Implement a first-class error for reexported component functions (byt…

…ecodealliance#4348)

Currently I don't know how we can reasonably implement this. Given all
the signatures of how we call functions and how functions are called on
the host there's no real feasible way that I know of to hook these two
up "seamlessly". This means that a component which reexports an imported
function can't be run in Wasmtime.

One of the main reasons for this is that when calling a component
function Wasmtime wants to lower arguments first and then have them
lifted when the host is called. With a reexport though there's not
actually anything to lower into so we'd sort of need something similar
to a table on the side or maybe a linear memory and that seems like it'd
get quite complicated quite quickly for not really all that much
benefit. As-such for now this simply returns a first-class error (rather
than the current panic) in situations like this.

crates/environ/src/component/translate/inline.rs

@@ -141,7 +141,9 @@ pub(super) fn run(
                 ComponentFuncDef::Lifted { ty, func, options } => {
                     Export::LiftedFunction { ty, func, options }
                 }
-                ComponentFuncDef::Import(_) => unimplemented!("reexporting a function import"),
+                ComponentFuncDef::Import(_) => {
+                    bail!("component export `{name}` is a reexport of an imported function which is not implemented")
+                }
             },
 
             ComponentItemDef::Instance(_) => unimplemented!("exporting an instance to the host"),

tests/misc_testsuite/component-model/import.wast

@@ -0,0 +1,5 @@
+(assert_invalid
+  (component
+    (import "host-return-two" (func $f (result u32)))
+    (export "x" (func $f)))
+  "component export `x` is a reexport of an imported function which is not implemented")