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Diffstat (limited to 'src/tools/clippy/src/docs/format_push_string.txt')
-rw-r--r-- | src/tools/clippy/src/docs/format_push_string.txt | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/src/tools/clippy/src/docs/format_push_string.txt b/src/tools/clippy/src/docs/format_push_string.txt new file mode 100644 index 000000000..ca409ebc7 --- /dev/null +++ b/src/tools/clippy/src/docs/format_push_string.txt @@ -0,0 +1,26 @@ +### What it does +Detects cases where the result of a `format!` call is +appended to an existing `String`. + +### Why is this bad? +Introduces an extra, avoidable heap allocation. + +### Known problems +`format!` returns a `String` but `write!` returns a `Result`. +Thus you are forced to ignore the `Err` variant to achieve the same API. + +While using `write!` in the suggested way should never fail, this isn't necessarily clear to the programmer. + +### Example +``` +let mut s = String::new(); +s += &format!("0x{:X}", 1024); +s.push_str(&format!("0x{:X}", 1024)); +``` +Use instead: +``` +use std::fmt::Write as _; // import without risk of name clashing + +let mut s = String::new(); +let _ = write!(s, "0x{:X}", 1024); +```
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