If we assume code is shared in AST form, but people still end up editing a text representation of the AST (rather than directly editing the AST through a visual

If we assume code is shared in AST form, but people still end up editing a text representation of the AST (rather than directly editing the AST through a visual interface, as in Scratch et al), then you just wouldn't be able to share your syntax errors - realistically you'd put them in a comment or something, or exchange out-of-band.