Fix invalid let syntax from LHS views (#53108)

When the frontend lowers an expression like `lhs .+= rhs`, it needs to
prevent evaluating the LHS more than once before re-writing to `lhs .=
lhs .+ rhs`. If the LHS was a `let` block — commonly generated by
`@views` and (since #53064) `@view` — the lowering pass had previously
been emitting an invalid `let` temporary. This very directly addresses
that case.

Fixes #53107.
Fixes #44356.

src/julia-syntax.scm

@@ -1653,8 +1653,10 @@
                  (let ((g (make-ssavalue)))
                    (begin (set! a (cons `(= ,g ,x) a))
                           g)))))
-        (cons (cons (car e) (map arg-to-temp (cdr e)))
-              (reverse a)))))
+        (if (eq? (car e) 'let)
+          (cons (arg-to-temp e) (reverse a))
+          (cons (cons (car e) (map arg-to-temp (cdr e)))
+                (reverse a))))))
 
 (define (lower-kw-call f args)
   (let* ((para (if (has-parameters? args) (cdar args) '()))

test/subarray.jl

@@ -637,6 +637,44 @@ end
         @test foo == [X, X]
     end
 
+    # Test as an assignment's left hand side
+    let x = [1,2,3,4]
+        @test Meta.@lower(@view(x[1]) = 1).head == :error
+        @test Meta.@lower(@view(x[1]) += 1).head == :error
+        @test Meta.@lower(@view(x[end]) = 1).head == :error
+        @test Meta.@lower(@view(x[end]) += 1).head == :error
+        @test Meta.@lower(@view(f(x)[end]) = 1).head == :error
+        @test Meta.@lower(@view(f(x)[end]) += 1).head == :error
+        @test (@view(x[1]) .+= 1) == fill(2)
+        @test x == [2,2,3,4]
+        @test (@view(reshape(x,2,2)[1,1]) .+= 10) == fill(12)
+        @test x == [12,2,3,4]
+        @test (@view(x[end]) .+= 1) == fill(5)
+        @test x == [12,2,3,5]
+        @test (@view(reshape(x,2,2)[end]) .+= 10) == fill(15)
+        @test x == [12,2,3,15]
+        @test (@view(reshape(x,2,2)[[begin],[begin,end]])::AbstractMatrix{Int} .+= [2]) == [14 5]
+        @test x == [14,2,5,15]
+
+        x = [1,2,3,4]
+        @test Meta.@lower(@views(x[[1]]) = 1).head == :error
+        @test Meta.@lower(@views(x[[1]]) += 1).head == :error
+        @test Meta.@lower(@views(x[[end]]) = 1).head == :error
+        @test Meta.@lower(@views(x[[end]]) += 1).head == :error
+        @test Meta.@lower(@views(f(x)[end]) = 1).head == :error
+        @test Meta.@lower(@views(f(x)[end]) += 1).head == :error
+        @test (@views(x[[1]]) .+= 1) == [2]
+        @test x == [2,2,3,4]
+        @test (@views(reshape(x,2,2)[[1],1]) .+= 10) == [12]
+        @test x == [12,2,3,4]
+        @test (@views(x[[end]]) .+= 1) == [5]
+        @test x == [12,2,3,5]
+        @test (@views(reshape(x,2,2)[[end]]) .+= 10) == [15]
+        @test x == [12,2,3,15]
+        @test (@views(reshape(x,2,2)[[begin],[begin,end]])::AbstractMatrix{Int} .+= [2]) == [14 5]
+        @test x == [14,2,5,15]
+    end
+
     # test @views macro
     @views let f!(x) = x[begin:end-1] .+= x[begin+1:end].^2
         x = [1,2,3,4]
@@ -663,6 +701,12 @@ end
         @test x == [5,8,12,9] && i == [4,3]
         @. x[3:end] = 0       # make sure @. works with end expressions in @views
         @test x == [5,8,0,0]
+        x[begin:end] .+= 1
+        @test x == [6,9,1,1]
+        x[[begin,2,end]] .-= [1,2,3]
+        @test x == [5,7,1,-2]
+        @. x[[begin,2,end]] .+= [1,2,3]
+        @test x == [6,9,1,1]
     end
     @views @test isa(X[1:3], SubArray)
     @test X[begin:end] == @views X[begin:end]

test/syntax.jl

@@ -3619,3 +3619,21 @@ end
     @test p("public[7] = 5") == Expr(:(=), Expr(:ref, :public, 7), 5)
     @test p("public() = 6") == Expr(:(=), Expr(:call, :public), Expr(:block, 6))
 end
+
+@testset "removing argument sideeffects" begin
+    # Allow let blocks in broadcasted LHSes, but only evaluate them once:
+    execs = 0
+    array = [1]
+    let x = array; execs += 1; x; end .+= 2
+    @test array == [3]
+    @test execs == 1
+    let; execs += 1; array; end .= 4
+    @test array == [4]
+    @test execs == 2
+    let x = array; execs += 1; x; end::Vector{Int} .+= 2
+    @test array == [6]
+    @test execs == 3
+    let; execs += 1; array; end::Vector{Int} .= 7
+    @test array == [7]
+    @test execs == 4
+end