What does 'let x = x' do in Rust?
I saw this code in the wild:
fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
let fields = fields;
What does the let fields = fields; line do? Why is it there?
rust
edited Feb 9 at 20:19
Shepmaster
161k16330474
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
add a comment |
I saw this code in the wild:
fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
let fields = fields;
What does the let fields = fields; line do? Why is it there?
rust
edited Feb 9 at 20:19
Shepmaster
161k16330474
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
add a comment |
I saw this code in the wild:
fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
let fields = fields;
What does the let fields = fields; line do? Why is it there?
rust
edited Feb 9 at 20:19
Shepmaster
161k16330474
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
I saw this code in the wild:
fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
let fields = fields;
What does the let fields = fields; line do? Why is it there?
rust
rust
edited Feb 9 at 20:19
Shepmaster
161k16330474
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
edited Feb 9 at 20:19
Shepmaster
161k16330474
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
edited Feb 9 at 20:19
Shepmaster
161k16330474
edited Feb 9 at 20:19
Shepmaster
161k16330474
edited Feb 9 at 20:19
Shepmaster
161k16330474
161k16330474
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
asked Feb 8 at 15:13
timtheliontimthelion
1,0781321
1,0781321
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
It makes fields immutable again.
fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.
"Downgrading" a mutable binding to immutable is quite common in Rust.
Another common way to do this is to use a block expression:
let fields = {
let mut fields = …;
fields.sort_by_key(…);
fields
};
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
34
Or "upgrading", depending upon your perspective.
– Synesso
Feb 9 at 3:06
6
IMO your another way to write that is the way to go: the mutable variable is scoped the time we need to use it, and then it is moved. It is better semantically.
– French Boiethios
Feb 13 at 8:48
@DarthBoiethios Does one or the other changes anything to compiled code? Like adding an additional, useless instruction? Or enabling more aggressive optimisations by the compiler based on immutability assumptions?
– iago-lito
Feb 27 at 19:22
1
@iago-lito Honestly, I'm not sure, but my uneducated guess is that is does not change anything.
– French Boiethios
Feb 27 at 19:47
1
@iago-lito Right now it actually does! However this is considered a bug and is likely to be fixed at some point.
– mcarton
Feb 27 at 20:13
|
show 1 more comment
The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
It makes fields immutable again.
fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.
"Downgrading" a mutable binding to immutable is quite common in Rust.
Another common way to do this is to use a block expression:
let fields = {
let mut fields = …;
fields.sort_by_key(…);
fields
};
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
34
Or "upgrading", depending upon your perspective.
– Synesso
Feb 9 at 3:06
6
IMO your another way to write that is the way to go: the mutable variable is scoped the time we need to use it, and then it is moved. It is better semantically.
– French Boiethios
Feb 13 at 8:48
@DarthBoiethios Does one or the other changes anything to compiled code? Like adding an additional, useless instruction? Or enabling more aggressive optimisations by the compiler based on immutability assumptions?
– iago-lito
Feb 27 at 19:22
1
@iago-lito Honestly, I'm not sure, but my uneducated guess is that is does not change anything.
– French Boiethios
Feb 27 at 19:47
1
@iago-lito Right now it actually does! However this is considered a bug and is likely to be fixed at some point.
– mcarton
Feb 27 at 20:13
|
show 1 more comment
It makes fields immutable again.
fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.
"Downgrading" a mutable binding to immutable is quite common in Rust.
Another common way to do this is to use a block expression:
let fields = {
let mut fields = …;
fields.sort_by_key(…);
fields
};
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
34
Or "upgrading", depending upon your perspective.
– Synesso
Feb 9 at 3:06
6
IMO your another way to write that is the way to go: the mutable variable is scoped the time we need to use it, and then it is moved. It is better semantically.
– French Boiethios
Feb 13 at 8:48
@DarthBoiethios Does one or the other changes anything to compiled code? Like adding an additional, useless instruction? Or enabling more aggressive optimisations by the compiler based on immutability assumptions?
– iago-lito
Feb 27 at 19:22
1
@iago-lito Honestly, I'm not sure, but my uneducated guess is that is does not change anything.
– French Boiethios
Feb 27 at 19:47
1
@iago-lito Right now it actually does! However this is considered a bug and is likely to be fixed at some point.
– mcarton
Feb 27 at 20:13
|
show 1 more comment
It makes fields immutable again.
fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.
"Downgrading" a mutable binding to immutable is quite common in Rust.
Another common way to do this is to use a block expression:
let fields = {
let mut fields = …;
fields.sort_by_key(…);
fields
};
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
It makes fields immutable again.
fields was previously defined as mutable (let mut fields = …;), to be used with sort_by_key which sorts in-place and requires the target to be mutable. The author has chosen here to explicitly prevent further mutability.
"Downgrading" a mutable binding to immutable is quite common in Rust.
Another common way to do this is to use a block expression:
let fields = {
let mut fields = …;
fields.sort_by_key(…);
fields
};
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
edited Feb 8 at 15:32
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
answered Feb 8 at 15:19
mcartonmcarton
7,63322738
7,63322738
34
Or "upgrading", depending upon your perspective.
– Synesso
Feb 9 at 3:06
6
IMO your another way to write that is the way to go: the mutable variable is scoped the time we need to use it, and then it is moved. It is better semantically.
– French Boiethios
Feb 13 at 8:48
@DarthBoiethios Does one or the other changes anything to compiled code? Like adding an additional, useless instruction? Or enabling more aggressive optimisations by the compiler based on immutability assumptions?
– iago-lito
Feb 27 at 19:22
1
@iago-lito Honestly, I'm not sure, but my uneducated guess is that is does not change anything.
– French Boiethios
Feb 27 at 19:47
1
@iago-lito Right now it actually does! However this is considered a bug and is likely to be fixed at some point.
– mcarton
Feb 27 at 20:13
|
show 1 more comment
34
Or "upgrading", depending upon your perspective.
– Synesso
Feb 9 at 3:06
6
IMO your another way to write that is the way to go: the mutable variable is scoped the time we need to use it, and then it is moved. It is better semantically.
– French Boiethios
Feb 13 at 8:48
@DarthBoiethios Does one or the other changes anything to compiled code? Like adding an additional, useless instruction? Or enabling more aggressive optimisations by the compiler based on immutability assumptions?
– iago-lito
Feb 27 at 19:22
1
@iago-lito Honestly, I'm not sure, but my uneducated guess is that is does not change anything.
– French Boiethios
Feb 27 at 19:47
1
@iago-lito Right now it actually does! However this is considered a bug and is likely to be fixed at some point.
– mcarton
Feb 27 at 20:13
34
34
Or "upgrading", depending upon your perspective.
– Synesso
Feb 9 at 3:06
Or "upgrading", depending upon your perspective.
– Synesso
Feb 9 at 3:06
6
6
IMO your another way to write that is the way to go: the mutable variable is scoped the time we need to use it, and then it is moved. It is better semantically.
– French Boiethios
Feb 13 at 8:48
IMO your another way to write that is the way to go: the mutable variable is scoped the time we need to use it, and then it is moved. It is better semantically.
– French Boiethios
Feb 13 at 8:48
@DarthBoiethios Does one or the other changes anything to compiled code? Like adding an additional, useless instruction? Or enabling more aggressive optimisations by the compiler based on immutability assumptions?
– iago-lito
Feb 27 at 19:22
@DarthBoiethios Does one or the other changes anything to compiled code? Like adding an additional, useless instruction? Or enabling more aggressive optimisations by the compiler based on immutability assumptions?
– iago-lito
Feb 27 at 19:22
1
1
@iago-lito Honestly, I'm not sure, but my uneducated guess is that is does not change anything.
– French Boiethios
Feb 27 at 19:47
@iago-lito Honestly, I'm not sure, but my uneducated guess is that is does not change anything.
– French Boiethios
Feb 27 at 19:47
1
1
@iago-lito Right now it actually does! However this is considered a bug and is likely to be fixed at some point.
– mcarton
Feb 27 at 20:13
@iago-lito Right now it actually does! However this is considered a bug and is likely to be fixed at some point.
– mcarton
Feb 27 at 20:13
|
show 1 more comment
The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
add a comment |
The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
add a comment |
The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
The statement let var = var; makes var immutable and bound to its current value. fields was declared as mut earlier.
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
answered Feb 8 at 15:18
Govind ParmarGovind Parmar
13k53764
13k53764
add a comment |
add a comment |
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