Make diagnostics that contain type names more user-friendly and clear

[winterqt]

Please describe the bug

Take the following on top of #1007:

diff --git a/std/src/std/uri.inko b/std/src/std/uri.inko
index 3ddc8d240a..ecc76c7377 100644
--- a/std/src/std/uri.inko
+++ b/std/src/std/uri.inko
@@ -1188,7 +1188,7 @@
   #
   # Path.from_slice(comp.slice(0, comp.size - 1).iter) # => Path('a/b/%20')
   # ```
-  fn pub static from_iter[B: Bytes, I: Iter[B]](iter: move I) -> Path {
+  fn pub static from_iter[B: Bytes](iter: move Iter[B]) -> Path {
     let buf = ByteArray.new
     let mut len = 0
 

test/std/test_uri.inko:346:28 error(invalid-type): expected a value of type 'Iter[B]', found 'Iter[String]'

This should instead say something along the lines of traits are forbidden here, did you mean to put it in the generic bounds?. I found it deeply confusing until I realized what was actually going on here. 😅

Operating system

macOS

Inko version

cf08927

[yorickpeterse]

When you use iter: move Iter[B] what that means is that iter is some value boxed as an Iter trait, for which all calls use dynamic dispatch. This is basically Box<dyn SomeTrait> from Rust. Acquiring such a value requires an explicit cast, and only works if the input value is a heap allocated value.

This approach is something you'd only use in specific cases, but not here.

As for the error: what's happening here is that the Iter[String] is a std.array.Iter[String] which is a concrete type, while the expected type is a Iter[B] boxed trait. This is indeed rather confusing due to the matching names, but is a more general problem, as we don't include namespaces in types in diagnostic messages anywhere. For example, if you pass a Foo from module A to something in module B that expects a different Foo, you'd get a diagnostic along the lines of "expected a value of type Foo but found a value of type Foo", which is indeed confusing.

Better diagnostics is definitely something we need to work on 😄