Add foreign formatting directive detection. #37613
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cc @rust-lang/lang @rust-lang/libs -- not sure what the right jurisdiction here is :) This seems like an amazing change to me! |
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Sounds like a good idea to me! |
alexcrichton
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I'm going to unilaterally usurp this from the lang team and declare it a libs PR! This seems like a large enough feature though that we'd want to consider it carefully, though, in which case... @rfcbot fcp merge |
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Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged teams: No concerns currently listed. Once these reviewers reach consensus, this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up! See this document for info about what commands tagged team members can give me. |
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This looks reasonable to me, but although it contains lots of unit tests, I'd like to see at least one compile-fail test showing that the compiler does indeed emit these errors. |
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@brson I looked at modifying an existing compile-fail test for |
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@DanielKeep You should probably use an |
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This teaches `format_args!` how to interpret format printf- and
shell-style format directives. This is used in cases where there are
unused formatting arguments, and the reason for that *might* be because
the programmer is trying to use the wrong kind of formatting string.
This was prompted by an issue encountered by simulacrum on the #rust IRC
channel. In short: although `println!` told them that they weren't using
all of the conversion arguments, the problem was in using printf-syle
directives rather than ones `println!` would undertand.
Where possible, `format_args!` will tell the programmer what they should
use instead. For example, it will suggest replacing `%05d` with `{:0>5}`,
or `%2$.*3$s` with `{1:.3$}`. Even if it cannot suggest a replacement,
it will explicitly note that Rust does not support that style of directive,
and direct the user to the `std::fmt` documentation.
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Added a |
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Thanks @DanielKeep ! |
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🔔 This is now entering its final comment period, as per the review above. 🔔 psst @alexcrichton, I wasn't able to add the |
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@bors: r+ |
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📌 Commit 455723c has been approved by |
…ur-format, r=alexcrichton
Add foreign formatting directive detection.
This teaches `format_args!` how to interpret format printf- and
shell-style format directives. This is used in cases where there are
unused formatting arguments, and the reason for that *might* be because
the programmer is trying to use the wrong kind of formatting string.
This was prompted by an issue encountered by simulacrum on the #rust IRC
channel. In short: although `println!` told them that they weren't using
all of the conversion arguments, the problem was in using printf-syle
directives rather than ones `println!` would undertand.
Where possible, `format_args!` will tell the programmer what they should
use instead. For example, it will suggest replacing `%05d` with `{:0>5}`,
or `%2$.*3$s` with `{1:.3$}`. Even if it cannot suggest a replacement,
it will explicitly note that Rust does not support that style of directive,
and direct the user to the `std::fmt` documentation.
-----
**Example**: given:
```rust
fn main() {
println!("%.*3$s %s!\n", "Hello,", "World", 4);
println!("%1$*2$.*3$f", 123.456);
}
```
The compiler outputs the following:
```text
error: multiple unused formatting arguments
--> local/fmt.rs:2:5
|
2 | println!("%.*3$s %s!\n", "Hello,", "World", 4);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: argument never used
--> local/fmt.rs:2:30
|
2 | println!("%.*3$s %s!\n", "Hello,", "World", 4);
| ^^^^^^^^
note: argument never used
--> local/fmt.rs:2:40
|
2 | println!("%.*3$s %s!\n", "Hello,", "World", 4);
| ^^^^^^^
note: argument never used
--> local/fmt.rs:2:49
|
2 | println!("%.*3$s %s!\n", "Hello,", "World", 4);
| ^
= help: `%.*3$s` should be written as `{:.2$}`
= help: `%s` should be written as `{}`
= note: printf formatting not supported; see the documentation for `std::fmt`
= note: this error originates in a macro outside of the current crate
error: argument never used
--> local/fmt.rs:6:29
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6 | println!("%1$*2$.*3$f", 123.456);
| ^^^^^^^
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= help: `%1$*2$.*3$f` should be written as `{0:1$.2$}`
= note: printf formatting not supported; see the documentation for `std::fmt`
```
Rollup of 30 pull requests - Successful merges: #37190, #37368, #37481, #37503, #37527, #37535, #37551, #37584, #37600, #37613, #37615, #37659, #37662, #37669, #37682, #37688, #37690, #37692, #37693, #37694, #37695, #37696, #37698, #37699, #37705, #37708, #37709, #37716, #37724, #37727 - Failed merges: #37640, #37689, #37717
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☔ The latest upstream changes (presumably #37730) made this pull request unmergeable. Please resolve the merge conflicts. |
This teaches
format_args!how to interpret format printf- andshell-style format directives. This is used in cases where there are
unused formatting arguments, and the reason for that might be because
the programmer is trying to use the wrong kind of formatting string.
This was prompted by an issue encountered by simulacrum on the #rust IRC
channel. In short: although
println!told them that they weren't usingall of the conversion arguments, the problem was in using printf-syle
directives rather than ones
println!would undertand.Where possible,
format_args!will tell the programmer what they shoulduse instead. For example, it will suggest replacing
%05dwith{:0>5},or
%2$.*3$swith{1:.3$}. Even if it cannot suggest a replacement,it will explicitly note that Rust does not support that style of directive,
and direct the user to the
std::fmtdocumentation.Example: given:
The compiler outputs the following: