Bug: Warnings "Invalid face reference: lsp-flycheck-info-unnecessary" when using lsp-ui-sideline

While editing Python files when using lsp-mode together with pyright, lsp-ui, lsp-ui-sideline and flycheck, warning messages like "Invalid face reference: lsp-flycheck-info-unnecessary" occur repeatedly in the "*Messages*" buffer.

This can be seen by e.g. creating a buffer with an unused import, saving it, and moving the cursor on and off the import line when the sideline is in use.

It occurs because lsp-flycheck-info-unnecessary is not a face (the corresponding face is lsp-flycheck-info-unnecessary-face) but is treated as such by lsp-ui-sideline--diagnostics.

Both are generated by lsp--diagnostics-flycheck-level, which generates a "level" from an original flycheck level (e.g. 'error, 'warning) and a list of tags.

Some of the levels returned from lsp-diagnostics--flycheck-calculate-level are themselves faces but those constructed with tags are not.

However lsp-ui-sideline--diagnostics may set a face variable directly from such a level and pass it to add-face-text-property. https://github.com/emacs-lsp/lsp-ui/blob/0dd39900c8ed8145d207985cb2f65cedd1ffb410/lsp-ui-sideline.el#L469-L474

When that occurs this triggers an "Invalid face" warnings. This can be fixed by replacing https://github.com/emacs-lsp/lsp-ui/blob/0dd39900c8ed8145d207985cb2f65cedd1ffb410/lsp-ui-sideline.el#L470

(face (let ((local-face
             (flycheck-error-level-error-list-face
              (if (eq level 'info) 'success level))))
        (if (facep local-face)
            local-face
          (error "Could not determine face for %s" level))))

Related from lsp-mode

I am seeing the same error in a LaTeX file. I am using a clean environment using the following command: M-x lsp-start-plain.

Steps to reproduce:

create this LaTeX file

\documentclass{article}

\begin{document}
\section{One section}
\label{sec:one-section}

\end{document}

emacs -Q
evaluate (package-initialize)
M-x lsp-start-plain
find-file the above LaTeX file

lsp. I am using the latest (5.13.0) texlab:

LSP :: Connected to [texlab2:1806426/starting /home/ramon/tmp].
LSP :: texlab2:1806426 initialized successfully in folders: (/home/ramon/tmp

Move point to '\label{sec:one-section}`. Lots of warnings appear in the Messages buffer:
```
Invalid face reference: lsp-flycheck-info-unnecessary [131 times]
```

emacs-lsp / lsp-ui

Bug: Warnings "Invalid face reference: lsp-flycheck-info-unnecessary" when using lsp-ui-sideline #761