Syntactically permit attributes on 'if' expressions

src/librustc_ast_passes/ast_validation.rs

@@ -399,6 +399,15 @@ impl<'a> AstValidator<'a> {
             }
         }
     }
+
+    fn check_attr_on_if_expr(&self, attrs: &[Attribute]) {
+        if let [a0, ..] = attrs {
+            // Just point to the first attribute in there...
+            self.err_handler()
+                .struct_span_err(a0.span, "attributes are not yet allowed on `if` expressions")
+                .emit();
+        }
+    }
 }
 
 enum GenericPosition {
@@ -515,6 +524,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 )
                 .emit();
             }
+            ExprKind::If(..) => self.check_attr_on_if_expr(&*expr.attrs),
             _ => {}
         }
 

src/librustc_parse/parser/expr.rs

@@ -676,20 +676,11 @@ impl<'a> Parser<'a> {
             expr.map(|mut expr| {
                 attrs.extend::<Vec<_>>(expr.attrs.into());
                 expr.attrs = attrs;
-                self.error_attr_on_if_expr(&expr);
                 expr
             })
         })
     }
 
-    fn error_attr_on_if_expr(&self, expr: &Expr) {
-        if let (ExprKind::If(..), [a0, ..]) = (&expr.kind, &*expr.attrs) {
-            // Just point to the first attribute in there...
-            self.struct_span_err(a0.span, "attributes are not yet allowed on `if` expressions")
-                .emit();
-        }
-    }
-
     fn parse_dot_or_call_expr_with_(&mut self, mut e: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
         loop {
             if self.eat(&token::Question) {

src/test/pretty/if-attr.rs

@@ -0,0 +1,28 @@
+// pp-exact
+
+#[cfg(FALSE)]
+fn simple_attr() {
+
+    #[attr]
+    if true { }
+
+    #[allow_warnings]
+    if true { }
+}
+
+#[cfg(FALSE)]
+fn if_else_chain() {
+
+    #[first_attr]
+    if true { } else if false { } else { }
+}
+
+#[cfg(FALSE)]
+fn if_let() {
+
+    #[attr]
+    if let Some(_) = Some(true) { }
+}
+
+
+fn main() { }

src/test/ui/parser/attr-stmt-expr-attr-bad.rs

@@ -38,8 +38,6 @@ fn main() {}
 //~^ ERROR an inner attribute is not permitted in this context
 #[cfg(FALSE)] fn e() { let _ = #[attr] &mut #![attr] 0; }
 //~^ ERROR an inner attribute is not permitted in this context
-#[cfg(FALSE)] fn e() { let _ = #[attr] if 0 {}; }
-//~^ ERROR attributes are not yet allowed on `if` expressions
 #[cfg(FALSE)] fn e() { let _ = if 0 #[attr] {}; }
 //~^ ERROR expected `{`, found `#`
 #[cfg(FALSE)] fn e() { let _ = if 0 {#![attr]}; }
@@ -51,14 +49,11 @@ fn main() {}
 #[cfg(FALSE)] fn e() { let _ = if 0 {} else {#![attr]}; }
 //~^ ERROR an inner attribute is not permitted in this context
 #[cfg(FALSE)] fn e() { let _ = if 0 {} else #[attr] if 0 {}; }
-//~^ ERROR attributes are not yet allowed on `if` expressions
-//~| ERROR expected `{`, found `#`
+//~^ ERROR expected `{`, found `#`
 #[cfg(FALSE)] fn e() { let _ = if 0 {} else if 0 #[attr] {}; }
 //~^ ERROR expected `{`, found `#`
 #[cfg(FALSE)] fn e() { let _ = if 0 {} else if 0 {#![attr]}; }
 //~^ ERROR an inner attribute is not permitted in this context
-#[cfg(FALSE)] fn e() { let _ = #[attr] if let _ = 0 {}; }
-//~^ ERROR attributes are not yet allowed on `if` expressions
 #[cfg(FALSE)] fn e() { let _ = if let _ = 0 #[attr] {}; }
 //~^ ERROR expected `{`, found `#`
 #[cfg(FALSE)] fn e() { let _ = if let _ = 0 {#![attr]}; }
@@ -70,8 +65,7 @@ fn main() {}
 #[cfg(FALSE)] fn e() { let _ = if let _ = 0 {} else {#![attr]}; }
 //~^ ERROR an inner attribute is not permitted in this context
 #[cfg(FALSE)] fn e() { let _ = if let _ = 0 {} else #[attr] if let _ = 0 {}; }
-//~^ ERROR attributes are not yet allowed on `if` expressions
-//~| ERROR expected `{`, found `#`
+//~^ ERROR expected `{`, found `#`
 #[cfg(FALSE)] fn e() { let _ = if let _ = 0 {} else if let _ = 0 #[attr] {}; }
 //~^ ERROR expected `{`, found `#`
 #[cfg(FALSE)] fn e() { let _ = if let _ = 0 {} else if let _ = 0 {#![attr]}; }