Add a hint when failing to import a module in cases where a user might want to use a relative import (#40984)

* Add a hint when failing to import a module in cases where a user might want to use a relative import

Author: KristofferC

base/loading.jl

@@ -975,10 +975,20 @@ function require(into::Module, mod::Symbol)
         if uuidkey === nothing
             where = PkgId(into)
             if where.uuid === nothing
+                hint, dots = begin
+                    if isdefined(into, mod) && getfield(into, mod) isa Module
+                        true, "."
+                    elseif isdefined(parentmodule(into), mod) && getfield(parentmodule(into), mod) isa Module
+                        true, ".."
+                    else
+                        false, ""
+                    end
+                end
+                hint_message = hint ? ", maybe you meant `import/using $(dots)$(mod)`" : ""
+                start_sentence = hint ? "Otherwise, run" : "Run"
                 throw(ArgumentError("""
-                    Package $mod not found in current path:
-                    - Run `import Pkg; Pkg.add($(repr(String(mod))))` to install the $mod package.
-                    """))
+                    Package $mod not found in current path$hint_message.
+                    - $start_sentence `import Pkg; Pkg.add($(repr(String(mod))))` to install the $mod package."""))
             else
                 s = """
                 Package $(where.name) does not have $mod in its dependencies:

test/errorshow.jl

@@ -803,3 +803,86 @@ if Sys.isapple() || (Sys.islinux() && Sys.ARCH === :x86_64)
         end
     end
 end  # Sys.isapple()
+
+@testset "error message hints relative modules #40959" begin
+    m = Module()
+    expr = :(module Foo
+        module Bar
+        end
+
+        using Bar
+    end)
+    try
+        Base.eval(m, expr)
+    catch err
+        err_str = sprint(showerror, err)
+        @test contains(err_str, "maybe you meant `import/using .Bar`")
+    end
+
+    m = Module()
+    expr = :(module Foo
+        Bar = 3
+
+        using Bar
+    end)
+    try
+        Base.eval(m, expr)
+    catch err
+        err_str = sprint(showerror, err)
+        @test !contains(err_str, "maybe you meant `import/using .Bar`")
+    end
+
+    m = Module()
+    expr = :(module Foo
+        using Bar
+    end)
+    try
+        Base.eval(m, expr)
+    catch err
+        err_str = sprint(showerror, err)
+        @test !contains(err_str, "maybe you meant `import/using .Bar`")
+    end
+
+    m = Module()
+    expr = :(module Foo
+        module Bar end
+        module Buzz
+            using Bar
+        end
+    end)
+    try
+        Base.eval(m, expr)
+    catch err
+        err_str = sprint(showerror, err)
+        @test contains(err_str, "maybe you meant `import/using ..Bar`")
+    end
+
+    m = Module()
+    expr = :(module Foo
+        Bar = 3
+        module Buzz
+            using Bar
+        end
+    end)
+    try
+        Base.eval(m, expr)
+    catch err
+        err_str = sprint(showerror, err)
+        @test !contains(err_str, "maybe you meant `import/using ..Bar`")
+    end
+
+    m = Module()
+    expr = :(module Foo
+        module Bar end
+        module Buzz
+            module Bar end
+            using Bar
+        end
+    end)
+    try
+        Base.eval(m, expr)
+    catch err
+        err_str = sprint(showerror, err)
+        @test contains(err_str, "maybe you meant `import/using .Bar`")
+    end
+end