feat: generate rust-project.json for LSP

Add a CMake module that writes rust-project.json on every configure so
rust-analyzer can resolve the no-Cargo crate layout, sysroot crates,
and proc-macro attributes without Cargo.

Co-Authored-By: qwen (lmstudio/qwen3.8-27b@q3_k_xl): LSP setup

cmake/RustLSP.cmake

@@ -0,0 +1,36 @@
+# Generate rust-project.json for rust-analyzer (LSP) support.
+#
+# This project has no Cargo manifest, so rust-analyzer cannot discover the
+# crate layout on its own. The generated file describes the three crates and
+# points at the toolchain sysroot so core/alloc resolve. It is regenerated on
+# every configure; the output is git-ignored.
+
+if(NOT RUSTC)
+    find_program(RUSTC rustc REQUIRED)
+endif()
+
+execute_process(
+    COMMAND "${RUSTC}" --print sysroot
+    OUTPUT_VARIABLE RUST_SYSROOT_RAW
+    RESULT_VARIABLE RUST_SYSROOT_RESULT
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+)
+
+if(NOT RUST_SYSROOT_RESULT EQUAL 0 OR NOT RUST_SYSROOT_RAW)
+    message(FATAL_ERROR "Failed to query the rustc sysroot")
+endif()
+
+# Use forward slashes so the JSON needs no backslash escaping.
+string(REPLACE "\\" "/" RUST_SYSROOT "${RUST_SYSROOT_RAW}")
+set(RUST_SYSROOT_SRC "${RUST_SYSROOT}/lib/rustlib/src/rust/library")
+
+if(NOT IS_DIRECTORY "${RUST_SYSROOT_SRC}")
+    message(WARNING "rust-src not found at ${RUST_SYSROOT_SRC}. "
+        "Run `rustup component add rust-src` for full LSP support.")
+endif()
+
+configure_file(
+    "${CMAKE_CURRENT_LIST_DIR}/rust-project.json.in"
+    "${CMAKE_CURRENT_SOURCE_DIR}/rust-project.json"
+    @ONLY
+)

cmake/rust-project.json.in

@@ -0,0 +1,28 @@
+{
+    "sysroot": "@RUST_SYSROOT@",
+    "sysroot_src": "@RUST_SYSROOT_SRC@",
+    "crates": [
+        {
+            "display_name": "rslibc",
+            "root_module": "src/lib.rs",
+            "edition": "2021",
+            "deps": []
+        },
+        {
+            "display_name": "rslibc_macros",
+            "root_module": "macros/src/lib.rs",
+            "edition": "2021",
+            "is_proc_macro": true,
+            "deps": []
+        },
+        {
+            "display_name": "rslibc_example",
+            "root_module": "runtime/main.rs",
+            "edition": "2021",
+            "deps": [
+                { "crate": 0, "name": "rslibc" },
+                { "crate": 1, "name": "rslibc_macros" }
+            ]
+        }
+    ]
+}