Rollup merge of #73670 - davidhewitt:format-args-capture, r=varkor

Add `format_args_capture` feature

This is the initial implementation PR for [RFC 2795](rust-lang/rfcs#2795).

Note that, as dicussed in the tracking issue (#67984), the feature gate has been called `format_args_capture`.

Next up I guess I need to add documentation for this feature. I've not written any docs before for rustc / std so I would appreciate suggestions on where I should add docs.

Author: Manishearth

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

@@ -0,0 +1,47 @@
+# `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/librustc_builtin_macros/format.rs

@@ -107,6 +107,9 @@ struct Context<'a, 'b> {
     arg_spans: Vec<Span>,
     /// All the formatting arguments that have formatting flags set, in order for diagnostics.
     arg_with_formatting: Vec<parse::FormatSpec<'a>>,
+
+    /// Whether this format string came from a string literal, as opposed to a macro.
+    is_literal: bool,
 }
 
 /// Parses the arguments from the given list of tokens, returning the diagnostic
@@ -498,10 +501,55 @@ impl<'a, 'b> Context<'a, 'b> {
                         self.verify_arg_type(Exact(idx), ty)
                     }
                     None => {
-                        let msg = format!("there is no argument named `{}`", name);
-                        let sp = *self.arg_spans.get(self.curpiece).unwrap_or(&self.fmtsp);
-                        let mut err = self.ecx.struct_span_err(sp, &msg[..]);
-                        err.emit();
+                        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 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());
+                            self.arg_unique_types.push(Vec::new());
+                            self.args.push(
+                                self.ecx.expr_ident(self.fmtsp, Ident::new(name, self.fmtsp)),
+                            );
+                            self.names.insert(name, idx);
+                            self.verify_arg_type(Exact(idx), ty)
+                        } else {
+                            let msg = format!("there is no argument named `{}`", name);
+                            let sp = if self.is_literal {
+                                *self.arg_spans.get(self.curpiece).unwrap_or(&self.fmtsp)
+                            } else {
+                                self.fmtsp
+                            };
+                            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?",
+                                    name
+                                ));
+                                err.note(
+                                    "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();
+                        }
                     }
                 }
             }
@@ -951,6 +999,7 @@ pub fn expand_preparsed_format_args(
         invalid_refs: Vec::new(),
         arg_spans,
         arg_with_formatting: Vec::new(),
+        is_literal: parser.is_literal,
     };
 
     // This needs to happen *after* the Parser has consumed all pieces to create all the spans

src/librustc_feature/active.rs

@@ -567,6 +567,9 @@ 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),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/librustc_parse_format/lib.rs

@@ -190,7 +190,7 @@ pub struct Parser<'a> {
     /// Whether the source string is comes from `println!` as opposed to `format!` or `print!`
     append_newline: bool,
     /// Whether this formatting string is a literal or it comes from a macro.
-    is_literal: bool,
+    pub is_literal: bool,
     /// Start position of the current line.
     cur_line_start: usize,
     /// Start and end byte offset of every line of the format string. Excludes

src/librustc_span/symbol.rs

@@ -342,6 +342,7 @@ symbols! {
         forbid,
         format_args,
         format_args_nl,
+        format_args_capture,
         from,
         From,
         from_desugaring,

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

@@ -0,0 +1,6 @@
+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

@@ -0,0 +1,18 @@
+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

@@ -0,0 +1,6 @@
+#![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-macro-hygiene.stderr

@@ -0,0 +1,22 @@
+error: there is no argument named `foo`
+  --> $DIR/format-args-capture-macro-hygiene.rs:4:13
+   |
+LL |     format!(concat!("{foo}"));
+   |             ^^^^^^^^^^^^^^^^
+   |
+   = note: did you intend to capture a variable `foo` from the surrounding scope?
+   = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: there is no argument named `bar`
+  --> $DIR/format-args-capture-macro-hygiene.rs:5:13
+   |
+LL |     format!(concat!("{ba", "r} {}"), 1);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: did you intend to capture a variable `bar` from the surrounding scope?
+   = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+

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

@@ -0,0 +1,22 @@
+#![feature(format_args_capture)]
+
+fn main() {
+    format!("{} {foo} {} {bar} {}", 1, 2, 3);
+    //~^ ERROR: cannot find value `foo` in this scope
+    //~^^ ERROR: cannot find value `bar` in this scope
+
+    format!("{foo}");                //~ ERROR: cannot find value `foo` in this scope
+
+    format!("{valuea} {valueb}", valuea=5, valuec=7);
+    //~^ ERROR cannot find value `valueb` in this scope
+    //~^^ ERROR named argument never used
+
+    format!(r##"
+
+        {foo}
+
+    "##);
+    //~^^^^^ ERROR: cannot find value `foo` in this scope
+
+    panic!("{foo} {bar}", bar=1);    //~ ERROR: cannot find value `foo` in this scope
+}