Allow ModuleTranslationState to be constructed with signatures.

This is needed to allow SpiderMonkey to provide function signature types
to the wasm translator when it uses Cranelift as a backend without
using the wasm translator to parse the entire module. There is perhaps a
better long-term design here where we allow an embedding that already
parses the Wasm module (such as SpiderMonkey) to provide information in
a more principled way, this suffices for now.

Patch is inspired by Ben's patch in
https://bugzilla.mozilla.org/show_bug.cgi?id=1641504, but does not
expose `wasmparser` types directly, instead using Cranelift types across
the API boundary.

cranelift/wasm/src/state/module_state.rs

@@ -1,6 +1,9 @@
+use crate::environ::{WasmError, WasmResult};
 use crate::translation_utils::SignatureIndex;
+use cranelift_codegen::ir::{types, Type};
 use cranelift_entity::PrimaryMap;
 use std::boxed::Box;
+use std::vec::Vec;
 
 /// Map of signatures to a function's parameter and return types.
 pub(crate) type WasmTypes =
@@ -21,6 +24,21 @@ pub struct ModuleTranslationState {
     pub(crate) wasm_types: WasmTypes,
 }
 
+fn cranelift_to_wasmparser_type(ty: Type) -> WasmResult<wasmparser::Type> {
+    Ok(match ty {
+        types::I32 => wasmparser::Type::I32,
+        types::I64 => wasmparser::Type::I64,
+        types::F32 => wasmparser::Type::F32,
+        types::F64 => wasmparser::Type::F64,
+        _ => {
+            return Err(WasmError::Unsupported(format!(
+                "Cannot convert Cranelift type to Wasm signature: {:?}",
+                ty
+            )));
+        }
+    })
+}
+
 impl ModuleTranslationState {
     /// Creates a new empty ModuleTranslationState.
     pub fn new() -> Self {
@@ -28,4 +46,24 @@ impl ModuleTranslationState {
             wasm_types: PrimaryMap::new(),
         }
     }
+
+    /// Create a new ModuleTranslationState with the given function signatures,
+    /// provided in terms of Cranelift types. The provided slice of signatures
+    /// is indexed by signature number, and contains pairs of (args, results)
+    /// slices.
+    pub fn from_func_sigs(sigs: &[(&[Type], &[Type])]) -> WasmResult<Self> {
+        let mut wasm_types = PrimaryMap::with_capacity(sigs.len());
+        for &(ref args, ref results) in sigs {
+            let args: Vec<wasmparser::Type> = args
+                .iter()
+                .map(|&ty| cranelift_to_wasmparser_type(ty))
+                .collect::<Result<_, _>>()?;
+            let results: Vec<wasmparser::Type> = results
+                .iter()
+                .map(|&ty| cranelift_to_wasmparser_type(ty))
+                .collect::<Result<_, _>>()?;
+            wasm_types.push((args.into_boxed_slice(), results.into_boxed_slice()));
+        }
+        Ok(Self { wasm_types })
+    }
 }