deprecate const on local variables, which used to be ignored.

part of #5148

NEWS.md

@@ -63,6 +63,9 @@ Language changes
     (`need_to_handle_undef_sparam = Set{Any}(m.sig for m in Test.detect_unbound_args(Base, recursive=true))`)
     is equal (`==`) to some known set (`expected = Set()`). ([#23117])
 
+  * `const` declarations on local variables were previously ignored. They now give a
+    warning, so that this syntax can be disallowed or given a new meaning in a
+    future version ([#5148]).
 
 Breaking changes
 ----------------

doc/src/manual/variables-and-scoping.md

@@ -475,7 +475,8 @@ their values (or even their types) might change at almost any time. If a global
 not change, adding a `const` declaration solves this performance problem.
 
 Local constants are quite different. The compiler is able to determine automatically when a local
-variable is constant, so local constant declarations are not necessary, and are currently just ignored.
+variable is constant, so local constant declarations are not necessary, and in fact are currently
+not supported.
 
 Special top-level assignments, such as those performed by the `function` and `struct` keywords,
 are constant by default.

src/julia-syntax.scm

@@ -3115,11 +3115,7 @@ f(x) = yt(x)
                  (if (vinfo:never-undef vi)
                      '(null)
                      `(newvar ,(cadr e))))))
-          ((const)
-           (if (or (assq (cadr e) (car  (lam:vinfo lam)))
-                   (assq (cadr e) (cadr (lam:vinfo lam))))
-               '(null)
-               e))
+          ((const) e)
           ((isdefined) ;; convert isdefined expr to function for closure converted variables
            (let* ((sym (cadr e))
                   (vi (and (symbol? sym) (assq sym (car  (lam:vinfo lam)))))
@@ -3661,7 +3657,14 @@ f(x) = yt(x)
             ((local-def) #f)
             ((local) #f)
             ((implicit-global) #f)
-            ((const) (emit e))
+            ((const)
+             (if (or (assq (cadr e) (car  (lam:vinfo lam)))
+                     (assq (cadr e) (cadr (lam:vinfo lam))))
+                 (begin
+                   (syntax-deprecation #f (string "`const` declaration on local variable" (linenode-string current-loc))
+                                       "")
+                   '(null))
+                 (emit e)))
             ((isdefined) (if tail (emit-return e) e))
 
             ;; top level expressions returning values

test/TestHelpers.jl

@@ -24,8 +24,8 @@ function open_fake_pty()
         error("Unable to create a fake PTY in Windows")
     end
 
-    const O_RDWR = Base.Filesystem.JL_O_RDWR
-    const O_NOCTTY = Base.Filesystem.JL_O_NOCTTY
+    O_RDWR = Base.Filesystem.JL_O_RDWR
+    O_NOCTTY = Base.Filesystem.JL_O_NOCTTY
 
     fdm = ccall(:posix_openpt, Cint, (Cint,), O_RDWR|O_NOCTTY)
     fdm == -1 && error("Failed to open PTY master")

test/core.jl

@@ -438,7 +438,8 @@ function const_implies_local()
         x = 1
         local y
         let
-            const x = 0
+            # TODO: change back to `const` if that's ever allowed
+            local x = 0
             y = x
         end
         x, y

test/dates/io.jl

@@ -378,7 +378,7 @@ end
 
 # Issue: https://github.com/quinnj/TimeZones.jl/issues/19
 let
-    const Zulu = String
+    Zulu = String
 
     function Dates.tryparsenext(d::Dates.DatePart{'Z'}, str, i, len)
         Dates.tryparsenext_word(str, i, len, Dates.min_width(d), Dates.max_width(d))

test/misc.jl

@@ -576,7 +576,7 @@ if Sys.iswindows()
 
     let addr = cfunction(WeVirtualProtectThisToRWX, UInt64, (UInt64, UInt64))
         addr = addr-(UInt64(addr)%4096)
-        const PAGE_EXECUTE_READWRITE = 0x40
+        PAGE_EXECUTE_READWRITE = 0x40
         oldPerm = Ref{UInt32}()
         err18083 = ccall(:VirtualProtect,stdcall,Cint,
             (Ptr{Void}, Csize_t, UInt32, Ptr{UInt32}),

test/repl.jl

@@ -232,7 +232,7 @@ function AddCustomMode(repl, prompt)
     hp.mode_mapping[:foobar] = foobar_mode
     foobar_mode.hist = hp
 
-    const foobar_keymap = Dict{Any,Any}(
+    foobar_keymap = Dict{Any,Any}(
         '<' => function (s,args...)
             if isempty(s)
                 if !haskey(s.mode_state,foobar_mode)

test/replcompletions.jl

@@ -76,7 +76,7 @@ function temp_pkg_dir_noinit(fn::Function)
     # Used in tests below to set up and tear down a sandboxed package directory
     # Unlike the version in test/pkg.jl, this does not run Pkg.init so does not
     # clone METADATA (only pkg and libgit2-online tests should need internet access)
-    const tmpdir = joinpath(tempdir(),randstring())
+    tmpdir = joinpath(tempdir(),randstring())
     withenv("JULIA_PKGDIR" => tmpdir) do
         @test !isdir(Pkg.dir())
         try

test/subtype.jl

@@ -1094,12 +1094,12 @@ f20103(::Type{TT20103{X,Y}},x::X,y::Y) where {X,Y} = 1
 f20103(::Type{TT20103{X,X}},x::X) where {X} = 100
 @test_broken typeintersect(Type{NTuple{N,E}} where E where N, Type{NTuple{N,E} where N} where E) == Union{} # use @testintersect once fixed
 let ints = (Int, Int32, UInt, UInt32)
-    const Ints = Union{ints...}
+    Ints = Union{ints...}
     vecs = []
     for i = 2:4, t in ints
         push!(vecs, NTuple{i, t})
     end
-    const Vecs = Union{vecs...}
+    Vecs = Union{vecs...}
     T = Type{Tuple{V, I}} where V <: Vecs where I <: Ints
     @testintersect(T, T, T)
     test(a::Type{Tuple{V, I}}) where {V <: Vecs, I <: Ints} = I