Rollup merge of rust-lang#69201 - Aaron1011:feature/permit-if-attr, r=Centril

Permit attributes on 'if' expressions

Previously, attributes on 'if' expressions (e.g. `#[attr] if true {}`)
were disallowed during parsing. This made it impossible for macros to
perform any custom handling of such attributes (e.g. stripping them
away), since a compilation error would be emitted before they ever had a
chance to run.

This PR permits attributes on 'if' expressions ('if-attrs' from here on).
Both built-in attributes (e.g. `#[allow]`, `#[cfg]`) and proc-macro attributes are supported.

We still do *not* accept attributes on 'other parts' of an if-else
chain. That is, the following code snippet still fails to parse:

```rust
if true {} #[attr] else if false {} else #[attr] if false {} #[attr]
else {}
```

Closes rust-lang#68618

Author: Centril

src/librustc_parse/parser/expr.rs

@@ -718,20 +718,11 @@ 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 +1,37 @@
+// 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) { }
+}
+
+#[cfg(FALSE)]
+fn let_attr_if() {
+    let _ = #[attr] if let _ = 0 { };
+    let _ = #[attr] if true { };
+
+    let _ = #[attr] if let _ = 0 { } else { };
+    let _ = #[attr] if true { } else { };
+}
+
+
+fn main() { }

src/test/ui/if-attrs/bad-cfg.rs

@@ -0,0 +1,5 @@
+#![feature(stmt_expr_attributes)]
+
+fn main() {
+    let _ = #[cfg(FALSE)] if true {}; //~ ERROR removing an expression
+}

src/test/ui/if-attrs/bad-cfg.stderr

@@ -0,0 +1,8 @@
+error: removing an expression is not supported in this position
+  --> $DIR/bad-cfg.rs:4:13
+   |
+LL |     let _ = #[cfg(FALSE)] if true {};
+   |             ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/if-attrs/builtin-if-attr.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+fn main() {
+    #[allow(unused_variables)]
+    if true {
+        let a = 1;
+    } else if false {
+        let b = 1;
+    } else {
+        let c = 1;
+    }
+}

src/test/ui/if-attrs/cfg-false-if-attr.rs

@@ -0,0 +1,43 @@
+// check-pass
+
+#[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 bar() {
+    #[cfg(FALSE)]
+    if true {
+        let x: () = true; // Should not error due to the #[cfg(FALSE)]
+    }
+
+    #[cfg_attr(not(unset_attr), cfg(FALSE))]
+    if true {
+        let a: () = true; // Should not error due to the applied #[cfg(FALSE)]
+    }
+}
+
+macro_rules! custom_macro {
+    ($expr:expr) => {}
+}
+
+custom_macro! {
+    #[attr] if true {}
+}
+
+
+fn main() {}

src/test/ui/if-attrs/else-attrs.rs

@@ -0,0 +1,25 @@
+#[cfg(FALSE)]
+fn if_else_parse_error() {
+    if true {
+    } #[attr] else if false { //~ ERROR expected
+    }
+}
+
+#[cfg(FALSE)]
+fn else_attr_ifparse_error() {
+    if true {
+    } else #[attr] if false { //~ ERROR expected
+    } else {
+    }
+}
+
+#[cfg(FALSE)]
+fn else_parse_error() {
+    if true {
+    } else if false {
+    } #[attr] else { //~ ERROR expected
+    }
+}
+
+fn main() {
+}

src/test/ui/if-attrs/else-attrs.stderr

@@ -0,0 +1,27 @@
+error: expected expression, found keyword `else`
+  --> $DIR/else-attrs.rs:4:15
+   |
+LL |     } #[attr] else if false {
+   |               ^^^^ expected expression
+
+error: expected `{`, found `#`
+  --> $DIR/else-attrs.rs:11:12
+   |
+LL |     } else #[attr] if false {
+   |            ^ expected `{`
+   |
+help: try placing this code inside a block
+   |
+LL |     } else #[attr] { if false {
+LL |     } else {
+LL |     } }
+   |
+
+error: expected expression, found keyword `else`
+  --> $DIR/else-attrs.rs:20:15
+   |
+LL |     } #[attr] else {
+   |               ^^^^ expected expression
+
+error: aborting due to 3 previous errors
+

src/test/ui/if-attrs/gate-whole-expr.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+fn main() {
+    let x = 1;
+
+    #[cfg(FALSE)]
+    if false {
+        x = 2;
+    } else if true {
+        x = 3;
+    } else {
+        x = 4;
+    }
+    assert_eq!(x, 1);
+}

src/test/ui/if-attrs/let-chains-attr.rs

@@ -0,0 +1,13 @@
+// check-pass
+
+#![feature(let_chains)] //~ WARN the feature `let_chains` is incomplete
+
+#[cfg(FALSE)]
+fn foo() {
+    #[attr]
+    if let Some(_) = Some(true) && let Ok(_) = Ok(1) {
+    } else if let Some(false) = Some(true) {
+    }
+}
+
+fn main() {}