Auto merge of #74566 - lzutao:guard, r=petrochenkov

Gate if-let guard feature

Enhanced on #74315. That PR is in crater queue so I don't want to push to it.

Close  #74232
cc #51114

src/librustc_ast_passes/feature_gate.rs

@@ -613,11 +613,14 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     let spans = sess.parse_sess.gated_spans.spans.borrow();
     macro_rules! gate_all {
         ($gate:ident, $msg:literal) => {
-            for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
-                gate_feature_post!(&visitor, $gate, *span, $msg);
+            if let Some(spans) = spans.get(&sym::$gate) {
+                for span in spans {
+                    gate_feature_post!(&visitor, $gate, *span, $msg);
+                }
             }
         };
     }
+    gate_all!(if_let_guard, "`if let` guard is not implemented");
     gate_all!(let_chains, "`let` expressions in this position are experimental");
     gate_all!(async_closure, "async closures are unstable");
     gate_all!(generators, "yield syntax is experimental");

src/librustc_feature/active.rs

@@ -582,6 +582,9 @@ declare_features! (
     /// The smallest useful subset of `const_generics`.
     (active, min_const_generics, "1.47.0", Some(74878), None),
 
+    /// Allows `if let` guard in match arms.
+    (active, if_let_guard, "1.47.0", Some(51114), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -591,6 +594,7 @@ declare_features! (
 /// unanticipated results, such as compiler crashes. We warn the user about these
 /// to alert them.
 pub const INCOMPLETE_FEATURES: &[Symbol] = &[
+    sym::if_let_guard,
     sym::impl_trait_in_bindings,
     sym::generic_associated_types,
     sym::const_generics,

src/librustc_parse/parser/expr.rs

@@ -1858,7 +1858,19 @@ impl<'a> Parser<'a> {
         let attrs = self.parse_outer_attributes()?;
         let lo = self.token.span;
         let pat = self.parse_top_pat(GateOr::No)?;
-        let guard = if self.eat_keyword(kw::If) { Some(self.parse_expr()?) } else { None };
+        let guard = if self.eat_keyword(kw::If) {
+            let if_span = self.prev_token.span;
+            let cond = self.parse_expr()?;
+            if let ExprKind::Let(..) = cond.kind {
+                // Remove the last feature gating of a `let` expression since it's stable.
+                self.sess.gated_spans.ungate_last(sym::let_chains, cond.span);
+                let span = if_span.to(cond.span);
+                self.sess.gated_spans.gate(sym::if_let_guard, span);
+            }
+            Some(cond)
+        } else {
+            None
+        };
         let arrow_span = self.token.span;
         self.expect(&token::FatArrow)?;
         let arm_start_span = self.token.span;

src/librustc_span/symbol.rs

@@ -567,6 +567,7 @@ symbols! {
         i8,
         ident,
         if_let,
+        if_let_guard,
         if_while_or_patterns,
         ignore,
         impl_header_lifetime_elision,

src/test/ui/parser/struct-literal-in-match-guard.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+// Unlike `if` condition, `match` guards accept struct literals.
+// This is detected in <https://github.com/rust-lang/rust/pull/74566#issuecomment-663613705>.
+
+#[derive(PartialEq)]
+struct Foo {
+    x: isize,
+}
+
+fn foo(f: Foo) {
+    match () {
+        () if f == Foo { x: 42 } => {}
+        _ => {}
+    }
+}
+
+fn main() {}

src/test/ui/rfc-2294-if-let-guard/feature-gate.rs

@@ -0,0 +1,85 @@
+// gate-test-if_let_guard
+
+use std::ops::Range;
+
+fn _if_let_guard() {
+    match () {
+        () if let 0 = 1 => {}
+        //~^ ERROR `if let` guard is not implemented
+        //~| ERROR `let` expressions are not supported here
+
+        () if (let 0 = 1) => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+
+        () if (((let 0 = 1))) => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+
+        () if true && let 0 = 1 => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+
+        () if let 0 = 1 && true => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+
+        () if (let 0 = 1) && true => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+
+        () if true && (let 0 = 1) => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+
+        () if (let 0 = 1) && (let 0 = 1) => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+        //~| ERROR `let` expressions are not supported here
+
+        () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+        //~| ERROR `let` expressions are not supported here
+        //~| ERROR `let` expressions are not supported here
+        //~| ERROR `let` expressions are not supported here
+        //~| ERROR `let` expressions are not supported here
+
+        () if let Range { start: _, end: _ } = (true..true) && false => {}
+        //~^ ERROR `let` expressions in this position are experimental
+        //~| ERROR `let` expressions are not supported here
+        _ => {}
+    }
+}
+
+fn _macros() {
+    macro_rules! use_expr {
+        ($e:expr) => {
+            match () {
+                () if $e => {}
+                _ => {}
+            }
+        }
+    }
+    use_expr!((let 0 = 1 && 0 == 0));
+    //~^ ERROR `let` expressions in this position are experimental
+    //~| ERROR `let` expressions are not supported here
+    use_expr!((let 0 = 1));
+    //~^ ERROR `let` expressions in this position are experimental
+    //~| ERROR `let` expressions are not supported here
+    match () {
+        #[cfg(FALSE)]
+        () if let 0 = 1 => {}
+        //~^ ERROR `if let` guard is not implemented
+        _ => {}
+    }
+    use_expr!(let 0 = 1);
+    //~^ ERROR no rules expected the token `let`
+}
+
+fn main() {}