etc: Add rust-analyzer configs for eglot & helix

LSP configuration in editors like Emacs (eglot) and helix does not
use the same JSON format as vscode and vim do. It is not obvious
how to set up LSP for rustc in those editors and the dev guide currently
does not cover them. Adding sample configuration files for those editors
alongside the currently existing JSON one would be helpful.

src/etc/rust_analyzer_eglot.el

@@ -0,0 +1,29 @@
+((rustic-mode
+  .((eglot-workspace-configuration
+     . (:rust-analyzer
+        ( :check ( :invocationLocation "root"
+                   :invocationStrategy "once"
+                   :overrideCommand ["python3"
+                                     "x.py"
+                                     "check"
+                                     "--json-output"])
+          :linkedProjects ["Cargo.toml"
+                           "src/tools/x/Cargo.toml"
+                           "src/bootstrap/Cargo.toml"
+                           "src/tools/rust-analyzer/Cargo.toml"
+                           "compiler/rustc_codegen_cranelift/Cargo.toml"
+                           "compiler/rustc_codegen_gcc/Cargo.toml"]
+          :rustfmt ( :overrideCommand ["build/host/rustfmt/bin/rustfmt"
+                                       "--edition=2021"])
+          :procMacro ( :server "build/host/stage0/libexec/rust-analyzer-proc-macro-srv"
+                       :enable t)
+          :cargo ( :buildScripts ( :enable t
+                                   :invocationLocation "root"
+                                   :invocationStrategy "once"
+                                   :overrideCommand ["python3"
+                                                     "x.py"
+                                                     "check"
+                                                     "--json-output"])
+                   :sysrootSrc "./library"
+                   :extraEnv (:RUSTC_BOOTSTRAP "1"))
+          :rustc ( :source "./Cargo.toml" )))))))

src/etc/rust_analyzer_helix.toml

@@ -0,0 +1,49 @@
+[language-server.rust-analyzer.config]
+linkedProjects = [
+    "Cargo.toml",
+    "src/tools/x/Cargo.toml",
+    "src/bootstrap/Cargo.toml",
+    "src/tools/rust-analyzer/Cargo.toml",
+    "compiler/rustc_codegen_cranelift/Cargo.toml",
+    "compiler/rustc_codegen_gcc/Cargo.toml"
+]
+
+[language-server.rust-analyzer.config.check]
+invocationLocation = "root"
+invocationStrategy = "once"
+overrideCommand = [
+    "python3",
+    "x.py",
+    "check",
+    "--json-output",
+]
+
+[language-server.rust-analyzer.config.rustfmt]
+overrideCommand = [
+    "build-rust-analyzer/host/rustfmt/bin/rustfmt",
+    "--edition=2021"
+]
+
+[language-server.rust-analyzer.config.procMacro]
+server = "build-rust-analyzer/host/stage0/libexec/rust-analyzer-proc-macro-srv"
+enable = true
+
+[language-server.rust-analyzer.config.rustc]
+source = "./Cargo.toml"
+
+[language-server.rust-analyzer.config.cargo]
+sysrootSrc = "./library"
+
+[language-server.rust-analyzer.config.cargo.extraEnv]
+RUSTC_BOOTSTRAP = "1"
+
+[language-server.rust-analyzer.config.cargo.buildScripts]
+enable = true
+invocationLocation = "root"
+invocationStrategy = "once"
+overrideCommand = [
+    "python3",
+    "x.py",
+    "check",
+    "--json-output",
+]