Function estimated_compression_workspace_bytes_for_run 

pub fn estimated_compression_workspace_bytes_for_run(
    level: CompressionLevel,
    src_size_hint: Option<u64>,
    window_log: Option<u8>,
    long_distance_matching: bool,
    dictionary: Option<DictionarySizes>,
) -> usize

The same estimate for a run whose window, long-distance matching and dictionary are what the caller has actually asked for.

A window_log override enlarges the history the frame keeps, and long-distance matching adds a hash table of its own on top — neither of them visible in the level’s own preset. A dictionary goes further than adding to the figure: it decides it. The frame runs the dictionary’s own compression parameters, so a small source compressed against a large dictionary builds tables sized for the dictionary, which at the higher levels is the difference between tens of KiB and hundreds of MiB. None, false and None give the preset, which is what estimated_compression_workspace_bytes_for_source reports.

dictionary is what a caller weighing a run before it parses the blob can answer with DictionarySizes::raw_content on the blob’s own length: the content of a trained dictionary is smaller than the blob it came in, and overstating it can only move the estimate toward the copy-mode geometry, which is the larger of the two.