Make some more deepcopy_internal methods type stable (#54562)

Same as #54496 for some more
methods.
I added some more testcases than the changed methods. For
`SimpleVector`, I don't know how to construct an instance for a
testcase.

This could be backported to 1.11.

Author: lgoettgens

base/deepcopy.jl

@@ -37,7 +37,7 @@ deepcopy_internal(x::Module, stackdict::IdDict) = error("deepcopy of Modules not
 
 function deepcopy_internal(x::SimpleVector, stackdict::IdDict)
     if haskey(stackdict, x)
-        return stackdict[x]
+        return stackdict[x]::typeof(x)
     end
     y = Core.svec(Any[deepcopy_internal(x[i], stackdict) for i = 1:length(x)]...)
     stackdict[x] = y
@@ -46,7 +46,7 @@ end
 
 function deepcopy_internal(x::String, stackdict::IdDict)
     if haskey(stackdict, x)
-        return stackdict[x]
+        return stackdict[x]::typeof(x)
     end
     y = GC.@preserve x unsafe_string(pointer(x), sizeof(x))
     stackdict[x] = y
@@ -58,7 +58,7 @@ function deepcopy_internal(@nospecialize(x), stackdict::IdDict)
     nf = nfields(x)
     if ismutable(x)
         if haskey(stackdict, x)
-            return stackdict[x]
+            return stackdict[x]::typeof(x)
         end
         y = ccall(:jl_new_struct_uninit, Any, (Any,), T)
         stackdict[x] = y

test/copy.jl

@@ -248,9 +248,20 @@ end
     @test (@inferred Base.deepcopy_internal(zeros(), IdDict())) == zeros()
 end
 
-@testset "deepcopy_internal big" begin
+@testset "deepcopy_internal inference" begin
+    @inferred Base.deepcopy_internal(1, IdDict())
+    @inferred Base.deepcopy_internal(1.0, IdDict())
     @inferred Base.deepcopy_internal(big(1), IdDict())
     @inferred Base.deepcopy_internal(big(1.0), IdDict())
+    @inferred Base.deepcopy_internal('a', IdDict())
+    @inferred Base.deepcopy_internal("abc", IdDict())
+    @inferred Base.deepcopy_internal([1,2,3], IdDict())
+
+    # structs without custom deepcopy_internal method
+    struct Immutable2; x::Int; end
+    mutable struct Mutable2; x::Int; end
+    @inferred Base.deepcopy_internal(Immutable2(1), IdDict())
+    @inferred Base.deepcopy_internal(Mutable2(1), IdDict())
 end
 
 @testset "`copyto!`'s unaliasing" begin