Add ergonomic_clones feature flag

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -489,6 +489,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
+    gate_all!(ergonomic_clones, "`.use` calls are experimental");
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
     gate_all!(generic_const_items, "generic const items are experimental");
     gate_all!(guard_patterns, "guard patterns are experimental", "consider using match arm guards");

compiler/rustc_feature/src/unstable.rs

@@ -473,6 +473,8 @@ declare_features! (
     (unstable, doc_masked, "1.21.0", Some(44027)),
     /// Allows `dyn* Trait` objects.
     (incomplete, dyn_star, "1.65.0", Some(102425)),
+    /// Allows the .use postfix syntax `x.use` and use closures `use |x| { ... }`
+    (unstable, ergonomic_clones, "CURRENT_RUSTC_VERSION", Some(132290)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (unstable, exhaustive_patterns, "1.13.0", Some(51085)),
     /// Allows explicit tail calls via `become` expression.

compiler/rustc_span/src/symbol.rs

@@ -859,6 +859,7 @@ symbols! {
         eprint_macro,
         eprintln_macro,
         eq,
+        ergonomic_clones,
         ermsb_target_feature,
         exact_div,
         except,

tests/ui/feature-gates/feature-gate-ergonomic-clones.rs

@@ -0,0 +1,20 @@
+fn ergonomic_clone(x: i32) -> i32 {
+    x.use
+    //~^ ERROR expected identifier, found keyword `use`
+    //~| ERROR `i32` is a primitive type and therefore doesn't have fields [E0610]
+}
+
+fn ergonomic_closure_clone() {
+    let s1 = String::from("hi!");
+
+    let s2 = use || {
+        //~^ ERROR expected expression, found keyword `use`
+        s1
+    };
+
+    let s3 = use || {
+        s1
+    };
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-ergonomic-clones.stderr

@@ -0,0 +1,26 @@
+error: expected identifier, found keyword `use`
+  --> $DIR/feature-gate-ergonomic-clones.rs:2:7
+   |
+LL |     x.use
+   |       ^^^ expected identifier, found keyword
+   |
+help: escape `use` to use it as an identifier
+   |
+LL |     x.r#use
+   |       ++
+
+error: expected expression, found keyword `use`
+  --> $DIR/feature-gate-ergonomic-clones.rs:10:14
+   |
+LL |     let s2 = use || {
+   |              ^^^ expected expression
+
+error[E0610]: `i32` is a primitive type and therefore doesn't have fields
+  --> $DIR/feature-gate-ergonomic-clones.rs:2:7
+   |
+LL |     x.use
+   |       ^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0610`.