Test/docs/news

Author: Keno

NEWS.md

@@ -101,6 +101,7 @@ Standard library changes
 * TCP socket objects now expose `closewrite` functionality and support half-open mode usage ([#40783]).
 * Intersect returns a result with the eltype of the type-promoted eltypes of the two inputs ([#41769]).
 * `Iterators.countfrom` now accepts any type that defines `+`. ([#37747])
+* The `LazyString` and the `lazy"str"` macro were added to support delayed construction of error messages in error paths. ([#33711])
 
 #### InteractiveUtils
 * A new macro `@time_imports` for reporting any time spent importing packages and their dependencies ([#41612])

base/strings/lazy.jl

@@ -8,6 +8,9 @@ of functions).
 
 This type is designed to be cheap to construct at runtime, trying to offload
 as much work as possible to either the macro or later printing operations.
+
+!!! compat "Julia 1.8"
+    `LazyString` requires Julia 1.8 or later.
 """
 mutable struct LazyString <: AbstractString
     parts::Tuple
@@ -16,6 +19,16 @@ mutable struct LazyString <: AbstractString
     LazyString(args...) = new(args)
 end
 
+"""
+    lazy"str"
+
+Create a [`LazyString`](@ref) using regular string interpolation syntax.
+Note that interpolations are *evaluated* at LazyString construction time,
+but *printing* is delayed until the first access to the string.
+
+!!! compat "Julia 1.8"
+    `lazy"str"` requires Julia 1.8 or later.
+"""
 macro lazy_str(text)
     parts = Any[]
     lastidx = idx = 1

test/strings/basic.jl

@@ -1094,3 +1094,10 @@ end
     @test sprint(summary, SubString("foα", 2)) == "3-codeunit SubString{String}"
     @test sprint(summary, "") == "empty String"
 end
+
+@testset "LazyString" begin
+    @test repr(lazy"$(1+2) is 3") == "\"3 is 3\""
+    let d = Dict(lazy"$(1+2) is 3" => 3)
+        @test d["3 is 3"] == 3
+    end
+end