[PDLL] Add support for single line lambda-like patterns

This allows for defining simple patterns in a single line. The lambda
body of a Pattern expects a single operation rewrite statement:

```
Pattern => replace op<my_dialect.foo>(operands: ValueRange) with operands;
```

Differential Revision: https://reviews.llvm.org/D115835

mlir/lib/Tools/PDLL/Parser/Parser.cpp

@@ -106,6 +106,10 @@ private:
 
   FailureOr<ast::Decl *> parseTopLevelDecl();
   FailureOr<ast::NamedAttributeDecl *> parseNamedAttributeDecl();
+  FailureOr<ast::CompoundStmt *>
+  parseLambdaBody(function_ref<LogicalResult(ast::Stmt *&)> processStatementFn,
+                  bool expectTerminalSemicolon = true);
+  FailureOr<ast::CompoundStmt *> parsePatternLambdaBody();
   FailureOr<ast::Decl *> parsePatternDecl();
   LogicalResult parsePatternDeclMetadata(ParsedPatternMetadata &metadata);
 
@@ -547,6 +551,36 @@ FailureOr<ast::NamedAttributeDecl *> Parser::parseNamedAttributeDecl() {
   return ast::NamedAttributeDecl::create(ctx, name, attrValue);
 }
 
+FailureOr<ast::CompoundStmt *> Parser::parseLambdaBody(
+    function_ref<LogicalResult(ast::Stmt *&)> processStatementFn,
+    bool expectTerminalSemicolon) {
+  consumeToken(Token::equal_arrow);
+
+  // Parse the single statement of the lambda body.
+  SMLoc bodyStartLoc = curToken.getStartLoc();
+  pushDeclScope();
+  FailureOr<ast::Stmt *> singleStatement = parseStmt(expectTerminalSemicolon);
+  bool failedToParse =
+      failed(singleStatement) || failed(processStatementFn(*singleStatement));
+  popDeclScope();
+  if (failedToParse)
+    return failure();
+
+  SMRange bodyLoc(bodyStartLoc, curToken.getStartLoc());
+  return ast::CompoundStmt::create(ctx, bodyLoc, *singleStatement);
+}
+
+FailureOr<ast::CompoundStmt *> Parser::parsePatternLambdaBody() {
+  return parseLambdaBody([&](ast::Stmt *&statement) -> LogicalResult {
+    if (isa<ast::OpRewriteStmt>(statement))
+      return success();
+    return emitError(
+        statement->getLoc(),
+        "expected Pattern lambda body to contain a single operation "
+        "rewrite statement, such as `erase`, `replace`, or `rewrite`");
+  });
+}
+
 FailureOr<ast::Decl *> Parser::parsePatternDecl() {
   SMRange loc = curToken.getLoc();
   consumeToken(Token::kw_Pattern);
@@ -568,8 +602,15 @@ FailureOr<ast::Decl *> Parser::parsePatternDecl() {
   // Parse the pattern body.
   ast::CompoundStmt *body;
 
+  // Handle a lambda body.
+  if (curToken.is(Token::equal_arrow)) {
+    FailureOr<ast::CompoundStmt *> bodyResult = parsePatternLambdaBody();
+    if (failed(bodyResult))
+      return failure();
+    body = *bodyResult;
+  } else {
     if (curToken.isNot(Token::l_brace))
-    return emitError("expected `{` to start pattern body");
+      return emitError("expected `{` or `=>` to start pattern body");
     FailureOr<ast::CompoundStmt *> bodyResult = parseCompoundStmt();
     if (failed(bodyResult))
       return failure();
@@ -592,6 +633,7 @@ FailureOr<ast::Decl *> Parser::parsePatternDecl() {
                        "Pattern body was terminated by an operation "
                        "rewrite statement, but found trailing statements");
     }
+  }
 
   return createPatternDecl(loc, name, metadata, body);
 }

mlir/test/mlir-pdll/Parser/pattern-failure.pdll

@@ -1,6 +1,6 @@
 // RUN: not mlir-pdll %s -split-input-file 2>&1  | FileCheck %s
 
-// CHECK: expected `{` to start pattern body
+// CHECK: expected `{` or `=>` to start pattern body
 Pattern }
 
 // -----
@@ -27,6 +27,11 @@ Pattern {
 
 // -----
 
+// CHECK: expected Pattern lambda body to contain a single operation rewrite statement, such as `erase`, `replace`, or `rewrite`
+Pattern => op<>;
+
+// -----
+
 //===----------------------------------------------------------------------===//
 // Metadata
 //===----------------------------------------------------------------------===//

mlir/test/mlir-pdll/Parser/pattern.pdll

@@ -23,3 +23,11 @@ Pattern NamedPattern {
 Pattern NamedPattern with benefit(10), recursion {
   erase _: Op;
 }
+
+// -----
+
+// CHECK: Module
+// CHECK: `-PatternDecl {{.*}} Name<NamedPattern>
+// CHECK:   `-CompoundStmt
+// CHECK:     `-EraseStmt
+Pattern NamedPattern => erase _: Op;