lspbench: probe diagnostics and format against a BROKEN fixture

Plus docs/lsp-evaluation.md: the three backends measured head to head.

2 files changed, 34 insertions, 4 deletions

diff --git a/test/lspbench.zig b/test/lspbench.zig
index 73b82e2b..307bdc49 100644
--- a/test/lspbench.zig
+++ b/test/lspbench.zig
@@ -72,18 +72,23 @@ const anchors = [_]Anchor{
     .{ .file = "src/modal.zig", .needle = "pub fn ", .at = 7, .kind = .document_symbols, .expect = "" },
     // references to a symbol used in several places
     .{ .file = "src/lsp.zig", .needle = "pub const Kind", .at = 11, .kind = .references, .expect = "" },
-    // diagnostics on a file that should have none
-    .{ .file = "src/lsp.zig", .needle = "const std", .at = 6, .kind = .diagnostics, .expect = "" },
+    // Diagnostics and format probe a DELIBERATELY BROKEN fixture, never the
+    // real source. On a clean corpus the correct answer is nothing, which is
+    // byte-identical to "this backend has no diagnostics" — a blind spot that
+    // rewards emitting a filler row and punishes honesty. test/lspfixture has
+    // an unused local (semantic: needs a compiler front end, not a tokenizer)
+    // and a misformatted fn (needs a formatter), so both probes have real work.
+    .{ .file = "test/lspfixture/broken.zig", .needle = "unused_local", .at = 0, .kind = .diagnostics, .expect = "broken.zig" },
     // the remaining kinds, probed once each so the matrix is complete
     .{ .file = "src/lsp.zig", .needle = "lineCol(source", .at = 0, .kind = .declaration, .expect = "" },
     .{ .file = "src/lsp.zig", .needle = "out: *std.ArrayList(u8)", .at = 10, .kind = .type_definition, .expect = "" },
     .{ .file = "src/lsp.zig", .needle = "pub const Kind", .at = 11, .kind = .implementation, .expect = "" },
     .{ .file = "src/lsp.zig", .needle = "pub const Kind", .at = 11, .kind = .select_refs, .expect = "" },
-    .{ .file = "src/lsp.zig", .needle = "pub fn query", .at = 7, .kind = .format, .expect = "" },
+    .{ .file = "test/lspfixture/broken.zig", .needle = "badly_spaced", .at = 0, .kind = .format, .expect = "" },
     .{ .file = "src/lsp.zig", .needle = "pub fn query", .at = 7, .kind = .code_action, .expect = "" },
     .{ .file = "src/lsp.zig", .needle = "pub fn query", .at = 7, .kind = .rename, .expect = "" },
     .{ .file = "src/lsp.zig", .needle = "pub const Kind", .at = 11, .kind = .workspace_symbols, .expect = "" },
-    .{ .file = "src/lsp.zig", .needle = "const std", .at = 6, .kind = .workspace_diagnostics, .expect = "" },
+    .{ .file = "test/lspfixture/broken.zig", .needle = "unused_local", .at = 0, .kind = .workspace_diagnostics, .expect = "broken.zig" },
 };
 
 const Result = struct {
diff --git a/test/lspfixture/broken.zig b/test/lspfixture/broken.zig
new file mode 100644
index 00000000..22c86c9b
--- /dev/null
+++ b/test/lspfixture/broken.zig
@@ -0,0 +1,25 @@
+//! A deliberately defective file, for the lspbench diagnostics and format
+//! probes. It is NOT built and NOT imported by anything — `zig build` never
+//! sees it, and the snapshot corpus never opens it.
+//!
+//! Why it exists: on a clean, already-formatted corpus the CORRECT answer to
+//! "what is wrong with this file" is nothing, which the harness cannot tell
+//! apart from a backend that has no diagnostics at all. That blind spot
+//! rewards a backend for emitting a filler row and punishes one for being
+//! honest, so the probes point here instead, where there is something real to
+//! find:
+//!
+//!   - `unused_local` is a SEMANTIC error. A tokenizer cannot see it; it takes
+//!     a compiler front end (AstGen/ZIR, or `zig ast-check`).
+//!   - the spacing in `badly_spaced` is a FORMATTING difference, which only a
+//!     formatter reports.
+//!
+//! Keep both. Removing either silently turns a probe back into a blind one.
+pub fn hasUnusedLocal() u32 {
+    const unused_local = 41;
+    return 7;
+}
+
+pub fn badly_spaced (  a : u32 , b : u32 ) u32 {
+    return a+b;
+}