That mostly looks good.

include/SciLexer.h is generated from include/LexicalStyles.iface by scripts/LexillaGen.py and will be overwritten by build scripts. The new style definitions should be added to include/LexicalStyles.iface.

To avoid multiple definition problems, non-static definitions like StringType and NUM_RUST_KEYWORD_LISTS should be in an unnamed namespace. Most maintained lexers place almost the whole file except for the LexerModule definition in an unnamed namespace.

Since there is a non-ASCII character 'æ' encoded as UTF-8, there should be a UTF-8 coding declaration so SciTE will correctly display the file. See test/examples/cpp/130NonAsciiKeyword.cxx.

The styling test results in this pull request appear to be from before the new feature: examples/rust/Issue268.rs.styled doesn't contain any style 24 or 25 but running TestLexers produces these styles.