Document the pass-through specialization heuristic for Type, Vararg, and Function (#32817)

Author: iamed2

doc/src/devdocs/ast.md

@@ -503,7 +503,7 @@ These symbols appear in the `head` field of [`Expr`](@ref)s in lowered form.
         See [Working with LLVM](@ref Working-with-LLVM) for where these are derived from and how they get handled.
 
 
-### Method
+### [Method](@id ast-lowered-method)
 
 A unique'd container describing the shared metadata for a single method.
 

doc/src/devdocs/functions.md

@@ -210,7 +210,7 @@ This function further unpacks each *element* of `other`, expecting each one to c
 Naturally, a more efficient implementation is available if all splatted arguments are named tuples.
 Notice that the original `circle` function is passed through, to handle closures.
 
-## Compiler efficiency issues
+## [Compiler efficiency issues](@id compiler-efficiency-issues)
 
 Generating a new type for every function has potentially serious consequences for compiler resource
 use when combined with Julia's "specialize on all arguments by default" design. Indeed, the initial

doc/src/manual/performance-tips.md

@@ -481,6 +481,65 @@ c = (b + 1.0f0)::Complex{T}
 does not hinder performance (but does not help either) since the compiler can determine the type of `c`
 at the time `k` is compiled.
 
+### Be aware of when Julia avoids specializing
+
+As a heuristic, Julia avoids automatically specializing on argument type parameters in three
+specific cases: `Type`, `Function`, and `Vararg`. Julia will always specialize when the argument is
+used within the method, but not if the argument is just passed through to another function. This
+usually has no performance impact at runtime and
+[improves compiler performance](@ref compiler-efficiency-issues). If you find it does have a
+performance impact at runtime in your case, you can trigger specialization by adding a type
+parameter to the method declaration. Here are some examples:
+
+This will not specialize:
+
+```julia
+function f_type(t)  # or t::Type
+    x = ones(t, 10)
+    return sum(map(sin, x))
+end
+```
+
+but this will:
+
+```julia
+function g_type(t::Type{T}) where T
+    x = ones(T, 10)
+    return sum(map(sin, x))
+end
+```
+
+These will not specialize:
+
+```julia
+f_func(f, num) = ntuple(f, div(num, 2))
+g_func(g::Function, num) = ntuple(g, div(num, 2))
+```
+
+but this will:
+
+```julia
+h_func(h::H, num) where {H} = ntuple(h, div(num, 2))
+```
+
+This will not specialize:
+
+```julia
+f_vararg(x::Int...) = tuple(x...)
+```
+
+but this will:
+
+```julia
+g_vararg(x::Vararg{Int, N}) where {N} = tuple(x...)
+```
+
+Note that [`@code_typed`](@ref) and friends will always show you specialized code, even if Julia
+would not normally specialize that method call. You need to check the
+[method internals](@ref ast-lowered-method) if you want to see whether specializations are generated
+when argument types are changed, i.e., if `(@which f(...)).specializations` contains specializations
+for the argument in question.
+
 ## Break functions into multiple definitions
 
 Writing a function as many small definitions allows the compiler to directly call the most applicable