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

.gitignore

@@ -2,3 +2,4 @@
 /target
 .env
 /build
+/rust-project.json

AGENTS.md

@@ -19,6 +19,7 @@ The project contains:
   entry helpers
 - `macros/src/lib.rs`: host-side `#[rslibc_macros::main]` attribute macro
 - `runtime/main.rs`: minimal example entry point
+- `cmake/RustLSP.cmake`: generates `rust-project.json` for rust-analyzer
 - `CMakeLists.txt`: direct `rustc` library, macro, and executable commands
 
 ## No-Std Design
@@ -63,7 +64,13 @@ The project contains:
 ## Build System
 
 CMake is configured with `LANGUAGES NONE` and invokes `rustc` through three
-custom commands:
+custom commands. CMake 4.4's experimental native Rust support
+(`CMAKE_EXPERIMENTAL_RUST`) was evaluated and rejected: it compiles each
+source file as its own crate, so multi-module crates break; it links Rust
+libraries into executables as native `.lib` inputs instead of `--extern`;
+and it has no `proc-macro` crate type. Keep the direct `rustc` commands.
+
+The three custom commands are:
 
 1. Compile `macros/src/lib.rs` as a host procedural macro.
 2. Compile `src/lib.rs` and its modules to `librslibc.rlib`.
@@ -106,6 +113,19 @@ Run it on Windows:
 - `RelWithDebInfo`: `-C opt-level=3 -C debuginfo=2`
 - Other or unset configurations: `-C opt-level=0 -C debuginfo=2`
 
+## LSP
+
+rust-analyzer works without Cargo through a generated manifest:
+
+- `cmake/RustLSP.cmake` runs on every configure and writes
+  `rust-project.json` (git-ignored) describing the three crates and the
+  toolchain sysroot, so `core`, `alloc`, and proc-macro attributes resolve.
+- Requires the rustup components:
+  `rustup component add rust-analyzer rust-src`.
+- Open the repository root in the editor so rust-analyzer picks up
+  `rust-project.json`. Re-run CMake configure after changing the crate
+  layout or switching toolchains.
+
 ## Verification
 
 After changing the source or build configuration, run a clean release build

CMakeLists.txt

@@ -4,6 +4,9 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 find_program(RUSTC rustc REQUIRED)
 
+# Generate rust-project.json for rust-analyzer (LSP) support.
+include(cmake/RustLSP.cmake)
+
 set(RUST_MACRO_SOURCE "${CMAKE_CURRENT_SOURCE_DIR}/macros/src/lib.rs")
 set(RUST_MACRO "${CMAKE_CURRENT_BINARY_DIR}/rslibc_macros${CMAKE_SHARED_LIBRARY_SUFFIX}")
 set(RUST_LIBRARY_SOURCE "${CMAKE_CURRENT_SOURCE_DIR}/src/lib.rs")

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" }
+            ]
+        }
+    ]
+}