More precise decision to parse expression after return keyword

Author: dtolnay

src/expr.rs

@@ -957,6 +957,8 @@ pub(crate) fn requires_terminator(expr: &Expr) -> bool {
 #[cfg(feature = "parsing")]
 pub(crate) mod parsing {
     use super::*;
+    #[cfg(feature = "full")]
+    use crate::ext::IdentExt;
     use crate::parse::discouraged::Speculative;
     #[cfg(feature = "full")]
     use crate::parse::ParseBuffer;
@@ -1155,6 +1157,25 @@ pub(crate) mod parsing {
         }
     }
 
+    #[cfg(feature = "full")]
+    fn can_begin_expr(input: ParseStream) -> bool {
+        input.peek(Ident::peek_any) // value name or keyword
+            || input.peek(token::Paren) // tuple
+            || input.peek(token::Bracket) // array
+            || input.peek(token::Brace) // block
+            || input.peek(Lit) // literal
+            || input.peek(Token![!]) && !input.peek(Token![!=]) // operator not
+            || input.peek(Token![-]) && !input.peek(Token![-=]) && !input.peek(Token![->]) // unary minus
+            || input.peek(Token![*]) && !input.peek(Token![*=]) // dereference
+            || input.peek(Token![|]) && !input.peek(Token![|=]) // closure
+            || input.peek(Token![&]) && !input.peek(Token![&=]) // reference
+            || input.peek(Token![..]) // range notation
+            || input.peek(Token![<]) && !input.peek(Token![<=]) && !input.peek(Token![<<=]) // associated path
+            || input.peek(Token![::]) // global path
+            || input.peek(Lifetime) // labeled loop
+            || input.peek(Token![#]) // expression attributes
+    }
+
     #[cfg(feature = "full")]
     fn parse_expr(
         input: ParseStream,
@@ -2247,7 +2268,7 @@ pub(crate) mod parsing {
                 attrs: Vec::new(),
                 yield_token: input.parse()?,
                 expr: {
-                    if !input.is_empty() && !input.peek(Token![,]) && !input.peek(Token![;]) {
+                    if can_begin_expr(input) {
                         Some(input.parse()?)
                     } else {
                         None
@@ -2447,15 +2468,11 @@ pub(crate) mod parsing {
             break_token: input.parse()?,
             label: input.parse()?,
             expr: {
-                if input.is_empty()
-                    || input.peek(Token![,])
-                    || input.peek(Token![;])
-                    || !allow_struct.0 && input.peek(token::Brace)
-                {
-                    None
-                } else {
+                if can_begin_expr(input) && (allow_struct.0 || !input.peek(token::Brace)) {
                     let expr = ambiguous_expr(input, allow_struct)?;
                     Some(Box::new(expr))
+                } else {
+                    None
                 }
             },
         })
@@ -2467,16 +2484,16 @@ pub(crate) mod parsing {
             attrs: Vec::new(),
             return_token: input.parse()?,
             expr: {
-                if input.is_empty() || input.peek(Token![,]) || input.peek(Token![;]) {
-                    None
-                } else {
+                if can_begin_expr(input) {
                     // NOTE: return is greedy and eats blocks after it even when in a
                     // position where structs are not allowed, such as in if statement
                     // conditions. For example:
                     //
                     // if return { println!("A") } {} // Prints "A"
                     let expr = ambiguous_expr(input, allow_struct)?;
                     Some(Box::new(expr))
+                } else {
+                    None
                 }
             },
         })

tests/repo/mod.rs

@@ -56,10 +56,6 @@ static EXCLUDE_FILES: &[&str] = &[
     // https://github.com/dtolnay/syn/issues/1527
     "tests/ui/parser/struct-literal-in-match-guard.rs",
 
-    // TODO: precedence of return in match guard
-    // https://github.com/dtolnay/syn/issues/1528
-    "tests/ui/unreachable-code.rs",
-
     // Compile-fail expr parameter in const generic position: f::<1 + 2>()
     "tests/ui/const-generics/early/closing-args-token.rs",
     "tests/ui/const-generics/early/const-expression-parameter.rs",