Constant UNSAFE_TO_CONCAT

pub const UNSAFE_TO_CONCAT: u32 = 0x00000002;

Indicates that if input text is changed on one side of the beginning of the cluster this glyph is part of, then the shaping results for the other side might change.

Note that the absence of this flag will NOT by itself mean that it IS safe to concat text. Only two pieces of text both of which clear of this flag can be concatenated safely.

This can be used to optimize paragraph layout, by avoiding re-shaping of each line after line-breaking, by limiting the reshaping to a small piece around the breaking position only, even if the breaking position carries the UNSAFE_TO_BREAK or when hyphenation or other text transformation happens at line-break position, in the following way:

1. Iterate back from the line-break position until the first cluster start position that is NOT unsafe-to-concat,
2. shape the segment from there till the end of line, 3. check whether the resulting glyph-run also is clear of the unsafe-to-concat at its start-of-text position; if it is, just splice it into place and the line is shaped; If not, move on to a position further back that is clear of unsafe-to-concat and retry from there, and repeat.

At the start of next line a similar algorithm can be implemented. That is: 1. Iterate forward from the line-break position until the first cluster start position that is NOT unsafe-to-concat, 2. shape the segment from beginning of the line to that position, 3. check whether the resulting glyph-run also is clear of the unsafe-to-concat at its end-of-text position; if it is, just splice it into place and the beginning is shaped; If not, move on to a position further forward that is clear of unsafe-to-concat and retry up to there, and repeat.

A slight complication will arise in the implementation of the algorithm above, because while our buffer API has a way to return flags for position corresponding to start-of-text, there is currently no position corresponding to end-of-text. This limitation can be alleviated by shaping more text than needed and looking for unsafe-to-concat flag within text clusters.

The UNSAFE_TO_BREAK flag will always imply this flag. To use this flag, you must enable the buffer flag PRODUCE_UNSAFE_TO_CONCAT during shaping, otherwise the buffer flag will not be reliably produced.