First version!

src/main/java/eu/midnightdust/fastleaves/mixin/MixinLeavesBlock.java

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+package eu.midnightdust.fastleaves.mixin;
+
+import net.fabricmc.api.EnvType;
+import net.fabricmc.api.Environment;
+import net.minecraft.block.Block;
+import net.minecraft.block.BlockState;
+import net.minecraft.block.LeavesBlock;
+import net.minecraft.util.math.Direction;
+import org.spongepowered.asm.mixin.Mixin;
+
+@Mixin(LeavesBlock.class)
+@Environment(EnvType.CLIENT)
+public class MixinLeavesBlock extends Block {
+
+    public MixinLeavesBlock(Settings settings) {
+        super(settings);
+    }
+
+    @Override
+    @SuppressWarnings("deprecation")
+    public boolean isSideInvisible(BlockState state, BlockState neighborState, Direction offset) {
+        return neighborState.getBlock() instanceof LeavesBlock;
+    }
+}

src/main/resources/fabric.mod.json

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+{
+  "schemaVersion": 1,
+  "id": "fastleaves",
+  "version": "${version}",
+
+  "name": "Fast Leaves",
+  "description": "Adds culling to leaf blocks, providing a huge performance boost over vanilla.",
+  "authors": [
+    "Motschen",
+    "TeamMidnightDust"
+  ],
+  "contact": {
+    "homepage": "https://www.midnightdust.eu/",
+    "sources": "https://github.com/TeamMidnightDust/FastLeaves",
+    "issues": "https://github.com/TeamMidnightDust/FastLeaves/issues"
+  },
+
+  "license": "MIT",
+  "icon": "assets/fastleaves/icon.png",
+
+  "environment": "client",
+
+  "mixins": [
+    "fastleaves.mixins.json"
+  ]
+}

src/main/resources/fastleaves.mixins.json

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+{
+  "required": true,
+  "package": "eu.midnightdust.fastleaves.mixin",
+  "compatibilityLevel": "JAVA_8",
+  "client": [
+    "MixinLeavesBlock"
+  ],
+  "injectors": {
+    "defaultRequire": 1
+  }
+}