ISLE: add support for extended left-hand sides with if-let clauses. (#4072)

This PR adds support for `if-let` clauses, as proposed in
bytecodealliance/rfcs#21. These clauses augment the left-hand side
(pattern-matching phase) of rules in the ISLE instruction-selection DSL
with sub-patterns matching on sub-expressions. The ability to list
additional match operations to perform, out-of-line from the original
toplevel pattern, greatly simplifies some idioms. See the RFC for more
details and examples of use.

cranelift/isle/isle/isle_examples/fail/impure_expression.isle

@@ -0,0 +1,10 @@
+(type u32 (primitive u32))
+
+(decl ctor (u32) u32)
+(rule (ctor x) x)
+
+(decl entry (u32) u32)
+
+(rule (entry x)
+      (if-let y (ctor x))
+      y)

cranelift/isle/isle/isle_examples/fail/impure_rhs.isle

@@ -0,0 +1,15 @@
+(type u32 (primitive u32))
+
+(decl pure ctor (u32) u32)
+(decl impure (u32) u32)
+
+(decl entry (u32) u32)
+
+(rule (entry x)
+      (if-let y (ctor x))
+      y)
+
+(rule (ctor x)
+      (impure x))
+
+(rule (impure x) x)

cranelift/isle/isle/isle_examples/link/borrows.isle

@@ -0,0 +1,15 @@
+(type u32 (primitive u32))
+(type A extern (enum (B (x u32) (y u32))))
+
+(decl get_a (A) u32)
+(extern extractor get_a get_a)
+
+(decl pure u32_pure (u32) u32)
+(extern constructor u32_pure u32_pure)
+
+(decl entry (u32) u32)
+(rule (entry x @ (get_a a1))
+      (if-let (get_a a2) x)
+      (if-let (A.B p q) a1)
+      (if-let r (u32_pure p))
+      r)

cranelift/isle/isle/isle_examples/link/borrows_main.rs

@@ -0,0 +1,21 @@
+mod borrows;
+
+#[derive(Clone)]
+pub enum A {
+    B { x: u32, y: u32 },
+}
+
+struct Context(A);
+impl borrows::Context for Context {
+    fn get_a(&mut self, _: u32) -> Option<A> {
+        Some(self.0.clone())
+    }
+
+    fn u32_pure(&mut self, value: u32) -> Option<u32> {
+        Some(value + 1)
+    }
+}
+
+fn main() {
+    borrows::constructor_entry(&mut Context(A::B { x: 1, y: 2 }), 42);
+}

cranelift/isle/isle/isle_examples/link/iflets.isle

@@ -0,0 +1,29 @@
+(type u32 (primitive u32))
+
+(decl pure A (u32 u32) u32)
+(extern constructor A A)
+
+(decl B (u32 u32) u32)
+(extern extractor B B)
+
+(decl C (u32 u32 u32 u32) u32)
+
+(decl pure predicate () u32)
+(rule (predicate) 1)
+
+(rule (C a b c (B d e))
+      (if-let (B f g) d)
+      (if-let h (A a b))
+      (A h a))
+
+(rule (C a b c d)
+      (if (predicate))
+      42)
+
+(rule (C a b a b)
+      (if-let x (D a b))
+      x)
+
+(decl pure D (u32 u32) u32)
+(rule (D x 0) x)
+(rule (D 0 x) x)

cranelift/isle/isle/isle_examples/link/iflets_main.rs

@@ -0,0 +1,16 @@
+mod iflets;
+
+struct Context;
+impl iflets::Context for Context {
+    fn A(&mut self, a: u32, b: u32) -> Option<u32> {
+        Some(a + b)
+    }
+
+    fn B(&mut self, value: u32) -> Option<(u32, u32)> {
+        Some((value, value + 1))
+    }
+}
+
+fn main() {
+    iflets::constructor_C(&mut Context, 1, 2, 3, 4);
+}