feat: Standalone file support

Closes #25

README.md

@@ -118,8 +118,16 @@ RustJoinLines
 RustHoverActions
 RustMoveItemDown
 RustMoveItemUp
+RustStartStandaloneServerForBuffer 
 ```
 
+#### Standalone File Support
+rust-tools supports rust analyzer for standalone files (not in a cargo project).
+The language server is automatically started when you start a rust file which is
+not in a cargo file (nvim abc.rs). If you want to attach some other buffer to
+the standalone client (after opening nvim and switching to a new rust file),
+then use the ```RustStartStandaloneServerForBuffer``` command.
+
 #### Inlay Hints
 ![inlay hints](https://github.com/simrat39/rust-tools-demos/raw/master/inlay_hints.png)
 ```lua

ftplugin/rust.vim

@@ -0,0 +1 @@
+command RustStartStandaloneServerForBuffer :lua require('rust-tools.standalone').start_standalone_client()

lua/rust-tools.lua

@@ -1,6 +1,8 @@
 local vim = vim
 local nvim_lsp = require 'lspconfig'
 local config = require 'rust-tools.config'
+local utils = require('rust-tools.utils.utils')
+local lspconfig_utils = require('lspconfig.util')
 
 local M = {}
 
@@ -73,6 +75,25 @@ end
 
 local function setup_lsp() nvim_lsp.rust_analyzer.setup(config.options.server) end
 
+local function get_root_dir()
+    local fname = vim.api.nvim_buf_get_name(0)
+    local cargo_crate_dir = lspconfig_utils.root_pattern 'Cargo.toml'(fname)
+    local cmd = 'cargo metadata --no-deps --format-version 1'
+    if cargo_crate_dir ~= nil then
+        cmd = cmd .. ' --manifest-path ' ..
+                  lspconfig_utils.path.join(cargo_crate_dir, 'Cargo.toml')
+    end
+    local cargo_metadata = vim.fn.system(cmd)
+    local cargo_workspace_dir = nil
+    if vim.v.shell_error == 0 then
+        cargo_workspace_dir =
+            vim.fn.json_decode(cargo_metadata)['workspace_root']
+    end
+    return cargo_workspace_dir or cargo_crate_dir or
+               lspconfig_utils.root_pattern 'rust-project.json'(fname) or
+               lspconfig_utils.find_git_ancestor(fname)
+end
+
 function M.setup(opts)
     config.setup(opts)
 
@@ -88,6 +109,10 @@ function M.setup(opts)
     if config.options.tools.autoSetHints then
         require'rust-tools.inlay_hints'.setup_autocmd()
     end
+
+    if utils.is_bufnr_rust(0) and (get_root_dir() == nil) then
+        require('rust-tools.standalone').start_standalone_client(config.options.server.handlers)
+    end
 end
 
 return M

lua/rust-tools/standalone.lua

@@ -0,0 +1,39 @@
+local ra_config = require 'rust-tools.config'
+
+local M = {}
+
+function M.start_standalone_client(handlers)
+    local config = {
+        root_dir = require('lspconfig.util').path.dirname(
+            vim.api.nvim_buf_get_name(0)),
+        capabilities = ra_config.options.server.capabilities,
+        cmd = {'rust-analyzer'},
+        init_options = {detachedFiles = {vim.api.nvim_buf_get_name(0)}},
+        on_init = function(client)
+            vim.lsp.buf_attach_client(0, client.id)
+            vim.cmd "command! RustSetInlayHints :lua require('rust-tools.inlay_hints').set_inlay_hints()"
+            vim.cmd "command! RustDisableInlayHints :lua require('rust-tools.inlay_hints').disable_inlay_hints()"
+            vim.cmd "command! RustToggleInlayHints :lua require('rust-tools.inlay_hints').toggle_inlay_hints()"
+            vim.cmd "command! RustExpandMacro :lua require('rust-tools.expand_macro').expand_macro()"
+            vim.cmd "command! RustJoinLines :lua require('rust-tools.join_lines').join_lines()"
+            vim.cmd "command! RustHoverActions :lua require('rust-tools.hover_actions').hover_actions()"
+            vim.cmd "command! RustMoveItemDown :lua require('rust-tools.move_item').move_item()"
+            vim.cmd "command! RustMoveItemUp :lua require('rust-tools.move_item').move_item(true)"
+        end,
+        on_exit = function()
+            vim.cmd "delcommand RustSetInlayHints"
+            vim.cmd "delcommand RustDisableInlayHints"
+            vim.cmd "delcommand RustToggleInlayHints"
+            vim.cmd "delcommand RustExpandMacro"
+            vim.cmd "delcommand RustJoinLines"
+            vim.cmd "delcommand RustHoverActions"
+            vim.cmd "delcommand RustMoveItemDown"
+            vim.cmd "delcommand RustMoveItemUp"
+        end,
+        handlers = handlers
+    }
+
+    vim.lsp.start_client(config)
+end
+
+return M

lua/rust-tools/utils/utils.lua

@@ -29,4 +29,8 @@ function M.snippet_text_edits_to_text_edits(spe)
     end
 end
 
+function M.is_bufnr_rust(bufnr)
+    return vim.api.nvim_buf_get_option(bufnr, 'ft') == 'rust'
+end
+
 return M