better error message for rpad/lpad with zero-width padding (#56488)

Closes #45339 — throw a more informative `ArgumentError` message from
`rpad` and `lpad` if a zero-`textwidth` padding is passed (not a
`DivideError`).

If the padding character has `ncodeunits == 1`, suggests that maybe they
want `str * pad^max(0, npad - ncodeunits(str))` instead.

base/strings/util.jl

@@ -476,7 +476,12 @@ function lpad(
     n = Int(n)::Int
     m = signed(n) - Int(textwidth(s))::Int
     m ≤ 0 && return stringfn(s)
-    l = textwidth(p)
+    l = Int(textwidth(p))::Int
+    if l == 0
+        throw(ArgumentError("$(repr(p)) has zero textwidth" * (ncodeunits(p) != 1 ? "" :
+            "; maybe you want pad^max(0, npad - ncodeunits(str)) * str to pad by codeunits" *
+            (s isa AbstractString && codeunit(s) != UInt8 ? "?" : " (bytes)?"))))
+    end
     q, r = divrem(m, l)
     r == 0 ? stringfn(p^q, s) : stringfn(p^q, first(p, r), s)
 end
@@ -508,7 +513,12 @@ function rpad(
     n = Int(n)::Int
     m = signed(n) - Int(textwidth(s))::Int
     m ≤ 0 && return stringfn(s)
-    l = textwidth(p)
+    l = Int(textwidth(p))::Int
+    if l == 0
+        throw(ArgumentError("$(repr(p)) has zero textwidth" * (ncodeunits(p) != 1 ? "" :
+            "; maybe you want str * pad^max(0, npad - ncodeunits(str)) to pad by codeunits" *
+            (s isa AbstractString && codeunit(s) != UInt8 ? "?" : " (bytes)?"))))
+    end
     q, r = divrem(m, l)
     r == 0 ? stringfn(s, p^q) : stringfn(s, p^q, first(p, r))
 end

test/strings/util.jl

@@ -65,6 +65,13 @@ end
     @test rpad("⟨k|H₁|k̃⟩", 12) |> textwidth == 12
     @test lpad("⟨k|H₁|k⟩", 12) |> textwidth == 12
     @test rpad("⟨k|H₁|k⟩", 12) |> textwidth == 12
+    for pad in (rpad, lpad), p in ('\0', "\0", "\0\0", "\u302")
+        if ncodeunits(p) == 1
+            @test_throws r".*has zero textwidth.*maybe you want.*bytes.*" pad("foo", 10, p)
+        else
+            @test_throws r".*has zero textwidth$" pad("foo", 10, p)
+        end
+    end
 end
 
 @testset "string truncation (ltruncate, rtruncate, ctruncate)" begin