Support @constprop (#752)

Co-authored-by: Shuhei Kadowaki <aviatesk@gmail.com>

Project.toml

@@ -1,6 +1,6 @@
 name = "Compat"
 uuid = "34da2185-b29b-5c13-b0c7-acf172513d20"
-version = "3.35.0"
+version = "3.36.0"
 
 [deps]
 Base64 = "2a0f44e3-6c83-55bd-87e4-b1978d98bd5f"

README.md

@@ -54,6 +54,8 @@ changes in `julia`.
 
 ## Supported features
 
+* `Compat.@constprop :aggressive ex` and `Compat.@constprop :none ex` allow control over constant-propagation during inference on Julia versions that support this feature, and otherwise just pass back `ex`. ([#42125]) (since Compat 3.36)
+
 * `Returns(value)` returns `value` for any arguments ([#39794]) (since Compat 3.35)
 
 * The function `current_exceptions()` has been added to get the current
@@ -273,3 +275,4 @@ Note that you should specify the correct minimum version for `Compat` in the
 [#41076]: https://github.com/JuliaLang/julia/pull/41076
 [#34331]: https://github.com/JuliaLang/julia/pull/34331
 [#39794]: https://github.com/JuliaLang/julia/pull/39794
+[#42125]: https://github.com/JuliaLang/julia/pull/42125

src/Compat.jl

@@ -1121,6 +1121,30 @@ if VERSION < v"1.7.0-DEV.793"
     end
 end
 
+# https://github.com/JuliaLang/julia/pull/42125
+if !isdefined(Base, Symbol("@constprop"))
+    if isdefined(Base, Symbol("@aggressive_constprop"))
+        macro constprop(setting, ex)
+            if isa(setting, QuoteNode)
+                setting = setting.value
+            end
+            setting === :aggressive && return esc(:(Base.@aggressive_constprop $ex))
+            setting === :none && return esc(ex)
+            throw(ArgumentError("@constprop $setting not supported"))
+        end
+    else
+        macro constprop(setting, ex)
+            if isa(setting, QuoteNode)
+                setting = setting.value
+            end
+            setting === :aggressive || setting === :none || throw(ArgumentError("@constprop $setting not supported"))
+            return esc(ex)
+        end
+    end
+else
+    using Base: @constprop
+end
+
 include("iterators.jl")
 include("deprecated.jl")
 

test/runtests.jl

@@ -1158,4 +1158,16 @@ end
     val = [1,2,3]
     @test Returns(val)(1) === val
     @test sprint(show, Returns(1.0)) == "Returns{Float64}(1.0)"
-end
+end
+
+# https://github.com/JuliaLang/julia/pull/42125
+@testset "@constprop" begin
+    Compat.@constprop :aggressive aggf(x) = Symbol(x)
+    Compat.@constprop :none      nonef(x) = Symbol(x)
+    @test_throws Exception Meta.lower(@__MODULE__,
+        quote
+            Compat.@constprop :other brokenf(x) = Symbol(x)
+        end
+    )
+    @test aggf("hi") == nonef("hi") == :hi
+end