Reduce double errors for invalid let expressions

Previously some invalid let expressions would result in both a feature
error and a parsing error. Avoid this and ensure that we only emit the
parsing error when this happens.

Author: matthewjasper

compiler/rustc_parse/src/parser/expr.rs

@@ -2485,9 +2485,6 @@ impl<'a> Parser<'a> {
         }
         let expr = self.parse_expr_assoc_with(1 + prec_let_scrutinee_needs_par(), None.into())?;
         let span = lo.to(expr.span);
-        if is_recovered.is_none() {
-            self.sess.gated_spans.gate(sym::let_chains, span);
-        }
         Ok(self.mk_expr(span, ExprKind::Let(pat, expr, span, is_recovered)))
     }
 
@@ -3460,6 +3457,8 @@ impl MutVisitor for CondChecker<'_> {
                             .sess
                             .emit_err(errors::ExpectedExpressionFoundLet { span, reason }),
                     );
+                } else {
+                    self.parser.sess.gated_spans.gate(sym::let_chains, span);
                 }
             }
             ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, _, _) => {

tests/ui/expr/if/bad-if-let-suggestion.rs

@@ -4,7 +4,6 @@
 fn a() {
     if let x = 1 && i = 2 {}
     //~^ ERROR cannot find value `i` in this scope
-    //~| ERROR `let` expressions in this position are unstable
     //~| ERROR mismatched types
     //~| ERROR expected expression, found `let` statement
 }

tests/ui/expr/if/bad-if-let-suggestion.stderr

@@ -11,7 +11,7 @@ LL |     if let x = 1 && i = 2 {}
    |                     ^ not found in this scope
 
 error[E0425]: cannot find value `i` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:13:9
+  --> $DIR/bad-if-let-suggestion.rs:12:9
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -20,7 +20,7 @@ LL |     if (i + j) = i {}
    |         ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `j` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:13:13
+  --> $DIR/bad-if-let-suggestion.rs:12:13
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -29,7 +29,7 @@ LL |     if (i + j) = i {}
    |             ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `i` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:13:18
+  --> $DIR/bad-if-let-suggestion.rs:12:18
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -38,23 +38,14 @@ LL |     if (i + j) = i {}
    |                  ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `x` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:20:8
+  --> $DIR/bad-if-let-suggestion.rs:19:8
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
 ...
 LL |     if x[0] = 1 {}
    |        ^ help: a function with a similar name exists: `a`
 
-error[E0658]: `let` expressions in this position are unstable
-  --> $DIR/bad-if-let-suggestion.rs:5:8
-   |
-LL |     if let x = 1 && i = 2 {}
-   |        ^^^^^^^^^
-   |
-   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
-   = help: add `#![feature(let_chains)]` to the crate attributes to enable
-
 error[E0308]: mismatched types
   --> $DIR/bad-if-let-suggestion.rs:5:8
    |
@@ -66,7 +57,7 @@ help: you might have meant to compare for equality
 LL |     if let x = 1 && i == 2 {}
    |                        +
 
-error: aborting due to 8 previous errors
+error: aborting due to 7 previous errors
 
-Some errors have detailed explanations: E0308, E0425, E0658.
+Some errors have detailed explanations: E0308, E0425.
 For more information about an error, try `rustc --explain E0308`.

tests/ui/rfcs/rfc-2294-if-let-guard/feature-gate.rs

@@ -9,11 +9,9 @@ fn _if_let_guard() {
 
         () if (let 0 = 1) => {}
         //~^ ERROR expected expression, found `let` statement
-        //~| ERROR `let` expressions in this position are unstable
 
         () if (((let 0 = 1))) => {}
         //~^ ERROR expected expression, found `let` statement
-        //~| ERROR `let` expressions in this position are unstable
 
         () if true && let 0 = 1 => {}
         //~^ ERROR `if let` guards are experimental
@@ -25,25 +23,18 @@ fn _if_let_guard() {
 
         () if (let 0 = 1) && true => {}
         //~^ ERROR expected expression, found `let` statement
-        //~| ERROR `let` expressions in this position are unstable
 
         () if true && (let 0 = 1) => {}
         //~^ ERROR expected expression, found `let` statement
-        //~| ERROR `let` expressions in this position are unstable
 
         () if (let 0 = 1) && (let 0 = 1) => {}
         //~^ ERROR expected expression, found `let` statement
         //~| ERROR expected expression, found `let` statement
-        //~| ERROR `let` expressions in this position are unstable
-        //~| ERROR `let` expressions in this position are unstable
 
         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
         //~^ ERROR `if let` guards are experimental
         //~| ERROR `let` expressions in this position are unstable
         //~| ERROR `let` expressions in this position are unstable
-        //~| ERROR `let` expressions in this position are unstable
-        //~| ERROR `let` expressions in this position are unstable
-        //~| ERROR `let` expressions in this position are unstable
         //~| ERROR expected expression, found `let` statement
         //~| ERROR expected expression, found `let` statement
         //~| ERROR expected expression, found `let` statement