[patrick-immometrica]

render_block interprets the whole template into io::sink() and keeps the requested block from block_buffer. Everything after the block, often the bulk of the page, is still evaluated and then thrown away.

Block names are unique and the parser doesn't allow blocks inside for loops, so once the requested block has been rendered nothing that follows can change it. This sets a flag on the state when the block is captured, and the RenderBlock arm returns right after it, which unwinds through any outer blocks too.

One behaviour change: an error that only happens after the block (an undefined variable further down, say) no longer fails render_block. I think that's what you'd expect when asking for one block, and it's what the new test checks, for a plain template, a nested block and a child template using super(). An error inside the block itself still surfaces.

Before/after for a template with a small header block followed by a 2,000-row loop (release build, µs per call, three runs):

	before	after
render_block("page.html", "header")	149–407	0.1–0.3
render("page.html")	618–811	336–654

Full renders don't change; the spread there is just noise.