share a single rust-analyzer instance per workspace between multiple nvim instances

[playbahn]

Checked other resources

• This is not a bug report or feature request (Please use issues for those).
• This is a question about rustaceanvim, not rust-analyzer or Neovim.
• This is not a question about autocompletion.
• I have read the README.
• I have read the FAQ.
• I have searched for a solution using :h rustaceanvim.
• I have searched existing discussions.
• I have searched the existing issues/discussions of other plugins related to this question (if applicable).
• I have searched the existing rust-analyzer discussions (if applicable).
• I have added a very descriptive title to this question.

Description

I want to be able to open multiple nvim instances with same project, using the same connection to rust-analyzer. Right now, multiple RA servers are instantiated when opening multiple nvim instances.

Example Code

vim.env.LAZY_STDPATH = '.repro'
---@diagnostic disable-next-line: param-type-mismatch
load(vim.fn.system('curl -s https://raw.githubusercontent.com/folke/lazy.nvim/main/bootstrap.lua'))()

require('lazy.minit').repro {
  spec = {
    {
      'mrcjkb/rustaceanvim',
      version = '^6',
      init = function()
        -- Configure rustaceanvim here
        vim.g.rustaceanvim = {}
      end,
      lazy = false,
    },
  },
}

Neovim version (nvim -v)

NVIM v0.11.4

Operating system/version

Arch Linux

Output of :checkhealth rustaceanvim

With `minimal.lua`:

- LSP log level : WARN
- Log path: /home/playbahn/master/simd-basics/.repro/state/nvim/lsp.log
- Log size: 0 KB

vim.lsp: Active Clients
- rust-analyzer (id: 1)
  - Version: 1.92.0-nightly (1d23d06 2025-10-22)
  - Root directory: ~/master/simd-basics
  - Command: { "/usr/lib/rustup/bin/rust-analyzer", "--log-file", "/tmp/nvim.playbahn/ebLd8E/0-rust-analyzer.log" }
  - Settings: {
      ["rust-analyzer"] = {
        check = {
          command = "clippy",
          extraArgs = { "--no-deps" }
        },
        checkOnSave = true
      }
    }
  - Attached buffers: 1

vim.lsp: Enabled Configurations

vim.lsp: File Watcher
- file watching "(workspace/didChangeWatchedFiles)" disabled on all clients

vim.lsp: Position Encodings
- No buffers contain mixed position encodings

With normal config

==============================================================================
vim.lsp:                                                                    ✅

- LSP log level : WARN
- Log path: /home/playbahn/.local/state/nvim/lsp.log
- Log size: 1151 KB

vim.lsp: Active Clients ~
- rust-analyzer (id: 1)
  - Version: 1.92.0-nightly (1d23d06 2025-10-22)
  - Root directory: ~/master/simd-basics
  - Command: { "/usr/lib/rustup/bin/rust-analyzer", "--log-file", "/tmp/nvim.playbahn/VFuQAt/1-rust-analyzer.log" }
  - Settings: {
      ["rust-analyzer"] = {
        check = {
          command = "clippy",
          extraArgs = { "--no-deps" }
        },
        inlayHints = {
          bindingModeHints = {
            enable = true
          },
          closingBraceHints = {
            minLines = 0
          },
          closureCaptureHints = {
            enable = true
          },
          closureReturnTypeHints = {
            enable = "always"
          },
          expressionAdjustmentHints = {
            enable = "reborrow",
            hideOutsideUnsafe = true
          },
          lifetimeElisionHints = {
            enable = "skip_trivial",
            useParameterNames = true
          },
          maxLength = vim.NIL,
          semanticHighlighting = {
            punctuation = {
              enable = true,
              specialization = {
                enable = true
              }
            }
          },
          typing = {
            triggerChars = "=.{(><"
          }
        }
      }
    }
  - Attached buffers: 1

vim.lsp: Enabled Configurations ~
- bashls:
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      general = {
        positionEncodings = { "utf-8", "utf-16", "utf-32" }
      },
      textDocument = {
        callHierarchy = {
          dynamicRegistration = false
        },
        codeAction = {
          codeActionLiteralSupport = {
            codeActionKind = {
              valueSet = { "", "quickfix", "refactor", "refactor.extract", "refactor.inline", "refactor.rewrite", "source", "source.organizeImports" }
            }
          },
          dataSupport = true,
          dynamicRegistration = true,
          isPreferredSupport = true,
          resolveSupport = {
            properties = { "edit", "command" }
          }
        },
        codeLens = {
          dynamicRegistration = false,
          resolveSupport = {
            properties = { "command" }
          }
        },
        completion = {
          completionItem = {
            commitCharactersSupport = false,
            deprecatedSupport = true,
            documentationFormat = { "markdown", "plaintext" },
            insertReplaceSupport = true,
            insertTextModeSupport = {
              valueSet = { 1 }
            },
            labelDetailsSupport = true,
            preselectSupport = false,
            resolveSupport = {
              properties = { "documentation", "detail", "additionalTextEdits", "command", "data" }
            },
            snippetSupport = true,
            tagSupport = {
              valueSet = { 1 }
            }
          },
          completionItemKind = {
            valueSet = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 }
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          completionList = {
            itemDefaults = { "commitCharacters", "editRange", "insertTextFormat", "insertTextMode", "data" }
          },
          contextSupport = true,
          dynamicRegistration = false,
          insertTextMode = 1
        },
        declaration = {
          linkSupport = true
        },
        definition = {
          dynamicRegistration = true,
          linkSupport = true
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        diagnostic = {
          dynamicRegistration = false,
          tagSupport = {
            valueSet = { 1, 2 }
          }
        },
        documentHighlight = {
          dynamicRegistration = false
        },
        documentSymbol = {
          dynamicRegistration = false,
          hierarchicalDocumentSymbolSupport = true,
          symbolKind = {
            valueSet = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 }
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        },
        foldingRange = {
          dynamicRegistration = false,
          foldingRange = {
            collapsedText = true
          },
          foldingRangeKind = {
            valueSet = { "comment", "imports", "region" }
          },
          lineFoldingOnly = true
        },
        formatting = {
          dynamicRegistration = true
        },
        hover = {
          contentFormat = { "markdown", "plaintext" },
          dynamicRegistration = true
        },
        implementation = {
          linkSupport = true
        },
        inlayHint = {
          dynamicRegistration = true,
          resolveSupport = {
            properties = { "textEdits", "tooltip", "location", "command" }
          }
        },
        publishDiagnostics = {
          dataSupport = true,
          relatedInformation = true,
          tagSupport = {
            valueSet = { 1, 2 }
          }
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        rangeFormatting = {
          dynamicRegistration = true,
          rangesSupport = true
        },
        references = {
          dynamicRegistration = false
        },
        rename = {
          dynamicRegistration = true,
          prepareSupport = true
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        semanticTokens = {
          augmentsSyntaxTokens = true,
          dynamicRegistration = false,
          formats = { "relative" },
          multilineTokenSupport = false,
          overlappingTokenSupport = true,
          requests = {
            full = {
              delta = true
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            range = false
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          serverCancelSupport = false,
          tokenModifiers = { "declaration", "definition", "readonly", "static", "deprecated", "abstract", "async", "modification", "documentation", "defaultLibrary" },
          tokenTypes = { "namespace", "type", "class", "enum", "interface", "struct", "typeParameter", "parameter", "variable", "property", "enumMember", "event", "function", "method", "macro", "keyword", "modifier", "comment", "string", "number", "regexp", "operator", "decorator" }
        },
        signatureHelp = {
          dynamicRegistration = false,
          signatureInformation = {
            activeParameterSupport = true,
            documentationFormat = { "markdown", "plaintext" },
            parameterInformation = {
              labelOffsetSupport = true
            }
          }
        },
        synchronization = {
          didSave = true,
          dynamicRegistration = false,
          willSave = true,
          willSaveWaitUntil = true
        },
        typeDefinition = {
          linkSupport = true
        }
      },
      window = {
        showDocument = {
          support = true
        },
        showMessage = {
          messageActionItem = {
            additionalPropertiesSupport = true
          }
        },
        workDoneProgress = true
      },
      workspace = {
        applyEdit = true,
        configuration = true,
        didChangeConfiguration = {
          dynamicRegistration = false
        },
        didChangeWatchedFiles = {
          dynamicRegistration = false,
          relativePatternSupport = true
        },
        inlayHint = {
          refreshSupport = true
        },
        semanticTokens = {
          refreshSupport = true
        },
        symbol = {
          dynamicRegistration = false,
          symbolKind = {
            valueSet = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 }
          }
        },
        workspaceEdit = {
          resourceOperations = { "rename", "create", "delete" }
        },
        workspaceFolders = true
      }
    }
  - cmd: { "bash-language-server", "start" }
  - filetypes: bash, sh
  - root_markers: { ".git" }
  - settings: {
      bashIde = {
        globPattern = "*@(.sh|.inc|.bash|.command)"
      }
    }

- clangd:
  - capabilities: {
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      },
      offsetEncoding = { "utf-8", "utf-16" },
      textDocument = {
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          dynamicRegistration = false
        },
        codeAction = {
          codeActionLiteralSupport = {
            codeActionKind = {
              valueSet = { "", "quickfix", "refactor", "refactor.extract", "refactor.inline", "refactor.rewrite", "source", "source.organizeImports" }
            }
          },
          dataSupport = true,
          dynamicRegistration = true,
          isPreferredSupport = true,
          resolveSupport = {
            properties = { "edit", "command" }
          }
        },
        codeLens = {
          dynamicRegistration = false,
          resolveSupport = {
            properties = { "command" }
          }
        },
        completion = {
          completionItem = {
            commitCharactersSupport = false,
            deprecatedSupport = true,
            documentationFormat = { "markdown", "plaintext" },
            insertReplaceSupport = true,
            insertTextModeSupport = {
              valueSet = { 1 }
            },
            labelDetailsSupport = true,
            preselectSupport = false,
            resolveSupport = {
              properties = { "documentation", "detail", "additionalTextEdits", "command", "data" }
            },
            snippetSupport = true,
            tagSupport = {
              valueSet = { 1 }
            }
          },
          completionItemKind = {
            valueSet = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 }
          },
          completionList = {
            itemDefaults = { "commitCharacters", "editRange", "insertTextFormat", "insertTextMode", "data" }
          },
          contextSupport = true,
          dynamicRegistration = false,
          editsNearCursor = true,
          insertTextMode = 1
        },
        declaration = {
          linkSupport = true
        },
        definition = {
          dynamicRegistration = true,
          linkSupport = true
        },
        diagnostic = {
          dynamicRegistration = false,
          tagSupport = {
            valueSet = { 1, 2 }
          }
        },
        documentHighlight = {
          dynamicRegistration = false
        },
        documentSymbol = {
          dynamicRegistration = false,
          hierarchicalDocumentSymbolSupport = true,
          symbolKind = {
            valueSet = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 }
          }
        },
        foldingRange = {
          dynamicRegistration = false,
          foldingRange = {
            collapsedText = true
          },
          foldingRangeKind = {
            valueSet = { "comment", "imports", "region" }
          },
          lineFoldingOnly = true
        },
        formatting = {
          dynamicRegistration = true
        },
        hover = {
          contentFormat = { "markdown", "plaintext" },
          dynamicRegistration = true
        },
        implementation = {
          linkSupport = true
        },
        inlayHint = {
          dynamicRegistration = true,
          resolveSupport = {
            properties = { "textEdits", "tooltip", "location", "command" }
          }
        },
        publishDiagnostics = {
          dataSupport = true,
          relatedInformation = true,
          tagSupport = {
            valueSet = { 1, 2 }
          }
        },
        rangeFormatting = {
          dynamicRegistration = true,
          rangesSupport = true
        },
        references = {
          dynamicRegistration = false
        },
        rename = {
          dynamicRegistration = true,
          prepareSupport = true
        },
        semanticTokens = {
          augmentsSyntaxTokens = true,
          dynamicRegistration = false,
          formats = { "relative" },
          multilineTokenSupport = false,
          overlappingTokenSupport = true,
          requests = {
            full = {
              delta = true
            },
            range = false
          },
          serverCancelSupport = false,
          tokenModifiers = { "declaration", "definition", "readonly", "static", "deprecated", "abstract", "async", "modification", "documentation", "defaultLibrary" },
          tokenTypes = { "namespace", "type", "class", "enum", "interface", "struct", "typeParameter", "parameter", "variable", "property", "enumMember", "event", "function", "method", "macro", "keyword", "modifier", "comment", "string", "number", "regexp", "operator", "decorator" }
        },
        signatureHelp = {
          dynamicRegistration = false,
          signatureInformation = {
            activeParameterSupport = true,
            documentationFormat = { "markdown", "plaintext" },
            parameterInformation = {
              labelOffsetSupport = true
            }
          }
        },
        synchronization = {
          didSave = true,
          dynamicRegistration = false,
          willSave = true,
          willSaveWaitUntil = true
        },
        typeDefinition = {
          linkSupport = true
        }
      },
      window = {
        showDocument = {
          support = true
        },
        showMessage = {
          messageActionItem = {
            additionalPropertiesSupport = true
          }
        },
        workDoneProgress = true
      },
      workspace = {
        applyEdit = true,
        configuration = true,
        didChangeConfiguration = {
          dynamicRegistration = false
        },
        didChangeWatchedFiles = {
          dynamicRegistration = false,
          relativePatternSupport = true
        },
        inlayHint = {
          refreshSupport = true
        },
        semanticTokens = {
          refreshSupport = true
        },
        symbol = {
          dynamicRegistration = false,
          symbolKind = {
            valueSet = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 }
          }
        },
        workspaceEdit = {
          resourceOperations = { "rename", "create", "delete" }
        },
        workspaceFolders = true
      }
    }
  - cmd: { "clangd" }
  - filetypes: c, cpp, objc, objcpp, cuda
  - on_attach: <function @/home/playbahn/.local/share/nvim/lazy/nvim-lspconfig/lsp/clangd.lua:93>
  - on_init: <function @/home/playbahn/.local/share/nvim/lazy/nvim-lspconfig/lsp/clangd.lua:86>
  - root_markers: { ".clangd", ".clang-tidy", ".clang-format", "compile_commands.json", "compile_flags.txt", "configure.ac", ".git" }

- lua_ls:
  - capabilities: {
      general = {
        positionEncodings = { "utf-8", "utf-16", "utf-32" }
      },
      textDocument = {
        callHierarchy = {
          dynamicRegistration = false
        },
        codeAction = {
          codeActionLiteralSupport = {
            codeActionKind = {
              valueSet = { "", "quickfix", "refactor", "refactor.extract", "refactor.inline", "refactor.rewrite", "source", "source.organizeImports" }
            }
          },
          dataSupport = true,
          dynamicRegistration = true,
          isPreferredSupport = true,
          resolveSupport = {
            properties = { "edit", "command" }
          }
        },
        codeLens = {
          dynamicRegistration = false,
          resolveSupport = {
            properties = { "command" }
          }
        },
        completion = {
          completionItem = {
            commitCharactersSupport = false,
            deprecatedSupport = true,
            documentationFormat = { "markdown", "plaintext" },
            insertReplaceSupport = true,
            insertTextModeSupport = {
              valueSet = { 1 }
            },
            labelDetailsSupport = true,
            preselectSupport = false,
            resolveSupport = {
              properties = { "documentation", "detail", "additionalTextEdits", "command", "data" }
            },
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            tagSupport = {
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            }
          },
          completionItemKind = {
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          },
          completionList = {
            itemDefaults = { "commitCharacters", "editRange", "insertTextFormat", "insertTextMode", "data" }
          },
          contextSupport = true,
          dynamicRegistration = false,
          insertTextMode = 1
        },
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          linkSupport = true
        },
        definition = {
          dynamicRegistration = true,
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        diagnostic = {
          dynamicRegistration = false,
          tagSupport = {
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          }
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          hierarchicalDocumentSymbolSupport = true,
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          },
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          },
          lineFoldingOnly = true
        },
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          dynamicRegistration = true
        },
        hover = {
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        },
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        },
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        },
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          resourceOperations = { "rename", "create", "delete" }
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  - cmd: { "lua-language-server" }
  - filetypes: lua
  - root_markers: { ".luarc.json", ".luarc.jsonc", ".luacheckrc", ".stylua.toml", "stylua.toml", "selene.toml", "selene.yml", ".git" }
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          },
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        },
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        },
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            properties = { "textEdits", "tooltip", "location", "command" }
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        },
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          tagSupport = {
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        },
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          dynamicRegistration = false,
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          multilineTokenSupport = false,
          overlappingTokenSupport = true,
          requests = {
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          },
          serverCancelSupport = false,
          tokenModifiers = { "declaration", "definition", "readonly", "static", "deprecated", "abstract", "async", "modification", "documentation", "defaultLibrary" },
          tokenTypes = { "namespace", "type", "class", "enum", "interface", "struct", "typeParameter", "parameter", "variable", "property", "enumMember", "event", "function", "method", "macro", "keyword", "modifier", "comment", "string", "number", "regexp", "operator", "decorator" }
        },
        signatureHelp = {
          dynamicRegistration = false,
          signatureInformation = {
            activeParameterSupport = true,
            documentationFormat = { "markdown", "plaintext" },
            parameterInformation = {
              labelOffsetSupport = true
            }
          }
        },
        synchronization = {
          didSave = true,
          dynamicRegistration = false,
          willSave = true,
          willSaveWaitUntil = true
        },
        typeDefinition = {
          linkSupport = true
        }
      },
      window = {
        showDocument = {
          support = true
        },
        showMessage = {
          messageActionItem = {
            additionalPropertiesSupport = true
          }
        },
        workDoneProgress = true
      },
      workspace = {
        applyEdit = true,
        configuration = true,
        didChangeConfiguration = {
          dynamicRegistration = false
        },
        didChangeWatchedFiles = {
          dynamicRegistration = false,
          relativePatternSupport = true
        },
        inlayHint = {
          refreshSupport = true
        },
        semanticTokens = {
          refreshSupport = true
        },
        symbol = {
          dynamicRegistration = false,
          symbolKind = {
            valueSet = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 }
          }
        },
        workspaceEdit = {
          resourceOperations = { "rename", "create", "delete" }
        },
        workspaceFolders = true
      }
    }
  - cmd: { "/usr/bin/zls" }
  - filetypes: zig, zir
  - root_markers: { "zls.json", "build.zig", ".git" }
  - workspace_required: false


vim.lsp: File Watcher ~
- file watching "(workspace/didChangeWatchedFiles)" disabled on all clients

vim.lsp: Position Encodings ~
- No buffers contain mixed position encodings

[playbahn]

I tried using using the root_dir function from the nvim-lspconfig plugin word-for-word, but:

Error detected while processing BufReadPost Autocommands for "*":
Error executing lua callback: /usr/share/nvim/runtime/filetype.lua:36: BufReadPost Autocommands for "*"..FileType Autocommands for "*"..function <SNR>1_LoadFTPlugin[20]..
script /home/playbahn/.local/share/nvim/lazy/rustaceanvim/ftplugin/rust.lua: Vim(runtime):E5113: Error while calling lua chunk: /home/playbahn/.config/nvim/lua/plugins/ru
staceanvim.lua:56: Invalid 'buffer': Expected Lua number
stack traceback:
        [C]: in function 'nvim_buf_get_name'
        /home/playbahn/.config/nvim/lua/plugins/rustaceanvim.lua:56: in function 'root_dir'
        .../share/nvim/lazy/rustaceanvim/lua/rustaceanvim/cargo.lua:127: in function 'get_config_root_dir'
        ...are/nvim/lazy/rustaceanvim/lua/rustaceanvim/lsp/init.lua:169: in function 'start'
        ...hn/.local/share/nvim/lazy/rustaceanvim/ftplugin/rust.lua:64: in main chunk
        [C]: in function 'nvim_cmd'
        /usr/share/nvim/runtime/filetype.lua:36: in function </usr/share/nvim/runtime/filetype.lua:35>
        [C]: in function 'pcall'
        vim/shared.lua: in function <vim/shared.lua:0>
        [C]: in function '_with'
        /usr/share/nvim/runtime/filetype.lua:35: in function </usr/share/nvim/runtime/filetype.lua:10>
stack traceback:
        [C]: in function '_with'
        /usr/share/nvim/runtime/filetype.lua:35: in function </usr/share/nvim/runtime/filetype.lua:10>

I then tried looking at the rustaceanvim code, and though it does have mentions of root_dir I don't understand Lua enough to form a strong understanding. I then i could just just pass cwd() to root_dir and true to reuse_client but that seems very makeshift-y.

[playbahn]

So turns out root_dir for rustaceanvim expects filename: string. Right now I have:

local function root_dir(fname, on_dir)
    local reused_dir = is_library(fname)
    if reused_dir then
        on_dir(reused_dir)
        return
    end

    local cargo_crate_dir = vim.fs.root(fname, { 'Cargo.toml' })
    local cargo_workspace_root

    if cargo_crate_dir == nil then
        on_dir(
            vim.fs.root(fname, { 'rust-project.json' })
                or vim.fs.dirname(vim.fs.find('.git', { path = fname, upward = true })[1])
        )
        return
    end

    local cmd = {
        'cargo',
        'metadata',
        '--no-deps',
        '--format-version',
        '1',
        '--manifest-path',
        cargo_crate_dir .. '/Cargo.toml',
    }

    vim.system(cmd, { text = true }, function(output)
        if output.code == 0 then
            if output.stdout then
                local result = vim.json.decode(output.stdout)
                if result['workspace_root'] then
                    cargo_workspace_root = vim.fs.normalize(result['workspace_root'])
                end
            end

            on_dir(cargo_workspace_root or cargo_crate_dir)
        else
            vim.schedule(function()
                vim.notify(
                    ('[rust_analyzer] cmd failed with code %d: %s\n%s'):format(
                        output.code,
                        cmd,
                        output.stderr
                    )
                )
            end)
        end
    end)
end

But this causes:

Error executing callback:
/usr/share/nvim/runtime/lua/vim/_system.lua:90: E5560: vim.wait must not be called in a fast event context
stack traceback:
        [C]: in function 'wait'
        /usr/share/nvim/runtime/lua/vim/_system.lua:90: in function 'wait'
        .../share/nvim/lazy/rustaceanvim/lua/rustaceanvim/cargo.lua:53: in function 'get_cargo_metadata'
        .../share/nvim/lazy/rustaceanvim/lua/rustaceanvim/cargo.lua:88: in function 'on_dir'
        /home/playbahn/.config/nvim/lua/plugins/rustaceanvim.lua:94: in function 'on_exit'
        /usr/share/nvim/runtime/lua/vim/_system.lua:318: in function </usr/share/nvim/runtime/lua/vim/_system.lua:288>

[playbahn]

Looks like the problem is in here:

vim.system(cmd, { text = true }, function(output)
    if output.code == 0 then
        if output.stdout then
            local result = vim.json.decode(output.stdout)
            if result['workspace_root'] then
                cargo_workspace_root = vim.fs.normalize(result['workspace_root'])
            end
        end
                                                                                  
        on_dir(cargo_workspace_root or cargo_crate_dir)
    else
        vim.schedule(function()
            vim.notify(
                ('[rust_analyzer] cmd failed with code %d: %s\n%s'):format(
                    output.code,
                    cmd,
                    output.stderr
                )
            )
        end)
    end
end)

With the exact line being:

on_dir(cargo_workspace_root or cargo_crate_dir)

[playbahn]

Adding a :wait() at the end of vim.system() call also does not resolve the issue.

[playbahn]

Nvim's reuse_client docs say:

Predicate which decides if a client should be re-used. Used on all running clients. The default implementation re-uses a client if name and root_dir matches.

Then why is it not reusing?

[playbahn]

Playing around:

diff --git a/lua/rustaceanvim/cargo.lua b/lua/rustaceanvim/cargo.lua
index 448a20a..68a17d1 100644
--- a/lua/rustaceanvim/cargo.lua
+++ b/lua/rustaceanvim/cargo.lua
@@ -94,6 +94,7 @@ end
 ---@param file_name string
 ---@return string | nil root_dir The root directory of the active client for file_name (if there is one)
 local function get_mb_active_client_root(file_name)
+  vim.print("inside get_mb_active_client_root")
   ---@diagnostic disable-next-line: missing-parameter
   local cargo_home = vim.uv.os_getenv('CARGO_HOME') or vim.fs.joinpath(vim.env.HOME, '.cargo')
   local registry = vim.fs.joinpath(cargo_home, 'registry', 'src')
@@ -105,8 +106,13 @@ local function get_mb_active_client_root(file_name)
 
   for _, item in ipairs { toolchains, registry, checkouts } do
     item = os.normalize_path_on_windows(item)
+    vim.print("file_name: " .. file_name)
+    vim.print("item: " .. item)
+    vim.print("file_name:sub(1, #item): " .. file_name:sub(1, #item))
+    vim.print("file_name:sub(1, #item) == item: " .. tostring(file_name:sub(1, #item) == item))
     if file_name:sub(1, #item) == item then
       local clients = rust_analyzer.get_active_rustaceanvim_clients()
+      vim.print("clients: " .. clients)
       return clients and #clients > 0 and clients[#clients].config.root_dir or nil
     end
   end

resulting in:

inside get_mb_active_client_root
file_name: /home/playbahn/master/simd-basics/src/lib.rs
item: /home/playbahn/.rustup/toolchains
file_name:sub(1, #item): /home/playbahn/master/simd-basics
file_name:sub(1, #item) == item: false
file_name: /home/playbahn/master/simd-basics/src/lib.rs
item: /home/playbahn/.cargo/registry/src
file_name:sub(1, #item): /home/playbahn/master/simd-basics/
file_name:sub(1, #item) == item: false
file_name: /home/playbahn/master/simd-basics/src/lib.rs
item: /home/playbahn/.cargo/git/checkouts
file_name:sub(1, #item): /home/playbahn/master/simd-basics/s
file_name:sub(1, #item) == item: false

all the items here are directories. Shouldn't the approach be:

1. Get the directory containing Cargo.toml or rust-project.json, either in the same directory as file_name or climbing up.
2. Get client having the same root_dir as the one found just now, and reuse that client?

EDIT: What if its a Cargo workspace with a lot of crates?

[mrcjkb]

I want to be able to open multiple nvim instances with same project, using the same connection to rust-analyzer. Right now, multiple RA servers are instantiated when opening multiple nvim instances.

This is not supported by rustaceanvim or rust-analyzer.
You can use ra-mulitplex for that.

I've just realised that ra-multiplex has been archived and moved to lspmux.
I haven't looked into whether lspmux has any breaking changes of if it's still compatible with rustaceanvim yet (see #870).

[playbahn]

But again,

Nvim's reuse_client docs say:

Predicate which decides if a client should be re-used. Used on all running clients. The default implementation re-uses a client if name and root_dir matches.

Then why is it not reusing?

EDIT: What I understand from the docs is that Nvim already/is supposed to reuse clients if the conditions are fulfilled, and the user (or in this case rustacenavim) has to provide some glue for the whole thing.

[mrcjkb]

reuse_client runs (per buffer) on existing clients of the current Nvim instance.
It doesn't know about clients running in other Nvim instances.