add isimmutabletype

Author: stevengj

NEWS.md

@@ -210,6 +210,8 @@ Library improvements
 
   * New `iszero(x)` function to quickly check whether `x` is zero (or is all zeros, for an array) ([#19950]).
 
+  * New function `isimmutabletype(T)` ([#18168]).
+
   * `notify` now returns a count of tasks woken up ([#19841]).
 
 Compiler/Runtime improvements
@@ -883,6 +885,7 @@ Language tooling improvements
 [#17758]: https://github.com/JuliaLang/julia/issues/17758
 [#17785]: https://github.com/JuliaLang/julia/issues/17785
 [#18050]: https://github.com/JuliaLang/julia/issues/18050
+[#18168]: https://github.com/JuliaLang/julia/issues/18168
 [#18330]: https://github.com/JuliaLang/julia/issues/18330
 [#18339]: https://github.com/JuliaLang/julia/issues/18339
 [#18346]: https://github.com/JuliaLang/julia/issues/18346
@@ -919,4 +922,5 @@ Language tooling improvements
 [#19919]: https://github.com/JuliaLang/julia/issues/19919
 [#19944]: https://github.com/JuliaLang/julia/issues/19944
 [#19950]: https://github.com/JuliaLang/julia/issues/19950
+[#20079]: https://github.com/JuliaLang/julia/issues/20079
 [#20164]: https://github.com/JuliaLang/julia/issues/20164

base/exports.jl

@@ -946,6 +946,7 @@ export
     isbits,
     isequal,
     isimmutable,
+    isimmutabletype,
     isless,
     ifelse,
     lexless,

base/reflection.jl

@@ -205,6 +205,15 @@ isimmutable(x::ANY) = (@_pure_meta; (isa(x,Tuple) || !typeof(x).mutable))
 isstructtype(t::DataType) = (@_pure_meta; nfields(t) != 0 || (t.size==0 && !t.abstract))
 isstructtype(x) = (@_pure_meta; false)
 
+"""
+    isimmutabletype(T)
+
+Return `true` if the type `T` is immutable.  See [manual](:ref:`man-immutable-composite-types`)
+for a discussion of immutability.   See also [`isimmutable`](:func:`isimmutable`)
+for the corresponding function acting on values rather than types.
+"""
+isimmutabletype(t::ANY) = (@_pure_meta; isa(t, DataType) && !t.mutable)
+
 """
     isbits(T)
 

test/reflection.jl

@@ -159,6 +159,13 @@ not_const = 1
 
 @test isimmutable(1) == true
 @test isimmutable([]) == false
+@test isimmutable("abc") == true
+@test isimmutable((3,4,5)) == true
+@test isimmutabletype(Int) == true
+@test isimmutabletype(Vector{Int}) == false
+@test isimmutabletype(String) == true
+@test isimmutabletype(Tuple{Int}) == true
+
 
 ## find bindings tests
 @test ccall(:jl_get_module_of_binding, Any, (Any, Any), Base, :sin)==Base