Using go/types to get rid of all unused variables in CL 189798 was a
neat idea, but it was pretty expensive. go/types is a full typechecker,
which does a lot more work than we actually need. Moreover, we had to
run it multiple times, to catch variables that became unused after
removing existing unused variables.
Instead, write our own little detector for unused imports and variables.
It doesn't use ast.Walk, as we need to know what fields we're
inspecting. For example, in "foo := bar", "foo" is declared, and "bar"
is used, yet they both appear as simple *ast.Ident cases under ast.Walk.
The code is documented to explain how unused variables are detected in a
single syntax tree pass. Since this happens after we've generated a
complete go/ast.File, we don't need to worry about our own simplified
node types.
The generated code is the same, but rulegen is much faster and uses less
memory at its peak, so it should scale better with time.
With 'benchcmd Rulegen go run *.go' on perflock, we get:
name old time/op new time/op delta
Rulegen 4.00s ± 0% 3.41s ± 1% -14.70% (p=0.008 n=5+5)
name old user-time/op new user-time/op delta
Rulegen 14.1s ± 1% 10.6s ± 1% -24.62% (p=0.008 n=5+5)
name old sys-time/op new sys-time/op delta
Rulegen 318ms ±26% 263ms ± 9% ~ (p=0.056 n=5+5)
name old peak-RSS-bytes new peak-RSS-bytes delta
Rulegen 231MB ± 4% 181MB ± 3% -21.69% (p=0.008 n=5+5)
Change-Id: I8387d52818f6131357868ad348dac8c96d926191
Reviewed-on: https://go-review.googlesource.com/c/go/+/191782
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org>