .incoming or h == .outgoing;
     if (call_side and !c.caps.call_hier) {
         tr.note("STOP: {s} does not advertise callHierarchyProvider", .{specs[si].name});
-        return;
+        return lsp.none(cx.out, "{s} does not answer call hierarchy (no callHierarchyProvider)", .{specs[si].name});
     }
     if (!call_side and !c.caps.type_hier) {
         tr.note("STOP: {s} does not advertise typeHierarchyProvider", .{specs[si].name});
-        return;
+        return lsp.none(cx.out, "{s} does not answer type hierarchy (no typeHierarchyProvider)", .{specs[si].name});
     }
     const prepare: []const u8 = if (call_side) "textDocument/prepareCallHierarchy" else "textDocument/prepareTypeHierarchy";
     const follow: []const u8 = switch (h) {