ssr: Allow replacing expressions with statements

Now that statements can be matched and replaced (rust-lang#6587) some usecases
require expressions to be replaced with statements as well. This happens
when something that can ambiguously be an expression or statement like
`if` and loop blocks appear in the last position of a block, as trailing
expression. In this case a replacement pattern of the form `if
foo(){$a();}==>>$a();` will only substitute `if` blocks in the list of
statements but not if they (implicitly) end up in the trailing
expression, where they are not wrapped by an EXPR_STMT (but the pattern
and template are, as parsing only succeeds for the `stmt ==>> stmt`
case).

Instead of adding two rules that match an expression - and emit
duplicate matching errors - allow the template for expressions to be a
statement if it fails to parse as an expression.

Author: MarijnS95

crates/ssr/src/parsing.rs

@@ -73,12 +73,18 @@ impl ParsedRule {
             placeholders_by_stand_in: pattern.placeholders_by_stand_in(),
             rules: Vec::new(),
         };
-        builder.try_add(ast::Expr::parse(&raw_pattern), raw_template.map(ast::Expr::parse));
+
+        let raw_template_stmt = raw_template.map(ast::Stmt::parse);
+        if let raw_template_expr @ Some(Ok(_)) = raw_template.map(ast::Expr::parse) {
+            builder.try_add(ast::Expr::parse(&raw_pattern), raw_template_expr);
+        } else {
+            builder.try_add(ast::Expr::parse(&raw_pattern), raw_template_stmt.clone());
+        }
         builder.try_add(ast::Type::parse(&raw_pattern), raw_template.map(ast::Type::parse));
         builder.try_add(ast::Item::parse(&raw_pattern), raw_template.map(ast::Item::parse));
         builder.try_add(ast::Path::parse(&raw_pattern), raw_template.map(ast::Path::parse));
         builder.try_add(ast::Pat::parse(&raw_pattern), raw_template.map(ast::Pat::parse));
-        builder.try_add(ast::Stmt::parse(&raw_pattern), raw_template.map(ast::Stmt::parse));
+        builder.try_add(ast::Stmt::parse(&raw_pattern), raw_template_stmt);
         builder.build()
     }
 }
@@ -89,7 +95,11 @@ struct RuleBuilder {
 }
 
 impl RuleBuilder {
-    fn try_add<T: AstNode>(&mut self, pattern: Result<T, ()>, template: Option<Result<T, ()>>) {
+    fn try_add<T: AstNode, T2: AstNode>(
+        &mut self,
+        pattern: Result<T, ()>,
+        template: Option<Result<T2, ()>>,
+    ) {
         match (pattern, template) {
             (Ok(pattern), Some(Ok(template))) => self.rules.push(ParsedRule {
                 placeholders_by_stand_in: self.placeholders_by_stand_in.clone(),

crates/ssr/src/tests.rs

@@ -203,6 +203,53 @@ fn ssr_let_stmt_replace_expr() {
     );
 }
 
+#[test]
+fn ssr_blockexpr_replace_stmt_with_stmt() {
+    assert_ssr_transform(
+        "if $a() {$b;} ==>> $b;",
+        "{
+    if foo() {
+        bar();
+    }
+    Ok(())
+}",
+        expect![[r#"{
+    bar();
+    Ok(())
+}"#]],
+    );
+}
+
+#[test]
+fn ssr_blockexpr_match_trailing_expr() {
+    assert_matches(
+        "if $a() {$b;}",
+        "{
+    if foo() {
+        bar();
+    }
+}",
+        &["if foo() {
+        bar();
+    }"],
+    );
+}
+
+#[test]
+fn ssr_blockexpr_replace_trailing_expr_with_stmt() {
+    assert_ssr_transform(
+        "if $a() {$b;} ==>> $b;",
+        "{
+    if foo() {
+        bar();
+    }
+}",
+        expect![["{
+    bar();
+}"]],
+    );
+}
+
 #[test]
 fn ssr_function_to_method() {
     assert_ssr_transform(