chromium – what does the $: syntax mean in a Ninja build file?

encountered some unusual syntax in a Ninja build file and I’m trying to understand its meaning. Specifically, I’m confused about the $: part in the following lines:

build third_party/blink/renderer/core/animation$:animation: phony obj/third_party/blink/renderer/core/animation/libanimation.a
build third_party/blink/renderer/core/animation: phony obj/third_party/blink/renderer/core/animation/libanimation.a

Here is the complete context of the build file:

plaintext

pool build_toolchain_action_pool
  depth = 48

pool build_toolchain_link_pool
  depth = 41

# Include other necessary subninja files
# subninja toolchain.ninja

build third_party/blink/renderer/core/animation$:animation: phony obj/third_party/blink/renderer/core/animation/libanimation.a

build third_party/blink/renderer/core/animation: phony obj/third_party/blink/renderer/core/animation/libanimation.a

I understand that the phony targets do not generate actual files but serve to manage dependencies. However, the $: syntax in the target names is not something I am familiar with, and I couldn’t find any references in the official Ninja documentation.

My Questions:
What does the $: syntax mean in the context of these Ninja build file lines?
Is this a standard feature of Ninja or a project-specific customization?
How can I determine the purpose of this syntax if it’s a custom feature?
Any insights or references to relevant documentation would be greatly appreciated.