diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index a9adae86b9ef2..f32a1b9d6d40b 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -1659,10 +1659,10 @@ typed_vcat(::Type{T}) where {T} = Vector{T}()
 typed_hcat(::Type{T}) where {T} = Vector{T}()
 
 ## cat: special cases
-vcat(X::T...) where {T}         = T[ X[i] for i=1:length(X) ]
-vcat(X::T...) where {T<:Number} = T[ X[i] for i=1:length(X) ]
-hcat(X::T...) where {T}         = T[ X[j] for i=1:1, j=1:length(X) ]
-hcat(X::T...) where {T<:Number} = T[ X[j] for i=1:1, j=1:length(X) ]
+vcat(X::T...) where {T}         = T[ X[i] for i=eachindex(X) ]
+vcat(X::T...) where {T<:Number} = T[ X[i] for i=eachindex(X) ]
+hcat(X::T...) where {T}         = T[ X[j] for i=1:1, j=eachindex(X) ]
+hcat(X::T...) where {T<:Number} = T[ X[j] for i=1:1, j=eachindex(X) ]
 
 vcat(X::Number...) = hvcat_fill!(Vector{promote_typeof(X...)}(undef, length(X)), X)
 hcat(X::Number...) = hvcat_fill!(Matrix{promote_typeof(X...)}(undef, 1,length(X)), X)
diff --git a/base/errorshow.jl b/base/errorshow.jl
index a75f2f54b156d..fc9fc5c2aac32 100644
--- a/base/errorshow.jl
+++ b/base/errorshow.jl
@@ -196,7 +196,7 @@ function showerror(io::IO, ex::CanonicalIndexError)
     print(io, "CanonicalIndexError: ", ex.func, " not defined for ", ex.type)
 end
 
-typesof(@nospecialize args...) = Tuple{Any[ Core.Typeof(args[i]) for i in 1:length(args) ]...}
+typesof(@nospecialize args...) = Tuple{Any[Core.Typeof(arg) for arg in args]...}
 
 function print_with_compare(io::IO, @nospecialize(a::DataType), @nospecialize(b::DataType), color::Symbol)
     if a.name === b.name
@@ -273,7 +273,7 @@ function showerror(io::IO, ex::MethodError)
         arg_types_param = arg_types_param[3:end]
         san_arg_types_param = san_arg_types_param[3:end]
         keys = kwt.parameters[1]::Tuple
-        kwargs = Any[(keys[i], fieldtype(kwt, i)) for i in 1:length(keys)]
+        kwargs = Any[(keys[i], fieldtype(kwt, i)) for i in eachindex(keys)]
         arg_types = rewrap_unionall(Tuple{arg_types_param...}, arg_types)
     end
     if f === Base.convert && length(arg_types_param) == 2 && !is_arg_types
@@ -694,7 +694,7 @@ function show_reduced_backtrace(io::IO, t::Vector)
 
     push!(repeated_cycle, (0,0,0)) # repeated_cycle is never empty
     frame_counter = 1
-    for i in 1:length(displayed_stackframes)
+    for i in eachindex(displayed_stackframes)
         (frame, n) = displayed_stackframes[i]
 
         print_stackframe(io, frame_counter, frame, n, ndigits_max, STACKTRACE_FIXEDCOLORS, STACKTRACE_MODULECOLORS)
@@ -871,7 +871,7 @@ end
 function _collapse_repeated_frames(trace)
     kept_frames = trues(length(trace))
     last_frame = nothing
-    for i in 1:length(trace)
+    for i in eachindex(trace)
         frame::StackFrame, _ = trace[i]
         if last_frame !== nothing && frame.file == last_frame.file && frame.line == last_frame.line
             #=
@@ -916,7 +916,7 @@ function _collapse_repeated_frames(trace)
                 end
                 if length(last_params) > length(params)
                     issame = true
-                    for i = 1:length(params)
+                    for i = eachindex(params)
                         issame &= params[i] == last_params[i]
                     end
                     if issame
diff --git a/base/expr.jl b/base/expr.jl
index a58ae62c21a5f..8f84883fa2623 100644
--- a/base/expr.jl
+++ b/base/expr.jl
@@ -65,7 +65,7 @@ function copy_exprs(@nospecialize(x))
     end
     return x
 end
-copy_exprargs(x::Array{Any,1}) = Any[copy_exprs(@inbounds x[i]) for i in 1:length(x)]
+copy_exprargs(x::Array{Any,1}) = Any[copy_exprs(@inbounds x[i]) for i in eachindex(x)]
 
 @eval exprarray(head::Symbol, arg::Array{Any,1}) = $(Expr(:new, :Expr, :head, :arg))
 
diff --git a/base/intfuncs.jl b/base/intfuncs.jl
index 6df5b2c36124b..13ac4c9bfaf48 100644
--- a/base/intfuncs.jl
+++ b/base/intfuncs.jl
@@ -978,7 +978,7 @@ end
 
 Return an array with element type `T` (default `Int`) of the digits of `n` in the given
 base, optionally padded with zeros to a specified size. More significant digits are at
-higher indices, such that `n == sum(digits[k]*base^(k-1) for k=1:length(digits))`.
+higher indices, such that `n == sum(digits[k]*base^(k-1) for k in eachindex(digits))`.
 
 See also [`ndigits`](@ref), [`digits!`](@ref),
 and for base 2 also [`bitstring`](@ref), [`count_ones`](@ref).
diff --git a/base/loading.jl b/base/loading.jl
index 20946969f0238..bd985e431e497 100644
--- a/base/loading.jl
+++ b/base/loading.jl
@@ -1214,7 +1214,7 @@ function _include_from_serialized(pkg::PkgId, path::String, ocachepath::Union{No
         t_comp_before = cumulative_compile_time_ns()
     end
 
-    for i in 1:length(depmods)
+    for i in eachindex(depmods)
         dep = depmods[i]
         dep isa Module && continue
         _, depkey, depbuild_id = dep::Tuple{String, PkgId, UInt128}
@@ -1706,8 +1706,7 @@ function compilecache_path(pkg::PkgId;
         end
         staledeps, _, _ = staledeps::Tuple{Vector{Any}, Union{Nothing, String}, UInt128}
         # finish checking staledeps module graph
-        for i in 1:length(staledeps)
-            dep = staledeps[i]
+        for dep in staledeps
             dep isa Module && continue
             modpath, modkey, modbuild_id = dep::Tuple{String, PkgId, UInt128}
             modpaths = find_all_in_cache_path(modkey)
@@ -1897,7 +1896,7 @@ end
         try
             staledeps, ocachefile, newbuild_id = staledeps::Tuple{Vector{Any}, Union{Nothing, String}, UInt128}
             # finish checking staledeps module graph
-            for i in 1:length(staledeps)
+            for i in eachindex(staledeps)
                 dep = staledeps[i]
                 dep isa Module && continue
                 modpath, modkey, modbuild_id = dep::Tuple{String, PkgId, UInt128}
@@ -1929,7 +1928,7 @@ end
             end
             # finish loading module graph into staledeps
             # TODO: call all start_loading calls (in reverse order) before calling any _include_from_serialized, since start_loading will drop the loading lock
-            for i in 1:length(staledeps)
+            for i in eachindex(staledeps)
                 dep = staledeps[i]
                 dep isa Module && continue
                 modpath, modkey, modbuild_id, modcachepath, modstaledeps, modocachepath = dep::Tuple{String, PkgId, UInt128, String, Vector{Any}, Union{Nothing, String}}
diff --git a/base/multidimensional.jl b/base/multidimensional.jl
index b4625c12620e3..bd3641db4999c 100644
--- a/base/multidimensional.jl
+++ b/base/multidimensional.jl
@@ -1674,7 +1674,7 @@ function checkdims_perm(P::AbstractArray{TP,N}, B::AbstractArray{TB,N}, perm) wh
     length(perm) == N || throw(ArgumentError("expected permutation of size $N, but length(perm)=$(length(perm))"))
     isperm(perm) || throw(ArgumentError("input is not a permutation"))
     indsP = axes(P)
-    for i = 1:length(perm)
+    for i in eachindex(perm)
         indsP[i] == indsB[perm[i]] || throw(DimensionMismatch("destination tensor of incorrect size"))
     end
     nothing
diff --git a/base/reflection.jl b/base/reflection.jl
index 0af40a2e481a0..7f9772e5ec976 100644
--- a/base/reflection.jl
+++ b/base/reflection.jl
@@ -2425,7 +2425,7 @@ function hasmethod(f, t, kwnames::Tuple{Vararg{Symbol}}; world::UInt=get_world_c
     for kw in kws
         endswith(String(kw), "...") && return true
     end
-    kwnames = Symbol[kwnames[i] for i in 1:length(kwnames)]
+    kwnames = collect(kwnames)
     return issubset(kwnames, kws)
 end
 
diff --git a/base/show.jl b/base/show.jl
index 8db2826a2088c..cd8107f8c6125 100644
--- a/base/show.jl
+++ b/base/show.jl
@@ -681,7 +681,7 @@ function show_can_elide(p::TypeVar, wheres::Vector, elide::Int, env::SimpleVecto
         has_typevar(v.lb, p) && return false
         has_typevar(v.ub, p) && return false
     end
-    for i = 1:length(env)
+    for i = eachindex(env)
         i == skip && continue
         has_typevar(env[i], p) && return false
     end
@@ -1188,7 +1188,7 @@ end
 
 function show_at_namedtuple(io::IO, syms::Tuple, types::DataType)
     first = true
-    for i in 1:length(syms)
+    for i in eachindex(syms)
         if !first
             print(io, ", ")
         end
@@ -2382,7 +2382,7 @@ function show_unquoted(io::IO, ex::Expr, indent::Int, prec::Int, quote_level::In
         if get(io, beginsym, false)
             print(io, '(')
             ind = indent + indent_width
-            for i = 1:length(ex.args)
+            for i = eachindex(ex.args)
                 if i > 1
                     # if there was only a comment before the first semicolon, the expression would get parsed as a NamedTuple
                     if !(i == 2 && ex.args[1] isa LineNumberNode)
@@ -2904,7 +2904,7 @@ function dump(io::IOContext, x::SimpleVector, n::Int, indent)
     end
     print(io, "SimpleVector")
     if n > 0
-        for i = 1:length(x)
+        for i in eachindex(x)
             println(io)
             print(io, indent, "  ", i, ": ")
             if isassigned(x,i)
@@ -3021,7 +3021,7 @@ function dump(io::IOContext, x::DataType, n::Int, indent)
         end
         fields = fieldnames(x)
         fieldtypes = datatype_fieldtypes(x)
-        for idx in 1:length(fields)
+        for idx in eachindex(fields)
             println(io)
             print(io, indent, "  ")
             is_mut && isconst(x, idx) && print(io, "const ")
diff --git a/base/tuple.jl b/base/tuple.jl
index cc22ea75c041c..fc213410cfd7c 100644
--- a/base/tuple.jl
+++ b/base/tuple.jl
@@ -503,7 +503,7 @@ end
 _findfirst_rec(f, i::Int, ::Tuple{}) = nothing
 _findfirst_rec(f, i::Int, t::Tuple) = (@inline; f(first(t)) ? i : _findfirst_rec(f, i+1, tail(t)))
 function _findfirst_loop(f::Function, t)
-    for i in 1:length(t)
+    for i in eachindex(t)
         f(t[i]) && return i
     end
     return nothing
@@ -537,7 +537,7 @@ function _isequal(t1::Tuple{Any,Vararg{Any}}, t2::Tuple{Any,Vararg{Any}})
     return isequal(t1[1], t2[1]) && _isequal(tail(t1), tail(t2))
 end
 function _isequal(t1::Any32, t2::Any32)
-    for i = 1:length(t1)
+    for i in eachindex(t1, t2)
         if !isequal(t1[i], t2[i])
             return false
         end
@@ -568,7 +568,7 @@ function _eq_missing(t1::Tuple, t2::Tuple)
 end
 function _eq(t1::Any32, t2::Any32)
     anymissing = false
-    for i = 1:length(t1)
+    for i in eachindex(t1, t2)
         eq = (t1[i] == t2[i])
         if ismissing(eq)
             anymissing = true