'How to see Go func len() Implementation in VS Code?

My goal is to see the logic behind func len() in Golang. I use VS Code for IDE.

The problem is when I tried to do Go to Implementation, it returns No implementation found for 'len'.

What I've did:

Install Go extension in VS Code, and install Go binary.
I can do Go to Definition, the IDE will open builtin.go. However, builtin.go only contains func len(v Type) int without a body.

Sources

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