How to enable inlay hint in Eglot + Clangd

Question

Using Emacs 30, eglot is built in emacs and now it supports inlay hints capability of LSPs. Now I want to use clangd with eglot with inlay hints enabled, but it seems that it does not work out of the box and requires some configurations. These are what I have tried without luck:

(add-hook 'eglot-managed-mode-hook #'eglot-inlay-hints-mode)

Which does nothing special

Creating a global clangd config file in ~/.config/clangd/config.yaml with these configurations:

Hover:
  ShowAKA: Yes
Diagnostics:
  UnusedIncludes: Strict
InlayHints:
  Designators: Yes
  Enabled: Yes
  ParameterNames: Yes
  DeducedTypes: Yes

It also does nothing. Appending this to .emacs to forcefully start clangd with config file enabled does nothing:

(with-eval-after-load 'eglot
  (add-to-list 'eglot-server-programs
           '(c-mode . ("clangd" "--enable-config"))
               `(c++-mode . ("clangd" "--enable-config"))))

Checking eglot output buffer, I've observed that Inlayhints capablity is sent as json-false. Here's the capabilities part:

            :capabilities
            (:workspace
            (:applyEdit t :executeCommand
                (:dynamicRegistration :json-false)
                :workspaceEdit
                (:documentChanges t)
                :didChangeWatchedFiles
                (:dynamicRegistration t)
                :symbol
                (:dynamicRegistration :json-false)
                :configuration t :workspaceFolders t)
            :textDocument
            (:synchronization
            (:dynamicRegistration :json-false :willSave t :willSaveWaitUntil t :didSave t)
            :completion
            (:dynamicRegistration :json-false :completionItem
                        (:snippetSupport t :deprecatedSupport t :resolveSupport
                                (:properties
                                ["documentation" "details" "additionalTextEdits"])
                                :tagSupport
                                (:valueSet
                                [1]))
                        :contextSupport t)
            :hover
            (:dynamicRegistration :json-false :contentFormat
                        ["plaintext"])
            :signatureHelp
            (:dynamicRegistration :json-false :signatureInformation
                        (:parameterInformation
                        (:labelOffsetSupport t)
                        :activeParameterSupport t))
            :references
            (:dynamicRegistration :json-false)
            :definition
            (:dynamicRegistration :json-false :linkSupport t)
            :declaration
            (:dynamicRegistration :json-false :linkSupport t)
            :implementation
            (:dynamicRegistration :json-false :linkSupport t)
            :typeDefinition
            (:dynamicRegistration :json-false :linkSupport t)
            :documentSymbol
            (:dynamicRegistration :json-false :hierarchicalDocumentSymbolSupport t :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]))
            :documentHighlight
            (:dynamicRegistration :json-false)
            :codeAction
            (:dynamicRegistration :json-false :codeActionLiteralSupport
                        (:codeActionKind
                        (:valueSet
                        ["quickfix" "refactor" "refactor.extract" "refactor.inline" "refactor.rewrite" "source" "source.organizeImports"]))
                        :isPreferredSupport t)
            :formatting
            (:dynamicRegistration :json-false)
            :rangeFormatting
            (:dynamicRegistration :json-false)
            :rename
            (:dynamicRegistration :json-false)
            :inlayHint
            (:dynamicRegistration :json-false)
            :publishDiagnostics
            (:relatedInformation :json-false :codeDescriptionSupport :json-false :tagSupport
                        (:valueSet
                        [1 2])))

How can I make it work?

Answer 1

Eglot requires clangd version to be atleast 15 to enable inlay-hints feature and it is enabled out of the box and does not require additional configurations.

Answer 2

Clangd has a separate option argument --inlay-hints - try adding this to command line in your configuration.