lsp-rust: add semantic token modifiers

This is more of a proof-of-concept than anything else.

It works for the "mutable" modifier, adding an underline. This matches
what the VsCode client does, but clashes with the lsp-mode default for
read vs write highlighted variables.

The "reference" modifier has the bold attribute set.

The rest have a no-operation face assigned, but need to be there to
get the font caluclation numbers right, and to be customized by users.

CHANGELOG.org

@@ -45,6 +45,7 @@
   * Add ~lsp-sorbet-as-add-on~ variable to allow running the Sorbet server as an add-on alongside Solargraph or others
   * Add [[https://github.com/nikeee/dot-language-server][dot-language-server]] (/a.k.a./ Graphviz) support.
   * Add [[https://github.com/FractalBoy/perl-language-server][PLS]] support (additional sever for Perl).
+  * Add some support for rust-analyzer semantic token modifiers.  Highlights mutable and reference variables.
 ** Release 8.0.0
   * Add ~lsp-clients-angular-node-get-prefix-command~ to get the Angular server from another location which is still has ~/lib/node_modules~ in it.
   * Set ~lsp-clients-angular-language-server-command~ after the first connection to speed up subsequent connections.

clients/lsp-rust.el

@@ -27,6 +27,7 @@
 (require 'lsp-mode)
 (require 'ht)
 (require 'dash)
+(require 'lsp-semantic-tokens)
 
 (defgroup lsp-rust nil
   "LSP support for Rust, using Rust Language Server or rust-analyzer."
@@ -926,6 +927,37 @@ or JSON objects in `rust-project.json` format."
 (lsp-defun lsp-rust--analyzer-debug-lens ((&Command :arguments? [args]))
   (lsp-rust-analyzer-debug args))
 
+(defun lsp-rust-analyzer--semantic-modifiers ()
+  "Mapping between rust-analyzer keywords and fonts to apply.
+The keywords are sent in the initialize response, in the semantic
+tokens legend."
+  '(
+    ("documentation" . lsp-rust-analyzer-documentation-modifier-face)
+    ("declaration" . lsp-rust-analyzer-declaration-modifier-face)
+    ("definition" . lsp-rust-analyzer-definition-modifier-face)
+    ("static" . lsp-rust-analyzer-static-modifier-face)
+    ("abstract" . lsp-rust-analyzer-abstract-modifier-face)
+    ("deprecated" . lsp-rust-analyzer-deprecated-modifier-face)
+    ("readonly" . lsp-rust-analyzer-readonly-modifier-face)
+    ("default_library" . lsp-rust-analyzer-default-library-modifier-face)
+    ("async" . lsp-rust-analyzer-async-modifier-face)
+    ("attribute" . lsp-rust-analyzer-attribute-modifier-face)
+    ("callable" . lsp-rust-analyzer-callable-modifier-face)
+    ("constant" . lsp-rust-analyzer-constant-modifier-face)
+    ("consuming" . lsp-rust-analyzer-consuming-modifier-face)
+    ("control_flow" . lsp-rust-analyzer-control-flow-modifier-face)
+    ("crate_root" . lsp-rust-analyzer-crate-root-modifier-face)
+    ("injected" . lsp-rust-analyzer-injected-modifier-face)
+    ("intra_doc_link" . lsp-rust-analyzer-intra-doc-link-modifier-face)
+    ("library" . lsp-rust-analyzer-library-modifier-face)
+    ("mutable" . lsp-rust-analyzer-mutable-modifier-face)
+    ("public" . lsp-rust-analyzer-public-modifier-face)
+    ("reference" . lsp-rust-analyzer-reference-modifier-face)
+    ("trait" . lsp-rust-analyzer-trait-modifier-face)
+    ("unsafe" . lsp-rust-analyzer-unsafe-modifier-face)
+    ))
+
+
 (lsp-register-client
  (make-lsp-client
   :new-connection (lsp-stdio-connection
@@ -947,6 +979,10 @@ or JSON objects in `rust-project.json` format."
                    (when lsp-rust-analyzer-server-display-inlay-hints
                      (lsp-rust-analyzer-inlay-hints-mode)))
   :ignore-messages nil
+
+  :semantic-tokens-faces-overrides `(:discard-default-modifiers t
+                                     :modifiers
+                                     ,(lsp-rust-analyzer--semantic-modifiers))
   :server-id 'rust-analyzer
   :custom-capabilities `((experimental . ((snippetTextEdit . ,(and lsp-enable-snippet (featurep 'yasnippet))))))
   :download-server-fn (lambda (_client callback error-callback _update?)
@@ -1290,6 +1326,125 @@ https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensio
   (interactive)
   (lsp-rust-analyzer-move-item "Down"))
 
+
+;; Semantic tokens
+
+;; Modifier faces
+(defface lsp-rust-analyzer-documentation-modifier-face
+  '((t nil))
+  "The face modification to use for documentation items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-declaration-modifier-face
+  '((t nil))
+  "The face modification to use for declaration items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-definition-modifier-face
+  '((t nil))
+  "The face modification to use for definition items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-static-modifier-face
+  '((t nil))
+  "The face modification to use for static items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-abstract-modifier-face
+  '((t nil))
+  "The face modification to use for abstract items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-deprecated-modifier-face
+  '((t nil))
+  "The face modification to use for deprecated items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-readonly-modifier-face
+  '((t nil))
+  "The face modification to use for readonly items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-default-library-modifier-face
+  '((t nil))
+  "The face modification to use for default-library items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-async-modifier-face
+  '((t nil))
+  "The face modification to use for async items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-attribute-modifier-face
+  '((t nil))
+  "The face modification to use for attribute items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-callable-modifier-face
+  '((t nil))
+  "The face modification to use for callable items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-constant-modifier-face
+  '((t nil))
+  "The face modification to use for constant items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-consuming-modifier-face
+  '((t nil))
+  "The face modification to use for consuming items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-control-flow-modifier-face
+  '((t nil))
+  "The face modification to use for control-flow items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-crate-root-modifier-face
+  '((t nil))
+  "The face modification to use for crate-root items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-injected-modifier-face
+  '((t nil))
+  "The face modification to use for injected items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-intra-doc-link-modifier-face
+  '((t nil))
+  "The face modification to use for intra-doc-link items.")
+  :group 'lsp-rust-analyzer
+
+(defface lsp-rust-analyzer-library-modifier-face
+  '((t nil))
+  "The face modification to use for library items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-mutable-modifier-face
+  '((t :underline t))
+  "The face modification to use for mutable items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-public-modifier-face
+  '((t nil))
+  "The face modification to use for public items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-reference-modifier-face
+  '((t :bold t))
+  "The face modification to use for reference items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-trait-modifier-face
+  '((t nil))
+  "The face modification to use for trait items."
+  :group 'lsp-rust-analyzer)
+
+(defface lsp-rust-analyzer-unsafe-modifier-face
+  '((t nil))
+  "The face modification to use for unsafe items."
+  :group 'lsp-rust-analyzer)
+
 (lsp-consistency-check lsp-rust)
 
 (provide 'lsp-rust)