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builtin: document when len and cap are constant

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The rules are subtle, but under some circumstances the result
can be constant. Mention this and refer to the appropriate
section of the specification.

Fixes #27588.

Change-Id: I4beaad036db87501378fb2ef48d216742d096933
Reviewed-on: https://go-review.googlesource.com/135519
Reviewed-by: Robert Griesemer <gri@golang.org>
This commit is contained in:
Rob Pike 2018-09-17 14:25:06 +10:00
parent c6118af558
commit 37db664c6c
1 changed files with 6 additions and 0 deletions
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6
src/builtin/builtin.go
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@ -152,6 +152,9 @@ func delete(m map[Type]Type1, key Type)
// String: the number of bytes in v. // String: the number of bytes in v.
// Channel: the number of elements queued (unread) in the channel buffer; // Channel: the number of elements queued (unread) in the channel buffer;
// if v is nil, len(v) is zero. // if v is nil, len(v) is zero.
// For some arguments, such as a string literal or a simple array expression, the
// result can be a constant. See the Go language specification's "Length and
// capacity" section for details.
func len(v Type) int func len(v Type) int
// The cap built-in function returns the capacity of v, according to its type: // The cap built-in function returns the capacity of v, according to its type:
@ -161,6 +164,9 @@ func len(v Type) int
// if v is nil, cap(v) is zero. // if v is nil, cap(v) is zero.
// Channel: the channel buffer capacity, in units of elements; // Channel: the channel buffer capacity, in units of elements;
// if v is nil, cap(v) is zero. // if v is nil, cap(v) is zero.
// For some arguments, such as a simple array expression, the result can be a
// constant. See the Go language specification's "Length and capacity" section for
// details.
func cap(v Type) int func cap(v Type) int
// The make built-in function allocates and initializes an object of type // The make built-in function allocates and initializes an object of type