X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/e2374eed1c3ae8d0063138ea011e86bbd42473ab..375923e203d323dadc639434ebc1f29530f4ac2a:/src/part06.rs diff --git a/src/part06.rs b/src/part06.rs index 26fa124..21fe644 100644 --- a/src/part06.rs +++ b/src/part06.rs @@ -1,113 +1,120 @@ -// Rust-101, Part 06: Copy -// ======================= +// Rust-101, Part 06: Copy, Lifetimes +// ================================== +// We continue to work on our `BigInt`, so we start by importing what we already established. use part05::BigInt; // With `BigInt` being about numbers, we should be able to write a version of `vec_min` -// that computes the minimum of a list of `BigInt`. We start by writing `min` for -// `BigInt`. Now our assumption of having no trailing zeros comes in handy! +// that computes the minimum of a list of `BigInt`. First, we have to write `min` for `BigInt`. impl BigInt { - fn min(self, other: Self) -> Self { - // Just to be sure, we first check that both operands actually satisfy our invariant. - // `debug_assert!` is a macro that checks that its argument (must be of type `bool`) - // is `true`, and panics otherwise. It gets removed in release builds, which you do with - // `cargo build --release`. + fn min_try1(self, other: Self) -> Self { + //@ Just to be sure, we first check that both operands actually satisfy our invariant. + //@ `debug_assert!` is a macro that checks that its argument (must be of type `bool`) is + //@ `true`, and panics otherwise. It gets removed in release builds, which you do with + //@ `cargo build --release`. debug_assert!(self.test_invariant() && other.test_invariant()); + // Now our assumption of having no trailing zeros comes in handy: // If the lengths of the two numbers differ, we already know which is larger. if self.data.len() < other.data.len() { self } else if self.data.len() > other.data.len() { other } else { - // **Exercise 05.1**: Fill in this code. - panic!("Not yet implemented."); + // **Exercise 06.1**: Fill in this code. + unimplemented!() } } } -// Now we can write `vec_min`. In order to make it type-check, we have to write it as follows. +// Now we can write `vec_min`. fn vec_min(v: &Vec) -> Option { let mut min: Option = None; + // If `v` is a shared reference to a vector, then the default for iterating over it is to call + // `iter`, the iterator that borrows the elements. for e in v { - min = Some(match min { - None => e.clone(), - Some(n) => e.clone().min(n) - }); + let e = e.clone(); + min = Some(match min { /*@*/ + None => e, /*@*/ + Some(n) => e.min_try1(n) /*@*/ + }); /*@*/ } min } -// Now, what's happening here? Why do we have to write `clone()`, and why did we not -// have to write that in our previous version? -// -// The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but -// the Option that it returns is *owned*. We can't just return one of the elements of `v`, -// as that would mean that it is no longer in the vector! In our code, this comes up when we update -// the intermediate variable `min`, which also has type `Option`. If you replace `e.clone()` -// in the `None` arm with `*e`, Rust will complain "Cannot move out of borrowed content". That's because -// `e` is a `&BigInt`. Assigning `min = Some(*e)` works just like a function call: Ownership of the -// underlying data is transferred from where `e` borrows from to `min`. But that's not allowed, since -// we just borrowed `e`, so we cannot empty it! We can, however, call `clone()` on it. Then we own -// the copy that was created, and hence we can store it in `min`.
-// Of course, making such a full copy is expensive, so we'd like to avoid it. We'll some to that soon. +//@ Now, what's happening here? Why do we have to to make a full (deep) copy of `e`, and why did we +//@ not have to do that in our previous version? +//@ +//@ The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but +//@ the Option that it returns is *owned*. We can't just return one of the elements of `v`, +//@ as that would mean that it is no longer in the vector! In our code, this comes up when we update +//@ the intermediate variable `min`, which also has type `Option`. If you get rid of the +//@ `e.clone()`, Rust will complain "Cannot move out of borrowed content". That's because +//@ `e` is a `&BigInt`. Assigning `min = Some(*e)` works just like a function call: Ownership of the +//@ underlying data is transferred from `e` to `min`. But that's not allowed, since +//@ we just borrowed `e`, so we cannot empty it! We can, however, call `clone` on it. Then we own +//@ the copy that was created, and hence we can store it in `min`.
+//@ Of course, making such a full copy is expensive, so we'd like to avoid it. We'll come to that +//@ in the next part. // ## `Copy` types -// But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work? -// We stored the minimal `i32` locally without cloning, and Rust did not complain. That's because there isn't -// really much of an "ownership" when it comes to types like `i32` or `bool`: If you move the value from one -// place to another, then both instance are "complete". We also say the value has been *duplicated*. This is in -// stark contrast to types like `Vec`, where moving the value results in both the old and the new vector to -// point to the same underlying buffer. We don't have two vectors, there's no duplication. -// -// Rust calls types that can be freely duplicated `Copy` types. `Copy` is another trait, and it -// is implemented for types like `i32` and `bool`. Remember how we defined the trait `Minimum` by writing -// `trait Minimum : Copy { ...`? This tells Rust that every type that implements `Minimum` must also -// implement `Copy`, and that's why the compiler accepted our generic `vec_min` in part 02. -// `Copy` is the first *marker trait* that we encounter: It does not provide any methods, but -// makes a promise about the behavior of the type - in this case, being duplicable. +//@ But before we go there, I should answer the second question I brought up above: Why did our old +//@ `vec_min` work? We stored the minimal `i32` locally without cloning, and Rust did not complain. +//@ That's because there isn't really much of an "ownership" when it comes to types like `i32` or +//@ `bool`: If you move the value from one place to another, then both instances are "complete". We +//@ also say the value has been *duplicated*. This is in stark contrast to types like `Vec`, +//@ where moving the value results in both the old and the new vector to point to the same +//@ underlying buffer. We don't have two vectors, there's no proper duplication. +//@ +//@ Rust calls types that can be easily duplicated `Copy` types. `Copy` is another trait, and it is +//@ implemented for types like `i32` and `bool`. Remember how we defined the trait `Minimum` by +//@ writing `trait Minimum : Copy { ...`? This tells Rust that every type that implements `Minimum` +//@ must also implement `Copy`, and that's why the compiler accepted our generic `vec_min` in part +//@ 02. `Copy` is the first *marker trait* that we encounter: It does not provide any methods, but +//@ makes a promise about the behavior of the type - in this case, being duplicable. -// If you try to implement `Copy` for `BigInt`, you will notice that Rust -// does not let you do that. A type can only be `Copy` if all its elements -// are `Copy`, and that's not the case for `BigInt`. However, we can make -// `SomethingOrNothing` copy if `T` is `Copy`. +//@ If you try to implement `Copy` for `BigInt`, you will notice that Rust does not let you do +//@ that. A type can only be `Copy` if all its elements are `Copy`, and that's not the case for +//@ `BigInt`. However, we can make `SomethingOrNothing` copy if `T` is `Copy`. use part02::{SomethingOrNothing,Something,Nothing}; -impl Copy for SomethingOrNothing{} -// Again, Rust can generate implementations of `Copy` automatically. If -// you add `#[derive(Copy,Clone)]` right before the definition of `SomethingOrNothing`, -// both `Copy` and `Clone` will automatically be implemented. +impl Copy for SomethingOrNothing {} +//@ Again, Rust can generate implementations of `Copy` automatically. If +//@ you add `#[derive(Copy,Clone)]` right before the definition of `SomethingOrNothing`, +//@ both `Copy` and `Clone` will automatically be implemented. -// ## An operational perspective -// Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain -// ownership passing and how `Copy` and `Clone` fit by looking at what happens on the machine.
-// When Rust code is executed, passing a value (like `i32` or `Vec`) to a function will always -// result in a shallow copy being performed: Rust just copies the bytes representing that value, and -// considers itself done. That's just like the default copy constructor in C++. Rust, however, will -// consider this a destructive operation: After copying the bytes elsewhere, the original value must -// no longer be used. After all, the two could not share a pointer! If, however, you mark a type `Copy`, -// then Rust will *not* consider a move destructive, and just like in C++, the old and new value -// can happily coexist. Now, Rust does not allow to to overload the copy constructor. This means that -// passing a value around will always be a fast operation, no allocation or any other kind of heap access -// will happen. In the situations where you would write a copy constructor in C++ (and hence -// incur a hidden cost on every copy of this type), you'd have the type *not* implement `Copy`, but only -// `Clone`. This makes the cost explicit. +//@ ## An operational perspective +//@ Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain +//@ ownership passing and how `Copy` and `Clone` fit in by looking at what happens on the machine. +//@
+//@ When Rust code is executed, passing a value (like `i32` or `Vec`) to a function will always +//@ result in a shallow copy being performed: Rust just copies the bytes representing that value, and +//@ considers itself done. That's just like the default copy constructor in C++. Rust, however, will +//@ consider this a destructive operation: After copying the bytes elsewhere, the original value must +//@ no longer be used. After all, the two could now share a pointer! If, however, you mark a type +//@ `Copy`, then Rust will *not* consider a move destructive, and just like in C++, the old and new +//@ value can happily coexist. Now, Rust does not allow you to overload the copy constructor. This +//@ means that passing a value around will always be a fast operation, no allocation or any other +//@ kind of heap access will happen. In the situations where you would write a copy constructor in +//@ C++ (and hence incur a hidden cost on every copy of this type), you'd have the type *not* +//@ implement `Copy`, but only `Clone`. This makes the cost explicit. // ## Lifetimes -// To fix the performance problems of `vec_min`, we need ti avoid using `clone()`. We'd like -// the return value to not be owned (remember that this was the source of our need for cloning), but *borrowed*. +//@ To fix the performance problems of `vec_min`, we need to avoid using `clone`. We'd like the +//@ return value to not be owned (remember that this was the source of our need for cloning), but +//@ *borrowed*. In other words, we want to return a shared reference to the minimal element. -// This is demonstrated by the function `head` that borrows the first element of a vector if it is non-empty. -// The type of the function says that it will either return nothing, or it will return a borrowed `T`. -// We can then borrow the first element of `v` and use it to construct the return value. +//@ The function `head` demonstrates how that could work: It returns a reference to the first +//@ element of a vector if it is non-empty. The type of the function says that it will either +//@ return nothing, or it will return a borrowed `T`. We can then obtain a reference to the first +//@ element of `v` and use it to construct the return value. fn head(v: &Vec) -> Option<&T> { if v.len() > 0 { - Some(&v[0]) + Some(&v[0]) /*@*/ } else { None } } - -// Now, coming back to `head` - here, we are returning a pointer to the first element. But doesn't -// that mean that callers have to be careful? Imagine `head` would be a C++ function, and we would -// write the following code. +// Technically, we are returning a pointer to the first element. But doesn't that mean that callers +// have to be careful? Imagine `head` would be a C++ function, and we would write the following +// code. /* int foo(std::vector v) { int *first = head(v); @@ -115,36 +122,47 @@ fn head(v: &Vec) -> Option<&T> { return *first; } */ -// This is very much like our very first motivating example for ownership, at the beginning of part 04. -// But this time, the bug is hidden behind the call to `head`. How does Rust solve this? If we translate -// the code above to Rust, it doesn't compile, so clearly we are good - but how and why? -// (Notice that have to explicitly assert using `unwrap` that `first` is not `None`, whereas the C++ code -// above would silently dereference a `NULL`-pointer. But that's another point.) +//@ This is very much like our very first motivating example for ownership, at the beginning of +//@ part 04: `push_back` could reallocate the buffer, making `first` an invalid pointer. Again, we +//@ have aliasing (of `first` and `v`) and mutation. But this time, the bug is hidden behind the +//@ call to `head`. How does Rust solve this? If we translate the code above to Rust, it doesn't +//@ compile, so clearly we are good - but how and why? +//@ (Notice that have to explicitly assert using //@ `unwrap` that `first` is not `None`, whereas +//@ the C++ code above would silently dereference a //@ `NULL`-pointer. But that's another point.) fn rust_foo(mut v: Vec) -> i32 { let first: Option<&i32> = head(&v); /* v.push(42); */ *first.unwrap() } -// To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that -// you borrowed your friend a `Vec`, or a book, is not good enough, unless you also agree on *how long* -// your friend can borrow. After all, you need to know when you can rely on owning your data (or book) again. -// -// Every borrow in Rust has an associated lifetime. The full type of `head` reads as follows: -// `fn<'a, T>(&'a Vec) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which represents how long the vector has -// been borrowed. The function type expresses that argument and return value have *the same lifetime*. -// -// When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose the scope -// where `first` is valid, which is the entire rest of the function. Because `head` ties the lifetime of its -// argument and return value together, this means that `&v` also has to borrow `v` for the entire duration of -// the function. So when we try to borrow `v` mutable for `push`, Rust complains that the two borrows (the one -// for `head`, and the one for `push`) overlap. Lucky us! Rust caught our mistake and made sure we don't crash the program. -// -// So, to sum this up: Lifetimes enable Rust to reason about *how long* a pointer has been borrowed. We can thus -// safely write functions like `head`, that return pointers into data they got as argument, and make sure they -// are used correctly, *while looking only at the function type*. At no point in our analysis of `rust_foo` did -// we have to look *into* `head`. That's, of course, crucial if we want to separate library code from application code. -// Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetimes elision*, -// where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](http://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision). +//@ To give the answer to this question, we have to talk about the *lifetime* of a reference. The +//@ point is, saying that you borrowed your friend a `Vec`, or a book, is not good enough, +//@ unless you also agree on *how long* your friend can borrow it. After all, you need to know when +//@ you can rely on owning your data (or book) again. +//@ +//@ Every reference in Rust has an associated lifetime, written `&'a T` for a reference with +//@ lifetime `'a` to something of type `T`. The full type of `head` reads as follows: `fn<'a, +//@ T>(&'a Vec) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which represents for how +//@ long the vector has been borrowed. The function type expresses that argument and return value +//@ have *the same lifetime*. +//@ +//@ When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose +//@ the scope where `first` is valid, which is the entire rest of the function. Because `head` ties +//@ the lifetime of its argument and return value together, this means that `&v` also has to borrow +//@ `v` for the entire duration of the function `rust_foo`. So when we try to create a unique +//@ reference to `v` for `push`, Rust complains that the two references (the one for `head`, and +//@ the one for `push`) overlap, so neither of them can be unique. Lucky us! Rust caught our +//@ mistake and made sure we don't crash the program. +//@ +//@ So, to sum this up: Lifetimes enable Rust to reason about *how long* a reference is valid, how +//@ long ownership has been borrowed. We can thus safely write functions like `head`, that return +//@ references into data they got as argument, and make sure they are used correctly, *while +//@ looking only at the function type*. At no point in our analysis of `rust_foo` did we have to +//@ look *into* `head`. That's, of course, crucial if we want to separate library code from +//@ application code. Most of the time, we don't have to explicitly add lifetimes to function +//@ types. This is thanks to *lifetime elision*, where Rust will automatically insert lifetimes we +//@ did not specify, following some simple, well-documented +//@ [rules](https://doc.rust- lang.org/stable/book/lifetimes.html#lifetime-elision). -// [index](main.html) | [previous](part05.html) | [next](main.html) +//@ [index](main.html) | [previous](part05.html) | [raw source](workspace/src/part06.rs) | +//@ [next](part07.html)