[wasmtime-api] Record original Trap from API callback. (#657)

* Record original Trap from API callback.

Fixes #645

* use TrapRegistry

* comment about magic number

crates/api/src/trampoline/trap.rs

@@ -0,0 +1,54 @@
+use std::cell::Cell;
+
+use super::binemit;
+use super::ir::{SourceLoc, TrapCode};
+use crate::r#ref::HostRef;
+use crate::Trap;
+use wasmtime_environ::TrapInformation;
+
+// Randomly selected user TrapCode magic number 13.
+pub const API_TRAP_CODE: TrapCode = TrapCode::User(13);
+
+thread_local! {
+    static RECORDED_API_TRAP: Cell<Option<HostRef<Trap>>> = Cell::new(None);
+}
+
+pub fn record_api_trap(trap: HostRef<Trap>) {
+    RECORDED_API_TRAP.with(|data| {
+        let trap = Cell::new(Some(trap));
+        data.swap(&trap);
+        assert!(
+            trap.take().is_none(),
+            "Only one API trap per thread can be recorded at a moment!"
+        );
+    });
+}
+
+pub fn take_api_trap() -> Option<HostRef<Trap>> {
+    RECORDED_API_TRAP.with(|data| data.take())
+}
+
+pub(crate) struct TrapSink {
+    pub traps: Vec<TrapInformation>,
+}
+
+impl TrapSink {
+    pub fn new() -> Self {
+        Self { traps: Vec::new() }
+    }
+}
+
+impl binemit::TrapSink for TrapSink {
+    fn trap(
+        &mut self,
+        code_offset: binemit::CodeOffset,
+        source_loc: SourceLoc,
+        trap_code: TrapCode,
+    ) {
+        self.traps.push(TrapInformation {
+            code_offset,
+            source_loc,
+            trap_code,
+        });
+    }
+}