Add TargetIsa::as_any for downcasting to specific ISA implementations

This is necessary when we would like to check specific ISA flags, e.g.

cranelift/codegen/src/isa/arm32/mod.rs

@@ -17,6 +17,7 @@ use crate::isa::{EncInfo, RegClass, RegInfo, TargetIsa};
 use crate::regalloc;
 use alloc::borrow::Cow;
 use alloc::boxed::Box;
+use core::any::Any;
 use core::fmt;
 use target_lexicon::{Architecture, Triple};
 
@@ -135,6 +136,10 @@ impl TargetIsa for Isa {
     fn unsigned_sub_overflow_condition(&self) -> ir::condcodes::IntCC {
         ir::condcodes::IntCC::UnsignedGreaterThanOrEqual
     }
+
+    fn as_any(&self) -> &dyn Any {
+        self as &dyn Any
+    }
 }
 
 impl fmt::Display for Isa {

cranelift/codegen/src/isa/mod.rs

@@ -66,6 +66,7 @@ use crate::settings::SetResult;
 use crate::timing;
 use alloc::borrow::Cow;
 use alloc::boxed::Box;
+use core::any::Any;
 use core::fmt;
 use core::fmt::{Debug, Formatter};
 use target_lexicon::{triple, Architecture, PointerWidth, Triple};
@@ -422,6 +423,10 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     fn get_mach_backend(&self) -> Option<&dyn MachBackend> {
         None
     }
+
+    /// Return an [Any] reference for downcasting to the ISA-specific implementation of this trait
+    /// with `isa.as_any().downcast_ref::<isa::foo::Isa>()`.
+    fn as_any(&self) -> &dyn Any;
 }
 
 impl Debug for &dyn TargetIsa {

cranelift/codegen/src/isa/riscv/mod.rs

@@ -17,6 +17,7 @@ use crate::isa::{EncInfo, RegClass, RegInfo, TargetIsa};
 use crate::regalloc;
 use alloc::borrow::Cow;
 use alloc::boxed::Box;
+use core::any::Any;
 use core::fmt;
 use target_lexicon::{PointerWidth, Triple};
 
@@ -130,6 +131,10 @@ impl TargetIsa for Isa {
     fn unsigned_sub_overflow_condition(&self) -> ir::condcodes::IntCC {
         unimplemented!()
     }
+
+    fn as_any(&self) -> &dyn Any {
+        self as &dyn Any
+    }
 }
 
 #[cfg(test)]

cranelift/codegen/src/isa/x86/mod.rs

@@ -23,6 +23,7 @@ use crate::result::CodegenResult;
 use crate::timing;
 use alloc::borrow::Cow;
 use alloc::boxed::Box;
+use core::any::Any;
 use core::fmt;
 use target_lexicon::{PointerWidth, Triple};
 
@@ -184,6 +185,10 @@ impl TargetIsa for Isa {
     fn create_systemv_cie(&self) -> Option<gimli::write::CommonInformationEntry> {
         Some(unwind::systemv::create_cie())
     }
+
+    fn as_any(&self) -> &dyn Any {
+        self as &dyn Any
+    }
 }
 
 impl fmt::Display for Isa {

cranelift/codegen/src/machinst/adapter.rs

@@ -10,6 +10,7 @@ use crate::settings::Flags;
 #[cfg(feature = "testing_hooks")]
 use crate::regalloc::RegDiversions;
 
+use core::any::Any;
 use std::borrow::Cow;
 use std::fmt;
 use target_lexicon::Triple;
@@ -127,4 +128,8 @@ impl TargetIsa for TargetIsaAdapter {
     fn unsigned_sub_overflow_condition(&self) -> ir::condcodes::IntCC {
         self.backend.unsigned_sub_overflow_condition()
     }
+
+    fn as_any(&self) -> &dyn Any {
+        self as &dyn Any
+    }
 }