Add UI coverage for closure for<> binder binding suggestions

Cover MachineApplicable rewrites, MaybeIncorrect capture cases, and
macro-expanded binders where structured suggestions must be suppressed.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Dnreikronos

tests/ui/feature-gates/feature-gate-closure_lifetime_binder-macro.rs

@@ -0,0 +1,20 @@
+//@ compile-flags: --error-format=json
+//@ forbid-output: MachineApplicable
+//@ forbid-output: MaybeIncorrect
+
+// Macro-expanded closures must not get a structured fn-pointer rewrite (hygiene + spans point
+// into the macro). Expect only the simple help.
+
+macro_rules! make {
+    ($x:ident) => {
+        for<'a> |x: &'a i32| -> i32 { *x + $x }
+        //~^ ERROR `for<...>` binders for closures are experimental
+        //~| HELP add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable
+        //~| HELP consider removing `for<...>`
+    };
+}
+
+fn main() {
+    let x = 1;
+    let _cl = make!(x);
+}

tests/ui/feature-gates/feature-gate-closure_lifetime_binder-macro.stderr

@@ -0,0 +1,43 @@
+{"$message_type":"diagnostic","message":"`for<...>` binders for closures are experimental","code":{"code":"E0658","explanation":"An unstable feature was used.
+
+Erroneous code example:
+
+```compile_fail,E0658
+use std::intrinsics; // error: use of unstable library feature `core_intrinsics`
+```
+
+If you're using a stable or a beta version of rustc, you won't be able to use
+any unstable features. In order to do so, please switch to a nightly version of
+rustc (by using [rustup]).
+
+If you're using a nightly version of rustc, just add the corresponding feature
+to be able to use it:
+
+```
+#![feature(core_intrinsics)]
+
+use std::intrinsics; // ok!
+```
+
+[rustup]: https://rust-lang.github.io/rustup/concepts/channels.html
+"},"level":"error","spans":[{"file_name":"$DIR/feature-gate-closure_lifetime_binder-macro.rs","byte_start":304,"byte_end":311,"line_start":10,"line_end":10,"column_start":9,"column_end":16,"is_primary":true,"text":[{"text":"        for<'a> |x: &'a i32| -> i32 { *x + $x }","highlight_start":9,"highlight_end":16}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":{"span":{"file_name":"$DIR/feature-gate-closure_lifetime_binder-macro.rs","byte_start":605,"byte_end":613,"line_start":19,"line_end":19,"column_start":15,"column_end":23,"is_primary":false,"text":[{"text":"    let _cl = make!(x);","highlight_start":15,"highlight_end":23}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null},"macro_decl_name":"make!","def_site_span":{"file_name":"$DIR/feature-gate-closure_lifetime_binder-macro.rs","byte_start":256,"byte_end":273,"line_start":8,"line_end":8,"column_start":1,"column_end":18,"is_primary":false,"text":[{"text":"macro_rules! make {","highlight_start":1,"highlight_end":18}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}}}],"children":[{"message":"see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable","code":null,"level":"help","spans":[],"children":[],"rendered":null},{"message":"this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"consider removing `for<...>`","code":null,"level":"help","spans":[],"children":[],"rendered":null}],"rendered":"error[E0658]: `for<...>` binders for closures are experimental
+  --> $DIR/feature-gate-closure_lifetime_binder-macro.rs:10:9
+   |
+LL |         for<'a> |x: &'a i32| -> i32 { *x + $x }
+   |         ^^^^^^^
+...
+LL |     let _cl = make!(x);
+   |               -------- in this macro invocation
+   |
+   = note: see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information
+   = help: add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: consider removing `for<...>`
+   = note: this error originates in the macro `make` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+"}
+{"$message_type":"diagnostic","message":"aborting due to 1 previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 1 previous error
+
+"}
+{"$message_type":"diagnostic","message":"For more information about this error, try `rustc --explain E0658`.","code":null,"level":"failure-note","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0658`.
+"}

tests/ui/feature-gates/feature-gate-closure_lifetime_binder-maybe-incorrect.rs

@@ -0,0 +1,34 @@
+//@ edition: 2024
+//@ compile-flags: --error-format=json
+//@ error-pattern: "suggestion_applicability":"MaybeIncorrect"
+
+// Capturing closures must not get a MachineApplicable rewrite. Cover plain captures, let-else
+// leakage, and let-chain shadowing — all should report MaybeIncorrect in JSON.
+
+fn main() {
+    let y = 1;
+    let _capture = for<'a> |x: &'a i32| -> i32 { *x + y };
+    //~^ ERROR `for<...>` binders for closures are experimental
+
+    let let_else_env = 1;
+    let _let_else = for<'a> |x: &'a i32| -> i32 {
+        //~^ ERROR `for<...>` binders for closures are experimental
+        let Some(_) = None::<i32> else {
+            let let_else_env = 0;
+            return let_else_env;
+        };
+        *x + let_else_env
+    };
+
+    let chain_env = 1;
+    let _let_chain = for<'a> |x: &'a i32| -> i32 {
+        //~^ ERROR `for<...>` binders for closures are experimental
+        if let Some(chain_env) = None::<i32>
+            && chain_env == 0
+        {
+            0
+        } else {
+            *x + chain_env
+        }
+    };
+}

tests/ui/feature-gates/feature-gate-closure_lifetime_binder-maybe-incorrect.stderr

@@ -0,0 +1,119 @@
+{"$message_type":"diagnostic","message":"`for<...>` binders for closures are experimental","code":{"code":"E0658","explanation":"An unstable feature was used.
+
+Erroneous code example:
+
+```compile_fail,E0658
+use std::intrinsics; // error: use of unstable library feature `core_intrinsics`
+```
+
+If you're using a stable or a beta version of rustc, you won't be able to use
+any unstable features. In order to do so, please switch to a nightly version of
+rustc (by using [rustup]).
+
+If you're using a nightly version of rustc, just add the corresponding feature
+to be able to use it:
+
+```
+#![feature(core_intrinsics)]
+
+use std::intrinsics; // ok!
+```
+
+[rustup]: https://rust-lang.github.io/rustup/concepts/channels.html
+"},"level":"error","spans":[{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":345,"byte_end":352,"line_start":10,"line_end":10,"column_start":20,"column_end":27,"is_primary":true,"text":[{"text":"    let _capture = for<'a> |x: &'a i32| -> i32 { *x + y };","highlight_start":20,"highlight_end":27}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable","code":null,"level":"help","spans":[],"children":[],"rendered":null},{"message":"this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"consider setting the binding type instead","code":null,"level":"help","spans":[{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":342,"byte_end":342,"line_start":10,"line_end":10,"column_start":17,"column_end":17,"is_primary":true,"text":[{"text":"    let _capture = for<'a> |x: &'a i32| -> i32 { *x + y };","highlight_start":17,"highlight_end":17}],"label":null,"suggested_replacement":": for<'a> fn(&'a i32) -> i32","suggestion_applicability":"MaybeIncorrect","expansion":null},{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":345,"byte_end":372,"line_start":10,"line_end":10,"column_start":20,"column_end":47,"is_primary":true,"text":[{"text":"    let _capture = for<'a> |x: &'a i32| -> i32 { *x + y };","highlight_start":20,"highlight_end":47}],"label":null,"suggested_replacement":"|x|","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"error[E0658]: `for<...>` binders for closures are experimental
+  --> $DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs:10:20
+   |
+LL |     let _capture = for<'a> |x: &'a i32| -> i32 { *x + y };
+   |                    ^^^^^^^
+   |
+   = note: see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information
+   = help: add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: consider setting the binding type instead
+   |
+LL -     let _capture = for<'a> |x: &'a i32| -> i32 { *x + y };
+LL +     let _capture: for<'a> fn(&'a i32) -> i32 = |x| { *x + y };
+   |
+
+"}
+{"$message_type":"diagnostic","message":"`for<...>` binders for closures are experimental","code":{"code":"E0658","explanation":"An unstable feature was used.
+
+Erroneous code example:
+
+```compile_fail,E0658
+use std::intrinsics; // error: use of unstable library feature `core_intrinsics`
+```
+
+If you're using a stable or a beta version of rustc, you won't be able to use
+any unstable features. In order to do so, please switch to a nightly version of
+rustc (by using [rustup]).
+
+If you're using a nightly version of rustc, just add the corresponding feature
+to be able to use it:
+
+```
+#![feature(core_intrinsics)]
+
+use std::intrinsics; // ok!
+```
+
+[rustup]: https://rust-lang.github.io/rustup/concepts/channels.html
+"},"level":"error","spans":[{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":496,"byte_end":503,"line_start":14,"line_end":14,"column_start":21,"column_end":28,"is_primary":true,"text":[{"text":"    let _let_else = for<'a> |x: &'a i32| -> i32 {","highlight_start":21,"highlight_end":28}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable","code":null,"level":"help","spans":[],"children":[],"rendered":null},{"message":"this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"consider setting the binding type instead","code":null,"level":"help","spans":[{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":493,"byte_end":493,"line_start":14,"line_end":14,"column_start":18,"column_end":18,"is_primary":true,"text":[{"text":"    let _let_else = for<'a> |x: &'a i32| -> i32 {","highlight_start":18,"highlight_end":18}],"label":null,"suggested_replacement":": for<'a> fn(&'a i32) -> i32","suggestion_applicability":"MaybeIncorrect","expansion":null},{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":496,"byte_end":523,"line_start":14,"line_end":14,"column_start":21,"column_end":48,"is_primary":true,"text":[{"text":"    let _let_else = for<'a> |x: &'a i32| -> i32 {","highlight_start":21,"highlight_end":48}],"label":null,"suggested_replacement":"|x|","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"error[E0658]: `for<...>` binders for closures are experimental
+  --> $DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs:14:21
+   |
+LL |     let _let_else = for<'a> |x: &'a i32| -> i32 {
+   |                     ^^^^^^^
+   |
+   = note: see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information
+   = help: add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: consider setting the binding type instead
+   |
+LL -     let _let_else = for<'a> |x: &'a i32| -> i32 {
+LL +     let _let_else: for<'a> fn(&'a i32) -> i32 = |x| {
+   |
+
+"}
+{"$message_type":"diagnostic","message":"`for<...>` binders for closures are experimental","code":{"code":"E0658","explanation":"An unstable feature was used.
+
+Erroneous code example:
+
+```compile_fail,E0658
+use std::intrinsics; // error: use of unstable library feature `core_intrinsics`
+```
+
+If you're using a stable or a beta version of rustc, you won't be able to use
+any unstable features. In order to do so, please switch to a nightly version of
+rustc (by using [rustup]).
+
+If you're using a nightly version of rustc, just add the corresponding feature
+to be able to use it:
+
+```
+#![feature(core_intrinsics)]
+
+use std::intrinsics; // ok!
+```
+
+[rustup]: https://rust-lang.github.io/rustup/concepts/channels.html
+"},"level":"error","spans":[{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":791,"byte_end":798,"line_start":24,"line_end":24,"column_start":22,"column_end":29,"is_primary":true,"text":[{"text":"    let _let_chain = for<'a> |x: &'a i32| -> i32 {","highlight_start":22,"highlight_end":29}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable","code":null,"level":"help","spans":[],"children":[],"rendered":null},{"message":"this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date","code":null,"level":"note","spans":[],"children":[],"rendered":null},{"message":"consider setting the binding type instead","code":null,"level":"help","spans":[{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":788,"byte_end":788,"line_start":24,"line_end":24,"column_start":19,"column_end":19,"is_primary":true,"text":[{"text":"    let _let_chain = for<'a> |x: &'a i32| -> i32 {","highlight_start":19,"highlight_end":19}],"label":null,"suggested_replacement":": for<'a> fn(&'a i32) -> i32","suggestion_applicability":"MaybeIncorrect","expansion":null},{"file_name":"$DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs","byte_start":791,"byte_end":818,"line_start":24,"line_end":24,"column_start":22,"column_end":49,"is_primary":true,"text":[{"text":"    let _let_chain = for<'a> |x: &'a i32| -> i32 {","highlight_start":22,"highlight_end":49}],"label":null,"suggested_replacement":"|x|","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"error[E0658]: `for<...>` binders for closures are experimental
+  --> $DIR/feature-gate-closure_lifetime_binder-maybe-incorrect.rs:24:22
+   |
+LL |     let _let_chain = for<'a> |x: &'a i32| -> i32 {
+   |                      ^^^^^^^
+   |
+   = note: see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information
+   = help: add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: consider setting the binding type instead
+   |
+LL -     let _let_chain = for<'a> |x: &'a i32| -> i32 {
+LL +     let _let_chain: for<'a> fn(&'a i32) -> i32 = |x| {
+   |
+
+"}
+{"$message_type":"diagnostic","message":"aborting due to 3 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 3 previous errors
+
+"}
+{"$message_type":"diagnostic","message":"For more information about this error, try `rustc --explain E0658`.","code":null,"level":"failure-note","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0658`.
+"}

tests/ui/feature-gates/feature-gate-closure_lifetime_binder-rustfix.fixed

@@ -0,0 +1,10 @@
+//@ run-rustfix
+//@ rustfix-only-machine-applicable
+
+// Verify the #160431 rewrite is MachineApplicable: rustfix applies it and the result compiles
+// without `#![feature(closure_lifetime_binder)]`.
+
+fn main() {
+    let _cl: for<'a> fn(&'a str) -> (&'a str, &'a str) = |x| { x.split_at(0) };
+    //~^ ERROR `for<...>` binders for closures are experimental
+}

tests/ui/feature-gates/feature-gate-closure_lifetime_binder-rustfix.rs

@@ -0,0 +1,10 @@
+//@ run-rustfix
+//@ rustfix-only-machine-applicable
+
+// Verify the #160431 rewrite is MachineApplicable: rustfix applies it and the result compiles
+// without `#![feature(closure_lifetime_binder)]`.
+
+fn main() {
+    let _cl = for<'a> |x: &'a str| -> (&'a str, &'a str) { x.split_at(0) };
+    //~^ ERROR `for<...>` binders for closures are experimental
+}

tests/ui/feature-gates/feature-gate-closure_lifetime_binder-rustfix.stderr

@@ -0,0 +1,18 @@
+error[E0658]: `for<...>` binders for closures are experimental
+  --> $DIR/feature-gate-closure_lifetime_binder-rustfix.rs:8:15
+   |
+LL |     let _cl = for<'a> |x: &'a str| -> (&'a str, &'a str) { x.split_at(0) };
+   |               ^^^^^^^
+   |
+   = note: see issue #97362 <https://github.com/rust-lang/rust/issues/97362> for more information
+   = help: add `#![feature(closure_lifetime_binder)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+help: consider setting the binding type instead
+   |
+LL -     let _cl = for<'a> |x: &'a str| -> (&'a str, &'a str) { x.split_at(0) };
+LL +     let _cl: for<'a> fn(&'a str) -> (&'a str, &'a str) = |x| { x.split_at(0) };
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.