Speed up first and only for various types (#52296)

Author: matthias314

base/abstractarray.jl

@@ -446,7 +446,7 @@ julia> firstindex(rand(3,4,5), 2)
 firstindex(a::AbstractArray) = (@inline; first(eachindex(IndexLinear(), a)))
 firstindex(a, d) = (@inline; first(axes(a, d)))
 
-first(a::AbstractArray) = a[first(eachindex(a))]
+@propagate_inbounds first(a::AbstractArray) = a[first(eachindex(a))]
 
 """
     first(coll)

base/dict.jl

@@ -738,6 +738,8 @@ end
 isempty(t::Dict) = (t.count == 0)
 length(t::Dict) = t.count
 
+@propagate_inbounds Iterators.only(t::Dict) = Iterators._only(t, first)
+
 @propagate_inbounds function Base.iterate(v::T, i::Int = v.dict.idxfloor) where T <: Union{KeySet{<:Any, <:Dict}, ValueIterator{<:Dict}}
     i == 0 && return nothing
     i = skip_deleted(v.dict, i)
@@ -1049,3 +1051,5 @@ iterate(dict::PersistentDict, state=nothing) = HAMT.iterate(dict.trie, state)
 length(dict::PersistentDict) = HAMT.length(dict.trie)
 isempty(dict::PersistentDict) = HAMT.isempty(dict.trie)
 empty(::PersistentDict, ::Type{K}, ::Type{V}) where {K, V} = PersistentDict{K, V}()
+
+@propagate_inbounds Iterators.only(dict::PersistentDict) = Iterators._only(dict, first)

base/iterators.jl

@@ -11,7 +11,7 @@ const Base = parentmodule(@__MODULE__)
 using .Base:
     @inline, Pair, Pairs, AbstractDict, IndexLinear, IndexStyle, AbstractVector, Vector,
     SizeUnknown, HasLength, HasShape, IsInfinite, EltypeUnknown, HasEltype, OneTo,
-    @propagate_inbounds, @isdefined, @boundscheck, @inbounds, Generator,
+    @propagate_inbounds, @isdefined, @boundscheck, @inbounds, Generator, IdDict,
     AbstractRange, AbstractUnitRange, UnitRange, LinearIndices, TupleOrBottom,
     (:), |, +, -, *, !==, !, ==, !=, <=, <, >, >=, missing,
     any, _counttuple, eachindex, ntuple, zero, prod, reduce, in, firstindex, lastindex,
@@ -1547,7 +1547,9 @@ Stacktrace:
 [...]
 ```
 """
-@propagate_inbounds function only(x)
+@propagate_inbounds only(x) = _only(x, iterate)
+
+@propagate_inbounds function _only(x, ::typeof(iterate))
     i = iterate(x)
     @boundscheck if i === nothing
         throw(ArgumentError("Collection is empty, must contain exactly 1 element"))
@@ -1559,6 +1561,15 @@ Stacktrace:
     return ret
 end
 
+@inline function _only(x, ::typeof(first))
+    @boundscheck if length(x) != 1
+        throw(ArgumentError("Collection must contain exactly 1 element"))
+    end
+    @inbounds first(x)
+end
+
+@propagate_inbounds only(x::IdDict) = _only(x, first)
+
 # Specific error messages for tuples and named tuples
 only(x::Tuple{Any}) = x[1]
 only(x::Tuple) = throw(

base/set.jl

@@ -175,6 +175,8 @@ rehash!(s::Set) = (rehash!(s.dict); s)
 
 iterate(s::Set, i...)       = iterate(KeySet(s.dict), i...)
 
+@propagate_inbounds Iterators.only(s::Set) = Iterators._only(s, first)
+
 # In case the size(s) is smaller than size(t) its more efficient to iterate through
 # elements of s instead and only delete the ones also contained in t.
 # The threshold for this decision boils down to a tradeoff between

base/strings/basic.jl

@@ -179,6 +179,8 @@ firstindex(s::AbstractString) = 1
 lastindex(s::AbstractString) = thisind(s, ncodeunits(s)::Int)
 isempty(s::AbstractString) = iszero(ncodeunits(s)::Int)
 
+@propagate_inbounds first(s::AbstractString) = s[firstindex(s)]
+
 function getindex(s::AbstractString, i::Integer)
     @boundscheck checkbounds(s, i)
     @inbounds return isvalid(s, i) ? (iterate(s, i)::NTuple{2,Any})[1] : string_index_err(s, i)

base/weakkeydict.jl

@@ -213,4 +213,6 @@ function iterate(t::WeakKeyDict{K,V}, state...) where {K, V}
     end
 end
 
+@propagate_inbounds Iterators.only(d::WeakKeyDict) = Iterators._only(d, first)
+
 filter!(f, d::WeakKeyDict) = filter_in_one_pass!(f, d)