Stabilize format_args_capture

Works as expected, and there are widespread reports of success with it,
as well as interest in it.

compiler/rustc_borrowck/src/lib.rs

@@ -3,7 +3,7 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(crate_visibility_modifier)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
 #![feature(let_else)]

compiler/rustc_builtin_macros/src/format.rs

@@ -527,17 +527,9 @@ impl<'a, 'b> Context<'a, 'b> {
                         self.verify_arg_type(Exact(idx), ty)
                     }
                     None => {
-                        let capture_feature_enabled = self
-                            .ecx
-                            .ecfg
-                            .features
-                            .map_or(false, |features| features.format_args_capture);
-
                         // For the moment capturing variables from format strings expanded from macros is
                         // disabled (see RFC #2795)
-                        let can_capture = capture_feature_enabled && self.is_literal;
+                        if self.is_literal {
-
-                        if can_capture {
                             // Treat this name as a variable to capture from the surrounding scope
                             let idx = self.args.len();
                             self.arg_types.push(Vec::new());
@@ -559,7 +551,6 @@ impl<'a, 'b> Context<'a, 'b> {
                             };
                             let mut err = self.ecx.struct_span_err(sp, &msg[..]);
 
-                            if capture_feature_enabled && !self.is_literal {
                             err.note(&format!(
                                 "did you intend to capture a variable `{}` from \
                                  the surrounding scope?",
@@ -569,13 +560,6 @@ impl<'a, 'b> Context<'a, 'b> {
                                 "to avoid ambiguity, `format_args!` cannot capture variables \
                                  when the format string is expanded from a macro",
                             );
-                            } else if self.ecx.parse_sess().unstable_features.is_nightly_build() {
-                                err.help(&format!(
-                                    "if you intended to capture `{}` from the surrounding scope, add \
-                                     `#![feature(format_args_capture)]` to the crate attributes",
-                                    name
-                                ));
-                            }
 
                             err.emit();
                         }

compiler/rustc_errors/src/lib.rs

@@ -6,7 +6,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(backtrace)]
 #![feature(if_let_guard)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(iter_zip)]
 #![feature(let_else)]
 #![feature(nll)]

compiler/rustc_expand/src/lib.rs

@@ -1,7 +1,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]
 #![feature(destructuring_assignment)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(if_let_guard)]
 #![feature(iter_zip)]
 #![feature(let_else)]

compiler/rustc_feature/src/accepted.rs

@@ -301,6 +301,8 @@ declare_features! (
     (accepted, relaxed_struct_unsize, "1.58.0", Some(81793), None),
     /// Allows dereferencing raw pointers during const eval.
     (accepted, const_raw_ptr_deref, "1.58.0", Some(51911), None),
+    /// Allows capturing variables in scope using format_args!
+    (accepted, format_args_capture, "1.58.0", Some(67984), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features

compiler/rustc_feature/src/active.rs

@@ -539,9 +539,6 @@ declare_features! (
     /// Be more precise when looking for live drops in a const context.
     (active, const_precise_live_drops, "1.46.0", Some(73255), None),
 
-    /// Allows capturing variables in scope using format_args!
-    (active, format_args_capture, "1.46.0", Some(67984), None),
-
     /// Allows `if let` guard in match arms.
     (active, if_let_guard, "1.47.0", Some(51114), None),
 

compiler/rustc_lint/src/lib.rs

@@ -30,7 +30,7 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(crate_visibility_modifier)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(iter_order_by)]
 #![feature(iter_zip)]
 #![feature(never_type)]

compiler/rustc_passes/src/lib.rs

@@ -7,7 +7,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(crate_visibility_modifier)]
 #![feature(in_band_lifetimes)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(iter_zip)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

compiler/rustc_resolve/src/lib.rs

@@ -13,7 +13,7 @@
 #![feature(drain_filter)]
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(iter_zip)]
 #![feature(let_else)]
 #![feature(never_type)]

compiler/rustc_typeck/src/lib.rs

@@ -58,7 +58,7 @@ This API is completely unstable and subject to change.
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(if_let_guard)]
 #![feature(in_band_lifetimes)]
 #![feature(is_sorted)]

library/alloc/src/lib.rs

@@ -105,7 +105,7 @@
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
 #![feature(inherent_ascii_escape)]
-#![feature(format_args_capture)]
+#![cfg_attr(bootstrap, feature(format_args_capture))]
 #![feature(inplace_iteration)]
 #![feature(iter_advance_by)]
 #![feature(iter_zip)]

src/doc/unstable-book/src/library-features/format-args-capture.md

@@ -1,47 +0,0 @@
-# `format_args_capture`
-
-The tracking issue for this feature is: [#67984]
-
-[#67984]: https://github.com/rust-lang/rust/issues/67984
-
-------------------------
-
-Enables `format_args!` (and macros which use `format_args!` in their implementation, such
-as `format!`, `print!` and `panic!`) to capture variables from the surrounding scope.
-This avoids the need to pass named parameters when the binding in question
-already exists in scope.
-
-```rust
-#![feature(format_args_capture)]
-
-let (person, species, name) = ("Charlie Brown", "dog", "Snoopy");
-
-// captures named argument `person`
-print!("Hello {person}");
-
-// captures named arguments `species` and `name`
-format!("The {species}'s name is {name}.");
-```
-
-This also works for formatting parameters such as width and precision:
-
-```rust
-#![feature(format_args_capture)]
-
-let precision = 2;
-let s = format!("{:.precision$}", 1.324223);
-
-assert_eq!(&s, "1.32");
-```
-
-A non-exhaustive list of macros which benefit from this functionality include:
-- `format!`
-- `print!` and `println!`
-- `eprint!` and `eprintln!`
-- `write!` and `writeln!`
-- `panic!`
-- `unreachable!`
-- `unimplemented!`
-- `todo!`
-- `assert!` and similar
-- macros in many thirdparty crates, such as `log`

src/test/ui/fmt/feature-gate-format-args-capture.rs

@@ -1,6 +0,0 @@
-fn main() {
-    format!("{foo}");                //~ ERROR: there is no argument named `foo`
-
-    // panic! doesn't hit format_args! unless there are two or more arguments.
-    panic!("{foo} {bar}", bar=1);    //~ ERROR: there is no argument named `foo`
-}

src/test/ui/fmt/feature-gate-format-args-capture.stderr

@@ -1,18 +0,0 @@
-error: there is no argument named `foo`
-  --> $DIR/feature-gate-format-args-capture.rs:2:14
-   |
-LL |     format!("{foo}");
-   |              ^^^^^
-   |
-   = help: if you intended to capture `foo` from the surrounding scope, add `#![feature(format_args_capture)]` to the crate attributes
-
-error: there is no argument named `foo`
-  --> $DIR/feature-gate-format-args-capture.rs:5:13
-   |
-LL |     panic!("{foo} {bar}", bar=1);
-   |             ^^^^^
-   |
-   = help: if you intended to capture `foo` from the surrounding scope, add `#![feature(format_args_capture)]` to the crate attributes
-
-error: aborting due to 2 previous errors
-

src/test/ui/fmt/format-args-capture-macro-hygiene.rs

@@ -1,5 +1,3 @@
-#![feature(format_args_capture)]
-
 fn main() {
     format!(concat!("{foo}"));         //~ ERROR: there is no argument named `foo`
     format!(concat!("{ba", "r} {}"), 1);     //~ ERROR: there is no argument named `bar`

src/test/ui/fmt/format-args-capture-missing-variables.rs

@@ -1,5 +1,3 @@
-#![feature(format_args_capture)]
-
 fn main() {
     format!("{} {foo} {} {bar} {}", 1, 2, 3);
     //~^ ERROR: cannot find value `foo` in this scope

src/test/ui/fmt/format-args-capture.rs

@@ -1,5 +1,4 @@
 // run-pass
-#![feature(format_args_capture)]
 #![feature(cfg_panic)]
 
 fn main() {

src/test/ui/fmt/ifmt-bad-arg.stderr

@@ -64,7 +64,6 @@ error: there is no argument named `foo`
 LL |     format!("{} {foo} {} {bar} {}", 1, 2, 3);
    |                 ^^^^^
    |
-   = help: if you intended to capture `foo` from the surrounding scope, add `#![feature(format_args_capture)]` to the crate attributes
 
 error: there is no argument named `bar`
   --> $DIR/ifmt-bad-arg.rs:27:26
@@ -72,7 +71,6 @@ error: there is no argument named `bar`
 LL |     format!("{} {foo} {} {bar} {}", 1, 2, 3);
    |                          ^^^^^
    |
-   = help: if you intended to capture `bar` from the surrounding scope, add `#![feature(format_args_capture)]` to the crate attributes
 
 error: there is no argument named `foo`
   --> $DIR/ifmt-bad-arg.rs:31:14
@@ -80,7 +78,6 @@ error: there is no argument named `foo`
 LL |     format!("{foo}");
    |              ^^^^^
    |
-   = help: if you intended to capture `foo` from the surrounding scope, add `#![feature(format_args_capture)]` to the crate attributes
 
 error: multiple unused formatting arguments
   --> $DIR/ifmt-bad-arg.rs:32:17
@@ -162,7 +159,6 @@ error: there is no argument named `valueb`
 LL |     format!("{valuea} {valueb}", valuea=5, valuec=7);
    |                       ^^^^^^^^
    |
-   = help: if you intended to capture `valueb` from the surrounding scope, add `#![feature(format_args_capture)]` to the crate attributes
 
 error: named argument never used
   --> $DIR/ifmt-bad-arg.rs:45:51
@@ -214,7 +210,6 @@ error: there is no argument named `foo`
 LL |         {foo}
    |         ^^^^^
    |
-   = help: if you intended to capture `foo` from the surrounding scope, add `#![feature(format_args_capture)]` to the crate attributes
 
 error: invalid format string: expected `'}'`, found `'t'`
   --> $DIR/ifmt-bad-arg.rs:75:1