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

Add `format_args_capture` feature

This is the initial implementation PR for [RFC 2795](https://github.com/rust-lang/rfcs/pull/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.
This commit is contained in:
Manish Goregaokar 2020-07-03 17:16:52 -07:00 committed by GitHub
commit 4a8d9ea80f
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13 changed files with 305 additions and 5 deletions

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@ -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`

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@ -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

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@ -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
// -------------------------------------------------------------------------

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@ -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

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

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@ -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`
}

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@ -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

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@ -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`
}

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@ -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

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@ -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
}

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@ -0,0 +1,52 @@
error: named argument never used
--> $DIR/format-args-capture-missing-variables.rs:10:51
|
LL | format!("{valuea} {valueb}", valuea=5, valuec=7);
| ------------------- ^ named argument never used
| |
| formatting specifier missing
error[E0425]: cannot find value `foo` in this scope
--> $DIR/format-args-capture-missing-variables.rs:4:13
|
LL | format!("{} {foo} {} {bar} {}", 1, 2, 3);
| ^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
error[E0425]: cannot find value `bar` in this scope
--> $DIR/format-args-capture-missing-variables.rs:4:13
|
LL | format!("{} {foo} {} {bar} {}", 1, 2, 3);
| ^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
error[E0425]: cannot find value `foo` in this scope
--> $DIR/format-args-capture-missing-variables.rs:8:13
|
LL | format!("{foo}");
| ^^^^^^^ not found in this scope
error[E0425]: cannot find value `valueb` in this scope
--> $DIR/format-args-capture-missing-variables.rs:10:13
|
LL | format!("{valuea} {valueb}", valuea=5, valuec=7);
| ^^^^^^^^^^^^^^^^^^^ not found in this scope
error[E0425]: cannot find value `foo` in this scope
--> $DIR/format-args-capture-missing-variables.rs:14:13
|
LL | format!(r##"
| _____________^
LL | |
LL | | {foo}
LL | |
LL | | "##);
| |_______^ not found in this scope
error[E0425]: cannot find value `foo` in this scope
--> $DIR/format-args-capture-missing-variables.rs:21:12
|
LL | panic!("{foo} {bar}", bar=1);
| ^^^^^^^^^^^^^ not found in this scope
error: aborting due to 7 previous errors
For more information about this error, try `rustc --explain E0425`.

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@ -0,0 +1,64 @@
// run-pass
// ignore-wasm32
// ignore-wasm64
#![feature(format_args_capture)]
fn main() {
named_argument_takes_precedence_to_captured();
panic_with_single_argument_does_not_get_formatted();
panic_with_multiple_arguments_is_formatted();
formatting_parameters_can_be_captured();
}
fn named_argument_takes_precedence_to_captured() {
let foo = "captured";
let s = format!("{foo}", foo="named");
assert_eq!(&s, "named");
let s = format!("{foo}-{foo}-{foo}", foo="named");
assert_eq!(&s, "named-named-named");
let s = format!("{}-{bar}-{foo}", "positional", bar="named");
assert_eq!(&s, "positional-named-captured");
}
fn panic_with_single_argument_does_not_get_formatted() {
// panic! with a single argument does not perform string formatting.
// RFC #2795 suggests that this may need to change so that captured arguments are formatted.
// For stability reasons this will need to part of an edition change.
let msg = std::panic::catch_unwind(|| {
panic!("{foo}");
}).unwrap_err();
assert_eq!(msg.downcast_ref::<&str>(), Some(&"{foo}"))
}
fn panic_with_multiple_arguments_is_formatted() {
let foo = "captured";
let msg = std::panic::catch_unwind(|| {
panic!("{}-{bar}-{foo}", "positional", bar="named");
}).unwrap_err();
assert_eq!(msg.downcast_ref::<String>(), Some(&"positional-named-captured".to_string()))
}
fn formatting_parameters_can_be_captured() {
let width = 9;
let precision = 3;
let x = 7.0;
let s = format!("{x:width$}");
assert_eq!(&s, " 7");
let s = format!("{x:<width$}");
assert_eq!(&s, "7 ");
let s = format!("{x:-^width$}");
assert_eq!(&s, "----7----");
let s = format!("{x:-^width$.precision$}");
assert_eq!(&s, "--7.000--");
}

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@ -63,18 +63,24 @@ 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
|
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
|
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
@ -155,6 +161,8 @@ 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
@ -205,6 +213,8 @@ 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