Document precision considerations of Duration-float methods#155133
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rust-bors[bot] merged 1 commit intorust-lang:mainfrom Apr 22, 2026
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Document precision considerations of Duration-float methods#155133rust-bors[bot] merged 1 commit intorust-lang:mainfrom
Duration-float methods#155133rust-bors[bot] merged 1 commit intorust-lang:mainfrom
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A `Duration` is essentially a 94-bit value (64-bit sec and ~30-bit ns), so there's some inherent loss when converting to floating-point for `mul_f64` and `div_f64`. We could go to greater lengths to compute these with more accuracy, like rust-lang#150933 or rust-lang#154107, but it's not clear that it's worth the effort. The least we can do is document that some rounding is to be expected, which this commit does with simple examples that only multiply or divide by `1.0`. This also changes the `f32` methods to just forward to `f64`, so we keep more of that duration precision, as the range is otherwise much more limited there.
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r? @scottmcm rustbot has assigned @scottmcm. Use Why was this reviewer chosen?The reviewer was selected based on:
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This looks good to me. |
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Ok, no new API guarantees, it's just a note informing users about limited precision/lossiness. This was also previously discussed in an api-meeting which was generally on board with the idea of just clarifying instead of wasting performance on slightly improving precision. LGTM @bors r+ |
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…uwer Rollup of 10 pull requests Successful merges: - #154794 (Add on_unmatch_args) - #155133 (Document precision considerations of `Duration`-float methods) - #154283 (Remove `nodes_in_current_session` field and related assertions) - #155374 (rustdoc: fix a few spots where emit isn't respected) - #155587 (Immediately feed visibility on DefId creation) - #155622 (c-variadic: `va_arg` fixes ) - #155629 (rustc_public: Add `constness` & `asyncness` in `FnDef`) - #155632 (Some metadata cleanups) - #155639 (BinOpAssign always returns unit) - #155647 (rustc-dev-guide subtree update)
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Rollup merge of #155133 - cuviper:duration_fp_docs, r=the8472 Document precision considerations of `Duration`-float methods A `Duration` is essentially a 94-bit value (64-bit sec and ~30-bit ns), so there's some inherent loss when converting to floating-point for `mul_f64` and `div_f64`. We could go to greater lengths to compute these with more accuracy, like #150933 or #154107, but it's not clear that it's worth the effort. The least we can do is document that some rounding is to be expected, which this commit does with simple examples that only multiply or divide by `1.0`. This also changes the `f32` methods to just forward to `f64`, so we keep more of that duration precision, as the range is otherwise much more limited there.
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…uwer Rollup of 10 pull requests Successful merges: - rust-lang/rust#154794 (Add on_unmatch_args) - rust-lang/rust#155133 (Document precision considerations of `Duration`-float methods) - rust-lang/rust#154283 (Remove `nodes_in_current_session` field and related assertions) - rust-lang/rust#155374 (rustdoc: fix a few spots where emit isn't respected) - rust-lang/rust#155587 (Immediately feed visibility on DefId creation) - rust-lang/rust#155622 (c-variadic: `va_arg` fixes ) - rust-lang/rust#155629 (rustc_public: Add `constness` & `asyncness` in `FnDef`) - rust-lang/rust#155632 (Some metadata cleanups) - rust-lang/rust#155639 (BinOpAssign always returns unit) - rust-lang/rust#155647 (rustc-dev-guide subtree update)
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A
Durationis essentially a 94-bit value (64-bit sec and ~30-bit ns),so there's some inherent loss when converting to floating-point for
mul_f64anddiv_f64. We could go to greater lengths to compute thesewith more accuracy, like #150933 or #154107,
but it's not clear that it's worth the effort. The least we can do is
document that some rounding is to be expected, which this commit does
with simple examples that only multiply or divide by
1.0.This also changes the
f32methods to just forward tof64, so we keepmore of that duration precision, as the range is otherwise much more
limited there.