Enforce that let-else's expression does not end in brace

src/classify.rs

@@ -1,4 +1,10 @@
 use crate::expr::Expr;
+use crate::generics::TypeParamBound;
+use crate::path::{Path, PathArguments};
+use crate::punctuated::Punctuated;
+use crate::ty::{ReturnType, Type};
+use proc_macro2::{Delimiter, TokenStream, TokenTree};
+use std::ops::ControlFlow;
 
 pub(crate) fn requires_terminator(expr: &Expr) -> bool {
     // see https://github.com/rust-lang/rust/blob/9a19e7604/compiler/rustc_ast/src/util/classify.rs#L7-L26
@@ -43,3 +49,130 @@ pub(crate) fn requires_terminator(expr: &Expr) -> bool {
         | Expr::Verbatim(_) => true
     }
 }
+
+/// Whether the expression's last token is `}`.
+#[cfg(feature = "parsing")]
+pub(crate) fn expr_trailing_brace(mut expr: &Expr) -> bool {
+    loop {
+        match expr {
+            Expr::Array(_) => return false,
+            Expr::Assign(e) => expr = &e.right,
+            Expr::Async(_) => return true,
+            Expr::Await(_) => return false,
+            Expr::Binary(e) => expr = &e.right,
+            Expr::Block(_) => return true,
+            Expr::Break(e) => match &e.expr {
+                Some(e) => expr = e,
+                None => return false,
+            },
+            Expr::Call(_) => return false,
+            Expr::Cast(e) => return type_trailing_brace(&e.ty),
+            Expr::Closure(e) => expr = &e.body,
+            Expr::Const(_) => return true,
+            Expr::Continue(_) => return false,
+            Expr::Field(_) => return false,
+            Expr::ForLoop(_) => return true,
+            Expr::Group(_) => return false,
+            Expr::If(_) => return true,
+            Expr::Index(_) => return false,
+            Expr::Infer(_) => return false,
+            Expr::Let(e) => expr = &e.expr,
+            Expr::Lit(_) => return false,
+            Expr::Loop(_) => return true,
+            Expr::Macro(e) => return e.mac.delimiter.is_brace(),
+            Expr::Match(_) => return true,
+            Expr::MethodCall(_) => return false,
+            Expr::Paren(_) => return false,
+            Expr::Path(_) => return false,
+            Expr::Range(e) => match &e.end {
+                Some(end) => expr = end,
+                None => return false,
+            },
+            Expr::Reference(e) => expr = &e.expr,
+            Expr::Repeat(_) => return false,
+            Expr::Return(e) => match &e.expr {
+                Some(e) => expr = e,
+                None => return false,
+            },
+            Expr::Struct(_) => return true,
+            Expr::Try(_) => return false,
+            Expr::TryBlock(_) => return true,
+            Expr::Tuple(_) => return false,
+            Expr::Unary(e) => expr = &e.expr,
+            Expr::Unsafe(_) => return true,
+            Expr::Verbatim(e) => return tokens_trailing_brace(e),
+            Expr::While(_) => return true,
+            Expr::Yield(e) => match &e.expr {
+                Some(e) => expr = e,
+                None => return false,
+            },
+        }
+    }
+}
+
+#[cfg(feature = "parsing")]
+fn type_trailing_brace(mut ty: &Type) -> bool {
+    fn last_type_in_path(path: &Path) -> Option<&Type> {
+        match &path.segments.last().unwrap().arguments {
+            PathArguments::None | PathArguments::AngleBracketed(_) => None,
+            PathArguments::Parenthesized(arg) => match &arg.output {
+                ReturnType::Default => None,
+                ReturnType::Type(_, ret) => Some(ret),
+            },
+        }
+    }
+
+    fn last_type_in_bounds(
+        bounds: &Punctuated<TypeParamBound, Token![+]>,
+    ) -> ControlFlow<bool, &Type> {
+        match bounds.last().unwrap() {
+            TypeParamBound::Trait(t) => match last_type_in_path(&t.path) {
+                Some(t) => ControlFlow::Continue(t),
+                None => ControlFlow::Break(false),
+            },
+            TypeParamBound::Lifetime(_) => ControlFlow::Break(false),
+            TypeParamBound::Verbatim(t) => ControlFlow::Break(tokens_trailing_brace(t)),
+        }
+    }
+
+    loop {
+        match ty {
+            Type::Array(_) => return false,
+            Type::BareFn(t) => match &t.output {
+                ReturnType::Default => return false,
+                ReturnType::Type(_, ret) => ty = ret,
+            },
+            Type::Group(_) => return false,
+            Type::ImplTrait(t) => match last_type_in_bounds(&t.bounds) {
+                ControlFlow::Break(trailing_brace) => return trailing_brace,
+                ControlFlow::Continue(t) => ty = t,
+            },
+            Type::Infer(_) => return false,
+            Type::Macro(t) => return t.mac.delimiter.is_brace(),
+            Type::Never(_) => return false,
+            Type::Paren(_) => return false,
+            Type::Path(t) => match last_type_in_path(&t.path) {
+                Some(t) => ty = t,
+                None => return false,
+            },
+            Type::Ptr(t) => ty = &t.elem,
+            Type::Reference(t) => ty = &t.elem,
+            Type::Slice(_) => return false,
+            Type::TraitObject(t) => match last_type_in_bounds(&t.bounds) {
+                ControlFlow::Break(trailing_brace) => return trailing_brace,
+                ControlFlow::Continue(t) => ty = t,
+            },
+            Type::Tuple(_) => return false,
+            Type::Verbatim(t) => return tokens_trailing_brace(t),
+        }
+    }
+}
+
+#[cfg(feature = "parsing")]
+fn tokens_trailing_brace(tokens: &TokenStream) -> bool {
+    if let Some(TokenTree::Group(last)) = tokens.clone().into_iter().last() {
+        last.delimiter() == Delimiter::Brace
+    } else {
+        false
+    }
+}

src/item.rs

@@ -921,7 +921,7 @@ pub(crate) mod parsing {
     };
     use crate::lifetime::Lifetime;
     use crate::lit::LitStr;
-    use crate::mac::{self, Macro, MacroDelimiter};
+    use crate::mac::{self, Macro};
     use crate::parse::discouraged::Speculative as _;
     use crate::parse::{Parse, ParseBuffer, ParseStream};
     use crate::pat::{Pat, PatType, PatWild};
@@ -2872,15 +2872,6 @@ pub(crate) mod parsing {
         }
     }
 
-    impl MacroDelimiter {
-        pub(crate) fn is_brace(&self) -> bool {
-            match self {
-                MacroDelimiter::Brace(_) => true,
-                MacroDelimiter::Paren(_) | MacroDelimiter::Bracket(_) => false,
-            }
-        }
-    }
-
     #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
     impl Parse for StaticMutability {
         fn parse(input: ParseStream) -> Result<Self> {

src/mac.rs

@@ -40,6 +40,14 @@ impl MacroDelimiter {
             MacroDelimiter::Bracket(token) => &token.span,
         }
     }
+
+    #[cfg(all(feature = "full", feature = "parsing"))]
+    pub(crate) fn is_brace(&self) -> bool {
+        match self {
+            MacroDelimiter::Brace(_) => true,
+            MacroDelimiter::Paren(_) | MacroDelimiter::Bracket(_) => false,
+        }
+    }
 }
 
 impl Macro {

src/stmt.rs

@@ -298,8 +298,8 @@ pub(crate) mod parsing {
             let eq_token: Token![=] = eq_token;
             let expr: Expr = input.parse()?;
 
-            let diverge = if let Some(else_token) = input.parse()? {
-                let else_token: Token![else] = else_token;
+            let diverge = if !classify::expr_trailing_brace(&expr) && input.peek(Token![else]) {
+                let else_token: Token![else] = input.parse()?;
                 let diverge = ExprBlock {
                     attrs: Vec::new(),
                     label: None,