Add `Base.isprecompilable` (#58805) (#246)
Alternative to https://github.com/JuliaLang/julia/pull/58146.
We want to compile a subset of the possible specializations of a
function. To this end, we have a number of manually written `precompile`
statements. Creating this list is, unfortunately, error-prone, and the
list is also liable to going stale. Thus we'd like to validate each
`precompile` statement in the list.
The simple answer is, of course, to actually run the `precompile`s, and
we naturally do so, but this takes time.
We would like a relatively quick way to check the validity of a
`precompile` statement.
This is a dev-loop optimization, to allow us to check "is-precompilable"
in unit tests.
We can't use `hasmethod` as it has both false positives (too loose):
```julia
julia> hasmethod(sum, (AbstractVector,))
true
julia> precompile(sum, (AbstractVector,))
false
julia> Base.isprecompilable(sum, (AbstractVector,)) # <- this PR
false
```
and also false negatives (too strict):
```julia
julia> bar(@nospecialize(x::AbstractVector{Int})) = 42
bar (generic function with 1 method)
julia> hasmethod(bar, (AbstractVector,))
false
julia> precompile(bar, (AbstractVector,))
true
julia> Base.isprecompilable(bar, (AbstractVector,)) # <- this PR
true
```
We can't use `hasmethod && isconcretetype` as it has false negatives
(too strict):
```julia
julia> has_concrete_method(f, argtypes) = all(isconcretetype, argtypes) && hasmethod(f, argtypes)
has_concrete_method (generic function with 1 method)
julia> has_concrete_method(bar, (AbstractVector,))
false
julia> has_concrete_method(convert, (Type{Int}, Int32))
false
julia> precompile(convert, (Type{Int}, Int32))
true
julia> Base.isprecompilable(convert, (Type{Int}, Int32)) # <- this PR
true
```
`Base.isprecompilable` is essentially `precompile` without the actual
compilation.
Co-authored-by: Kiran Pamnany <kpamnany@users.noreply.github.com>
