diff --git a/NEWS.md b/NEWS.md
index fa2aafffb2802..1d0700631ae48 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -57,6 +57,8 @@ New library functions
 * `Base.donotdelete` is now public. It prevents deadcode elimination of its arguments ([#55774]).
 * `Sys.sysimage_target()` returns the CPU target string used to build the current system image ([#58970]).
 * `Iterators.findeach` is a lazy version of `findall` ([#54124])
+* `Base.unsafe_substring` is an unexported, public constructor to build a `SubString` without checking for
+   valid string indices.
 
 New library features
 --------------------
diff --git a/base/public.jl b/base/public.jl
index e944e8f3915d6..e83ce7bab7af5 100644
--- a/base/public.jl
+++ b/base/public.jl
@@ -107,6 +107,8 @@ public
 
 # Strings
     escape_raw_string,
+    unsafe_substring,
+    unannotate,
 
 # IO
     # types
diff --git a/base/regex.jl b/base/regex.jl
index 2bd47c271ce75..f3c976bab705b 100644
--- a/base/regex.jl
+++ b/base/regex.jl
@@ -447,12 +447,10 @@ end
 
 function _annotatedmatch(m::RegexMatch{S}, str::AnnotatedString{S}) where {S<:AbstractString}
     RegexMatch{AnnotatedString{S}}(
-        (@inbounds SubString{AnnotatedString{S}}(
-            str, m.match.offset, m.match.ncodeunits, Val(:noshift))),
+        (@inbounds unsafe_substring(str, m.match.offset + 1, m.match.ncodeunits)),
         Union{Nothing,SubString{AnnotatedString{S}}}[
             if !isnothing(cap)
-                (@inbounds SubString{AnnotatedString{S}}(
-                    str, cap.offset, cap.ncodeunits, Val(:noshift)))
+                (@inbounds unsafe_substring(str, cap.offset + 1, cap.ncodeunits))
             end for cap in m.captures],
         m.offset, m.offsets, m.regex)
 end
diff --git a/base/strings/annotated.jl b/base/strings/annotated.jl
index 89cba6db42c8d..f98c2dee72ae7 100644
--- a/base/strings/annotated.jl
+++ b/base/strings/annotated.jl
@@ -158,6 +158,32 @@ eltype(::Type{<:AnnotatedString{S}}) where {S} = AnnotatedChar{eltype(S)}
 firstindex(s::AnnotatedString) = firstindex(s.string)
 lastindex(s::AnnotatedString) = lastindex(s.string)
 
+"""
+    unannotate(s::AnnotatedString{S})::S
+    unannotate(s::SubString{AnnotatedString{S}})::SubString{S}
+
+Get the underlying string of `s`, without copying.
+
+# Examples
+```jldoctest; setup=:(using Base: AnnotatedString)
+julia> s = AnnotatedString("abcde", [(1:3, :A, 4)])
+"abcde"
+
+julia> u = unannotate(s)
+"abcde"
+
+julia> typeof(u)
+String
+```
+"""
+unannotate(s::AnnotatedString) = s.string
+
+function unannotate(s::SubString{<:AnnotatedString})
+    start_index = first(parentindices(s)[1])
+    @inbounds unsafe_substring(parent(s).string, start_index, ncodeunits(s))
+end
+
+
 function getindex(s::AnnotatedString, i::Integer)
     @boundscheck checkbounds(s, i)
     @inbounds if isvalid(s, i)
@@ -204,16 +230,14 @@ cmp(a::AnnotatedString, b::AnnotatedString) = cmp(a.string, b.string)
 # To prevent substring equality from hitting the generic fallback
 
 function ==(a::SubString{<:AnnotatedString}, b::SubString{<:AnnotatedString})
-    SubString(a.string.string, a.offset, a.ncodeunits, Val(:noshift)) ==
-        SubString(b.string.string, b.offset, b.ncodeunits, Val(:noshift)) &&
-        annotations(a) == annotations(b)
+    unannotate(a) == unannotate(b) && annotations(a) == annotations(b)
 end
 
 ==(a::SubString{<:AnnotatedString}, b::AnnotatedString) =
-    annotations(a) == annotations(b) && SubString(a.string.string, a.offset, a.ncodeunits, Val(:noshift)) == b.string
+    annotations(a) == annotations(b) && unannotate(a) == b.string
 
 ==(a::SubString{<:AnnotatedString}, b::AbstractString) =
-    isempty(annotations(a)) && SubString(a.string.string, a.offset, a.ncodeunits, Val(:noshift)) == b
+    isempty(annotations(a)) && unannotate(a) == b
 
 ==(a::AbstractString, b::SubString{<:AnnotatedString}) = b == a
 
@@ -262,7 +286,7 @@ function annotatedstring(xs...)
                     push!(annotations, setindex(annot, rstart:rstop, :region))
                 end
             end
-            print(s, SubString(x.string.string, x.offset, x.ncodeunits, Val(:noshift)))
+            print(s, unannotate(x))
         elseif x isa AnnotatedChar
             for annot in x.annotations
                 push!(annotations, (region=1+size:1+size, annot...))
diff --git a/base/strings/substring.jl b/base/strings/substring.jl
index ec9944449bc0e..06caae5277413 100644
--- a/base/strings/substring.jl
+++ b/base/strings/substring.jl
@@ -36,18 +36,67 @@ struct SubString{T<:AbstractString} <: AbstractString
         end
         return new(s, i-1, nextind(s,j)-i)
     end
-    function SubString{T}(s::T, i::Int, j::Int, ::Val{:noshift}) where T<:AbstractString
-        @boundscheck if !(i == j == 0)
-            si, sj = i + 1, prevind(s, j + i + 1)
-            @inbounds isvalid(s, si) || string_index_err(s, si)
-            @inbounds isvalid(s, sj) || string_index_err(s, sj)
-        end
-        new(s, i, j)
+    # We don't expose this, because the exposed constructor needs to avoid constructing
+    # a SubString{SubString{T}} when passed a substring.
+    global function _unsafe_substring(s::T, offset::Int, ncodeunits::Int) where {T <: AbstractString}
+        new{T}(s, offset, ncodeunits)
+    end
+end
+
+function check_codeunit_bounds(s::AbstractString, first_index::Int, n_codeunits::Int)
+    last_index = first_index + n_codeunits - 1
+    bad_index = if first_index < 1
+        first_index
+    elseif last_index > ncodeunits(s)
+        last_index
+    else
+        return nothing
     end
+    throw(BoundsError(s, bad_index))
+end
+
+"""
+    unsafe_substring(s::AbstractString, first_index::Int, n_codeunits::Int)::SubString{typeof(s)}
+    unsafe_substring(s::SubString{S}, first_index::Int, n_codeunits::Int)::SubString{S}
+
+Create a substring of `s` spanning the codeunits `first_index:(first_index + n_codeunits - 1)`.
+
+If `first_index` < 1, or `first_index + n_codeunits - 1 > ncodeunits(s)`, throw a `BoundsError`.
+
+This function does check bounds, but does not validate that the arguments corresponds to valid
+start and end indices in `s`, and so the resulting substring may contain truncated characters.
+The presence of truncated characters is safe and well-defined for `String` and `SubString{String}`,
+but may not be permitted for custom subtypes of `AbstractString`.
+
+# Examples
+```jldoctest
+julia> s = "Hello, Bjørn!";
+
+julia> ss = unsafe_substring(s, 3, 10)
+"lo, Bjørn"
+
+julia> typeof(ss)
+SubString{String}
+
+julia> ss2 = unsafe_substring(ss, 2, 6)
+"o, Bj\\xc3"
+
+julia> typeof(ss2)
+SubString{String}
+```
+"""
+function unsafe_substring(s::AbstractString, first_index::Int, n_codeunits::Int)
+    @boundscheck @inline checkbounds(codeunits(s), first_index:(first_index + n_codeunits - 1))
+    return _unsafe_substring(s, first_index - 1, n_codeunits)
+end
+
+function unsafe_substring(s::SubString, first_index::Int, n_codeunits::Int)
+    @boundscheck @inline check_codeunit_bounds(s, first_index, n_codeunits)
+    string = s.string
+    return _unsafe_substring(string, first_index + s.offset - 1, n_codeunits)
 end
 
 @propagate_inbounds SubString(s::T, i::Int, j::Int) where {T<:AbstractString} = SubString{T}(s, i, j)
-@propagate_inbounds SubString(s::T, i::Int, j::Int, v::Val{:noshift}) where {T<:AbstractString} = SubString{T}(s, i, j, v)
 @propagate_inbounds SubString(s::AbstractString, i::Integer, j::Integer=lastindex(s)) = SubString(s, Int(i)::Int, Int(j)::Int)
 @propagate_inbounds SubString(s::AbstractString, r::AbstractUnitRange{<:Integer}) = SubString(s, first(r), last(r))
 
@@ -56,8 +105,9 @@ end
     SubString(s.string, s.offset+i, s.offset+j)
 end
 
-SubString(s::AbstractString) = SubString(s, 1, lastindex(s)::Int)
-SubString{T}(s::T) where {T<:AbstractString} = SubString{T}(s, 1, lastindex(s)::Int)
+SubString(s::AbstractString) = @inbounds unsafe_substring(s, 1, Int(ncodeunits(s))::Int)
+SubString{T}(s::T) where {T<:AbstractString} = SubString(s)
+SubString(s::SubString) = s
 
 @propagate_inbounds view(s::AbstractString, r::AbstractUnitRange{<:Integer}) = SubString(s, r)
 @propagate_inbounds maybeview(s::AbstractString, r::AbstractUnitRange{<:Integer}) = view(s, r)
diff --git a/base/strings/util.jl b/base/strings/util.jl
index 0573893481d3d..da149a8f49b03 100644
--- a/base/strings/util.jl
+++ b/base/strings/util.jl
@@ -377,7 +377,7 @@ end
     end
     off = s isa String ? 0 : s.offset
     par = s isa String ? s : s.string
-    @inbounds @inline SubString{String}(par, off, len, Val{:noshift}())
+    @inbounds unsafe_substring(s, 1, len)
 end
 """
     lstrip([pred=isspace,] str::AbstractString)::SubString
diff --git a/doc/src/base/strings.md b/doc/src/base/strings.md
index 15a7c0531de4a..a3930312ce3d5 100644
--- a/doc/src/base/strings.md
+++ b/doc/src/base/strings.md
@@ -15,6 +15,7 @@ Base.repeat(::AbstractChar, ::Integer)
 Base.repr(::Any)
 Core.String(::AbstractString)
 Base.SubString
+Base.unsafe_substring
 Base.LazyString
 Base.@lazy_str
 Base.transcode
@@ -110,4 +111,5 @@ Base.AnnotatedChar
 Base.annotatedstring
 Base.annotations
 Base.annotate!
+Base.unannotate
 ```
diff --git a/test/strings/annotated.jl b/test/strings/annotated.jl
index 7f53740b9eec1..022369f8e4772 100644
--- a/test/strings/annotated.jl
+++ b/test/strings/annotated.jl
@@ -77,6 +77,17 @@
 
     @test Bool === Base.infer_return_type(isvalid, Tuple{Base.AnnotatedString, Vararg})
     @test Int === Base.infer_return_type(ncodeunits, Tuple{Base.AnnotatedString})
+
+    @testset "unannotate" begin
+        s = "some string"
+        str = Base.AnnotatedString(s, [(2:5, :A, 3)])
+        @test Base.unannotate(str) === s
+
+        str2 = SubString(str, 2:9)
+        u = Base.unannotate(str2)
+        @test u isa SubString{String}
+        @test u == SubString(s, 2:9)
+    end
 end
 
 @testset "AnnotatedChar" begin
diff --git a/test/strings/basic.jl b/test/strings/basic.jl
index 214a14ed2443f..458b72d41f6b3 100644
--- a/test/strings/basic.jl
+++ b/test/strings/basic.jl
@@ -222,10 +222,17 @@ end
         @test (@views (x[3], x[1:2], x[[1,4]])) == ('c', "ab", "ad")
     end
 
-    @testset ":noshift constructor" begin
-        @test SubString("", 0, 0, Val(:noshift)) == ""
-        @test SubString("abcd", 0, 1, Val(:noshift)) == "a"
-        @test SubString("abcd", 0, 4, Val(:noshift)) == "abcd"
+    @testset "unsafe_substring" begin
+        s = "abcdefgøø"
+        @test unsafe_substring(s, 1, 11) == s
+        @test unsafe_substring(s, 1, 3) == "abc"
+        @test unsafe_substring(s, 3, 3) == "cde"
+        @test unsafe_substring(s, 5, 4) == String(codeunits(s)[5:8])
+        @test unsafe_substring(s, 1, 2) isa SubString{String}
+        @test unsafe_substring(unsafe_substring(s, 2, 8), 1, 3) isa SubString{String}
+
+        @test_throws BoundsError unsafe_substring(s, 0, 2)
+        @test_throws BoundsError unsafe_substring(s, 2, 11)
     end
 end