Disallow cast with trailing braced macro in let-else

Author: dtolnay

compiler/rustc_ast/src/util/classify.rs

@@ -113,6 +113,10 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
             | While(..)
             | ConstBlock(_) => break Some(expr),
 
+            Cast(_, ty) => {
+                break type_trailing_brace(ty).then_some(expr);
+            }
+
             MacCall(mac) => {
                 break (mac.args.delim == Delimiter::Brace).then_some(expr);
             }
@@ -131,7 +135,6 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
             | MethodCall(_)
             | Tup(_)
             | Lit(_)
-            | Cast(_, _)
             | Type(_, _)
             | Await(_, _)
             | Field(_, _)
@@ -148,3 +151,71 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
         }
     }
 }
+
+/// Whether the type's last token is `}`.
+fn type_trailing_brace(mut ty: &ast::Ty) -> bool {
+    loop {
+        match &ty.kind {
+            ast::TyKind::AnonStruct(..) | ast::TyKind::AnonUnion(..) => break true,
+
+            ast::TyKind::MacCall(mac) => break mac.args.delim == Delimiter::Brace,
+
+            ast::TyKind::Ptr(mut_ty) | ast::TyKind::Ref(_, mut_ty) => {
+                ty = &mut_ty.ty;
+            }
+
+            ast::TyKind::BareFn(fn_ty) => match &fn_ty.decl.output {
+                ast::FnRetTy::Default(_) => break false,
+                ast::FnRetTy::Ty(ret) => ty = ret,
+            },
+
+            ast::TyKind::Path(_, path) => match path_return_type(path) {
+                Some(trailing_ty) => ty = trailing_ty,
+                None => break false,
+            },
+
+            ast::TyKind::TraitObject(bounds, _) | ast::TyKind::ImplTrait(_, bounds, _) => {
+                match bounds.last() {
+                    Some(ast::GenericBound::Trait(bound, _)) => {
+                        match path_return_type(&bound.trait_ref.path) {
+                            Some(trailing_ty) => ty = trailing_ty,
+                            None => break false,
+                        }
+                    }
+                    Some(ast::GenericBound::Outlives(_)) | None => break false,
+                }
+            }
+
+            ast::TyKind::Slice(..)
+            | ast::TyKind::Array(..)
+            | ast::TyKind::Never
+            | ast::TyKind::Tup(..)
+            | ast::TyKind::Paren(..)
+            | ast::TyKind::Typeof(..)
+            | ast::TyKind::Infer
+            | ast::TyKind::ImplicitSelf
+            | ast::TyKind::CVarArgs
+            | ast::TyKind::Pat(..)
+            | ast::TyKind::Dummy
+            | ast::TyKind::Err(..) => break false,
+        }
+    }
+}
+
+/// Returns the trailing return type in the given path, if it has one.
+///
+/// ```ignore (illustrative)
+/// ::std::ops::FnOnce(&str) -> fn() -> *const c_void
+///                             ^^^^^^^^^^^^^^^^^^^^^
+/// ```
+fn path_return_type(path: &ast::Path) -> Option<&ast::Ty> {
+    let last_segment = path.segments.last()?;
+    let args = last_segment.args.as_ref()?;
+    match &**args {
+        ast::GenericArgs::Parenthesized(args) => match &args.output {
+            ast::FnRetTy::Default(_) => None,
+            ast::FnRetTy::Ty(ret) => Some(ret),
+        },
+        ast::GenericArgs::AngleBracketed(_) => None,
+    }
+}

tests/ui/parser/bad-let-else-statement.rs

@@ -205,7 +205,7 @@ fn t() {
         //~^ WARN irrefutable `let...else` pattern
         8
     } else {
-        // FIXME: right curly brace `}` before `else` in a `let...else` statement not allowed
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
         return;
     };
 }

tests/ui/parser/bad-let-else-statement.stderr

@@ -241,6 +241,20 @@ help: use parentheses instead of braces for this macro
 LL |     let bad = format_args! ("") else { return; };
    |                            ~  ~
 
+error: right curly brace `}` before `else` in a `let...else` statement not allowed
+  --> $DIR/bad-let-else-statement.rs:207:5
+   |
+LL |     } else {
+   |     ^
+   |
+help: wrap the expression in parentheses
+   |
+LL ~     let foo = (&std::ptr::null as &'static dyn std::ops::Fn() -> *const primitive! {
+LL |
+LL |         8
+LL ~     }) else {
+   |
+
 error: right curly brace `}` before `else` in a `let...else` statement not allowed
   --> $DIR/bad-let-else-statement.rs:190:25
    |
@@ -311,5 +325,5 @@ LL | |     } else {
    = note: this pattern will always match, so the `else` clause is useless
    = help: consider removing the `else` clause
 
-error: aborting due to 19 previous errors; 5 warnings emitted
+error: aborting due to 20 previous errors; 5 warnings emitted