delete-by-moving-to-trash is a variable defined in ‘src/fileio.c’.

Its value is nil

Specifies whether to use the system’s trash can. When non-nil, certain file deletion commands use the function ‘move-file-to-trash’ instead of deleting files outright. This includes interactive calls to ‘delete-file’ and ‘delete-directory’ and the Dired deletion commands.

  This variable was introduced, or its default value was changed, in
  version 23.1 of Emacs.
  You can customize this variable.

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