Improve diagnostics for let x += 1

src/librustc_parse/parser/stmt.rs

@@ -216,8 +216,28 @@ impl<'a> Parser<'a> {
  }
 
  /// Parses the RHS of a local variable declaration (e.g., '= 14;').
- fn parse_initializer(&mut self, skip_eq: bool) -> PResult<'a, Option<P<Expr>>> {
- if self.eat(&token::Eq) || skip_eq { Ok(Some(self.parse_expr()?)) } else { Ok(None) }
+ fn parse_initializer(&mut self, eq_optional: bool) -> PResult<'a, Option<P<Expr>>> {
+ let eq_consumed = match self.token.kind {
+ token::BinOpEq(..) => {
+ // Recover `let x <op>= 1` as `let x = 1`
+ self.struct_span_err(
+ self.token.span,
+ "can't reassign to an uninitialized variable",
+ )
+ .span_suggestion_short(
+ self.token.span,
+ "initialize the variable",
+ "=".to_string(),
+ Applicability::MaybeIncorrect,
+ )
+ .emit();
+ self.bump();
+ true
+ }
+ _ => self.eat(&token::Eq),
+ };
+
+ Ok(if eq_consumed || eq_optional { Some(self.parse_expr()?) } else { None })
  }
 
  /// Parses a block. No inner attributes are allowed.

src/test/ui/parser/let-binop.rs

@@ -0,0 +1,8 @@
+fn main() {
+ let a: i8 *= 1; //~ ERROR can't reassign to an uninitialized variable
+ let _ = a;
+ let b += 1; //~ ERROR can't reassign to an uninitialized variable
+ let _ = b;
+ let c *= 1; //~ ERROR can't reassign to an uninitialized variable
+ let _ = c;
+}

src/test/ui/parser/let-binop.stderr

@@ -0,0 +1,20 @@
+error: can't reassign to an uninitialized variable
+ --> $DIR/let-binop.rs:2:15
+ |
+LL | let a: i8 *= 1;
+ | ^^ help: initialize the variable
+
+error: can't reassign to an uninitialized variable
+ --> $DIR/let-binop.rs:4:11
+ |
+LL | let b += 1;
+ | ^^ help: initialize the variable
+
+error: can't reassign to an uninitialized variable
+ --> $DIR/let-binop.rs:6:11
+ |
+LL | let c *= 1;
+ | ^^ help: initialize the variable
+
+error: aborting due to 3 previous errors
+