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Specify that range patterns must be nonempty. #2093

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Specify that range patterns must be nonempty. #2093

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10 changes: 6 additions & 4 deletions src/patterns.md
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Expand Up @@ -554,10 +554,12 @@ It is written as `..=` followed by the upper bound.

For example, `..=10` will match any integer less than or equal to 10, such as 10, 1, 0, and for signed integer types, all negative values.

r[patterns.range.constraint-less-than]
The lower bound cannot be greater than the upper bound.
That is, in `a..=b`, a ≤ b must be the case.
For example, it is an error to have a range pattern `10..=0`.
r[patterns.range.constraint-nonempty]
A range pattern must match at least one possible value. In other words:
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@ehuss ehuss Nov 22, 2025

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The wording here seemed little strange to me. Does this make sense? Obviously range patterns that are outside the range of the scrutinee won't match the value at all.

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A range pattern must match at least one possible value. In other words:
A range pattern must be able to match at least one possible value. In other words:

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@theemathas theemathas Nov 22, 2025

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I wrote it this way to be consistent with the other lines in this section, for example:

The exclusive range pattern matches all values from the lower bound up to, but not including the upper bound.


* In `a..=b`, a ≤ b must be the case. For example, it is an error to have a range pattern `10..=0`, but `10..=10` is allowed.
* In `a..b`, a < b must be the case. For example, it is an error to have a range pattern `10..0` or `10..10`.
* In `..b`, b must not be the smallest value of its type. For example, it is an error to have a range pattern `..-128i8` or `..f64::NEG_INFINITY`.

r[patterns.range.bound]
A bound is written as one of:
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