The most widely used form of macros in Rust is the declarative macro. These are also sometimes referred to as “macros by example,” “macro_rules! macros,” or just plain “macros.” At their core, declarative macros allow you to write something similar to a Rust match expression. As discussed in Chapter 6, match expressions are control structures that take an expression, compare the resulting value of the expression to patterns, and then run the code associated with the matching pattern. Macros also compare a value to patterns that are associated with particular code: in this situation, the value is the literal Rust source code passed to the macro; the patterns are compared with the structure of that source code; and the code associated with each pattern, when matched, replaces the code passed to the macro. This all happens during compilation.

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The #[macro_export] annotation indicates that this macro should be made available whenever the crate in which the macro is defined is brought into scope. Without this annotation, the macro can’t be brought into scope.

Depth of fieldcalculator

However, they would need to write the implementation block for each type they wanted to use with hello_macro; we want to spare them from having to do this work.

In addition to the focal length, the F-number (aperture value) is another important figure that expresses the characteristics of the lens: the smaller the F-number, the more light-gathering power it will have, and will broaden your possibility. The F-number is calculated as 'focal length ÷ lens effective diameter'. The smaller the 'maximum aperture F value' when the aperture is at its widest, the more sensitive the lens is. When choosing a lens, it is better to mind the F-number as well as the focal length.

The function that defines a procedural macro takes a TokenStream as an input and produces a TokenStream as an output. The TokenStream type is defined by the proc_macro crate that is included with Rust and represents a sequence of tokens. This is the core of the macro: the source code that the macro is operating on makes up the input TokenStream, and the code the macro produces is the output TokenStream. The function also has an attribute attached to it that specifies which kind of procedural macro we’re creating. We can have multiple kinds of procedural macros in the same crate.

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Here, we have two parameters of type TokenStream. The first is for the contents of the attribute: the GET, "/" part. The second is the body of the item the attribute is attached to: in this case, fn index() {} and the rest of the function’s body.

Fundamentally, macros are a way of writing code that writes other code, which is known as metaprogramming. In Appendix C, we discuss the derive attribute, which generates an implementation of various traits for you. We’ve also used the println! and vec! macros throughout the book. All of these macros expand to produce more code than the code you’ve written manually.

We want our procedural macro to generate an implementation of our HelloMacro trait for the type the user annotated, which we can get by using #name. The trait implementation has the one function hello_macro, whose body contains the functionality we want to provide: printing Hello, Macro! My name is and then the name of the annotated type.

We’ve defined a macro that can take any number of arguments of any type and can generate code to create a vector containing the specified elements.

The 35-150mm F/2-2.8 Di III VXD (Model A058) is a high resolution travel zoom lens that covers everything from the 35mm wide angle to the 150mm telephoto focal length, the first zoom lens achieving an aperture of F2 at the wide angle end. It has a groundbreaking fast-aperture and utilizes the linear motor focus mechanism VXD (Voice-coil eXtreme-torque Drive), thereby achieving high speed, high precision autofocusing. The innovative lens design enabled us to greatly improve the lens's grip and functionality. The software, developed in-house, enables to easily customize functions and to update firmware.

The quote! macro lets us define the Rust code that we want to return. The compiler expects something different to the direct result of the quote! macro’s execution, so we need to convert it to a TokenStream. We do this by calling the into method, which consumes this intermediate representation and returns a value of the required TokenStream type.

To define a macro, you use the macro_rules! construct. Let’s explore how to use macro_rules! by looking at how the vec! macro is defined. Chapter 8 covered how we can use the vec! macro to create a new vector with particular values. For example, the following macro creates a new vector containing three integers:

The comma following $() indicates that a literal comma separator character could optionally appear after the code that matches the code in $(). The * specifies that the pattern matches zero or more of whatever precedes the *.

You might have noticed that we’re calling unwrap to cause the hello_macro_derive function to panic if the call to the syn::parse function fails here. It’s necessary for our procedural macro to panic on errors because proc_macro_derive functions must return TokenStream rather than Result to conform to the procedural macro API. We’ve simplified this example by using unwrap; in production code, you should provide more specific error messages about what went wrong by using panic! or expect.

The 17-70mm F/2.8 Di III-A VC RXD (Model B070) is a large-aperture standard zoom lens for APS-C format mirrorless cameras. With a focal length range of 17mm to 70mm (a full-frame equivalent of 25.5-105mm) for daily use, this achieves a 4.1x zoom. The optical design ensures high resolution and high contrast not just in the center of the image but also in corners and at the edges. The quiet AF drive motor and the VC image stabilization mechanism facilitate hand-held shooting. In addition, by counteracting focus breathing, the 17-70mm F2.8 empowers users' expression of their creative intentions to the fullest degree. This highly practical lens allows you to easily enjoy the high image quality of a large F2.8 aperture for both still and video shooting.

Function-like macros define macros that look like function calls. Similarly to macro_rules! macros, they’re more flexible than functions; for example, they can take an unknown number of arguments. However, macro_rules! macros can be defined only using the match-like syntax we discussed in the section “Declarative Macros with macro_rules! for General Metaprogramming” earlier. Function-like macros take a TokenStream parameter and their definition manipulates that TokenStream using Rust code as the other two types of procedural macros do. An example of a function-like macro is an sql! macro that might be called like so:

Here is some basic knowledge about focal length with interchangeable lenses that you will need to know when shooting with an SLR or mirrorless SLR camera. When shooting with an interchangeable lens camera, you can take beautiful and impressive photos by working with the angle of view, focus point and degree of bokeh. Particularly when choosing a lens, it is important to understand the characteristics of focal length as the angle of view changes depending on focal length. Let’s learn the basic knowledge and make use of it to take the photos as you imagine.

The stringify! macro used here is built into Rust. It takes a Rust expression, such as 1 + 2, and at compile time turns the expression into a string literal, such as "1 + 2". This is different than format! or println!, macros which evaluate the expression and then turn the result into a String. There is a possibility that the #name input might be an expression to print literally, so we use stringify!. Using stringify! also saves an allocation by converting #name to a string literal at compile time.

The important point about focal length is that the angle of view changes with its actual number. The angle of view is the angle of area captured by the camera through the lens, and the larger the angel of view, the wider the area is captured. Lens types can be roughly divided into three categories: wide-angle, standard, and telephoto. A standard lens can capture images at an angle of view close to that of the human field of view. Generally, lenses with a wider angle of view are called wide-angle lenses, and lenses with a narrower angle of view are called telephoto lenses. As a general rule of thumb for focal lengths, focal length smaller than 35mm are generally classified into wide-angle lens, lenses around 50mm are called standard lens, and lenses larger than 80mm are telephoto lens. Check out the differences in impression and composition caused by changing the angle of view. Here is an example of a portrait taken with the TAMRON 35-150mm F/2-2.8 Di III VXD (Model A058) at a different focal length. At the wide-angle end (35 mm), the beautiful impression of the figure standing stands out with the background. At the telephoto end (150 mm), on the other hand, the expression on the person's face is compelling and the background bokeh accentuates this image. In this way, by changing the focal length of a zoom lens, various angles of view can be created, and different impressions can be captured.

The quote! macro also provides some very cool templating mechanics: we can enter #name, and quote! will replace it with the value in the variable name. You can even do some repetition similar to the way regular macros work. Check out the quote crate’s docs for a thorough introduction.

We’ve used macros like println! throughout this book, but we haven’t fully explored what a macro is and how it works. The term macro refers to a family of features in Rust: declarative macros with macro_rules! and three kinds of procedural macros:

A function signature must declare the number and type of parameters the function has. Macros, on the other hand, can take a variable number of parameters: we can call println!("hello") with one argument or println!("hello {}", name) with two arguments. Also, macros are expanded before the compiler interprets the meaning of the code, so a macro can, for example, implement a trait on a given type. A function can’t, because it gets called at runtime and a trait needs to be implemented at compile time.

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We have a trait and its function. At this point, our crate user could implement the trait to achieve the desired functionality, like so:

The 70-300mm F/4.5-6.3 Di III RXD (Model A047) for full-frame mirrorless cameras is a telephoto zoom lens designed and created so photographers of all skill levels can enjoy high quality images comfortably. The 70-300mm F4.5-6.3 covers a broad telephoto zoom range yet is the small and lightest weight. With special emphasis on resolving power, TAMRON has deployed special lens elements appropriately arranged to correct chromatic aberration, generally very strong in a telephoto lens, as well as other aberrations. Users can enjoy high-resolution images combined with stunning bokeh qualities that are achievable only with a telephoto lens. The lens also incorporates the RXD, a high-speed precision AF drive system that is remarkably quiet. The 70-300mm F4.5-6.3 is a versatile lens for photographing landscapes, sports and other action, pets, wildlife, and more. The lens also demonstrates its potential for portrait shooting, casual snapshots, and scenarios that require you to be mobile and shoot handheld, like sporting events.

When creating procedural macros, the definitions must reside in their own crate with a special crate type. This is for complex technical reasons that we hope to eliminate in the future. In Listing 19-29, we show how to define a procedural macro, where some_attribute is a placeholder for using a specific macro variety.

The hello_macro_derive function will be called when a user of our library specifies #[derive(HelloMacro)] on a type. This is possible because we’ve annotated the hello_macro_derive function here with proc_macro_derive and specified the name HelloMacro, which matches our trait name; this is the convention most procedural macros follow.

Let’s look at the different kinds of procedural macros. We’ll start with a custom derive macro and then explain the small dissimilarities that make the other forms different.

Other than that, attribute-like macros work the same way as custom derive macros: you create a crate with the proc-macro crate type and implement a function that generates the code you want!

We need to declare the hello_macro_derive crate as a procedural macro crate. We’ll also need functionality from the syn and quote crates, as you’ll see in a moment, so we need to add them as dependencies. Add the following to the Cargo.toml file for hello_macro_derive:

The 17-70mm F/2.8 Di III-A VC RXD (Model B070) is a large-aperture standard zoom lens for APS-C format mirrorless cameras. With a focal length range of 17mm to 70mm (a full-frame equivalent of 25.5-105mm) for daily use, this achieves a 4.1x zoom. The optical design ensures high resolution and high contrast not just in the center of the image but also in corners and at the edges. The quiet AF drive motor and the VC image stabilization mechanism facilitate hand-held shooting. In addition, by counteracting focus breathing, the 17-70mm F2.8 empowers users' expression of their creative intentions to the fullest degree. This highly practical lens allows you to easily enjoy the high image quality of a large F2.8 aperture for both still and video shooting.

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The syn crate parses Rust code from a string into a data structure that we can perform operations on. The quote crate turns syn data structures back into Rust code. These crates make it much simpler to parse any sort of Rust code we might want to handle: writing a full parser for Rust code is no simple task.

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As explained above, focal length is a very important figure, but when choosing a lens you also need to be mindful of the sensor size on your camera body. With digital SLR cameras, light reaching the image sensor is stored as an electrical charge, and an image is created based on the color information obtained through the color filter. These image sensors come in mainly two types: 35mm full-frame, and APS-C. The size of a 35mm full-frame sensor is 36mm x 24mm. The larger the sensor size allows a wider area to be captured with greater sensitivity and its rich gradation. On the other hand, the size of an APS-C sensor is a step smaller at 23.5mm x 15.6mm* and captures narrower area. *APS-C image sensor sizes vary by camera. Therefore, you will need a lens that is compatible with the sensor size of the camera body. Camera lenses have a specific sensor size that they are compatible with, so choose a lens that is compatible with the camera you have. When APS-C camera is used, you will need to do some calculation to figure out the real coverage. The angle of view of each focal length depends on the sensor size. Generally, focal lengths and angles of view are based on 35mm full-frame size, so when selecting an APS-C compatible lens, it is easier to understand the angle of view when converted to full-frame size. Such conversion is known as '35mm equivalent'. In general, the focal length of APS-C lenses can be converted to the focal length of lenses for full-frame cameras by multiplying the focal length by 1.5 (for Sony, Nikon and 1.6 for Canon). The relationship between angle of view and focal length can be mapped as shown in the table below, which you can use as a reference when selecting a lens for APS-C.

We then start the macro definition with macro_rules! and the name of the macro we’re defining without the exclamation mark. The name, in this case vec, is followed by curly brackets denoting the body of the macro definition.

Note: The actual definition of the vec! macro in the standard library includes code to preallocate the correct amount of memory up front. That code is an optimization that we don’t include here to make the example simpler.

Metaprogramming is useful for reducing the amount of code you have to write and maintain, which is also one of the roles of functions. However, macros have some additional powers that functions don’t.

Our two crates are tightly related, so we create the procedural macro crate within the directory of our hello_macro crate. If we change the trait definition in hello_macro, we’ll have to change the implementation of the procedural macro in hello_macro_derive as well. The two crates will need to be published separately, and programmers using these crates will need to add both as dependencies and bring them both into scope. We could instead have the hello_macro crate use hello_macro_derive as a dependency and re-export the procedural macro code. However, the way we’ve structured the project makes it possible for programmers to use hello_macro even if they don’t want the derive functionality.

As ultra-telephoto zoom lenses allow you to enlarge the faraway subjects, they are best suited for shooting subjects you cannot get close to, such as wildlife, sporting events held at large venues, aircraft and trains. By combining the compression effects and bokeh that telephoto zooms are known for, you can also compose more appealing images.

Generally, on a camera lens there is an indication of focal length. For a product name such as “28-75mm F/2.8, ” this “xx mm” indicates the focal length. There are two types of lenses: prime lenses which have a single focal length, and zoom lenses that have a variable focal length. If the focal length is described with a range, such as “xx-xx mm,” it is a zoom lens. For a prime lens, only a single figure is indicated, such as “xx mm.” Example of a zoom lens: 28-75mm F/2.8 Di III VXD G2 (Model A063)Example of a prime lens: 20mm F/2.8 Di III OSD M1:2 (Model F050)

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Shallowdepth of field

Now let’s look at the pattern in the body of the code associated with this arm: temp_vec.push() within $()* is generated for each part that matches $() in the pattern zero or more times depending on how many times the pattern matches. The $x is replaced with each expression matched. When we call this macro with vec![1, 2, 3];, the code generated that replaces this macro call will be the following:

The second form of macros is the procedural macro, which acts more like a function (and is a type of procedure). Procedural macros accept some code as an input, operate on that code, and produce some code as an output rather than matching against patterns and replacing the code with other code as declarative macros do. The three kinds of procedural macros are custom derive, attribute-like, and function-like, and all work in a similar fashion.

depth offield中文

When light enters a lens, the point where the light gathers is called the focal point. While light travels in a straight ray, when it passes through a lens it refracts and is concentrated at a single point so called focal point. When a light passes through a lens and at this focal point it forms an image, when an image sensor is placed at this position, the image can be captured. A focal length is a distance from the center of the lens (principal point) to the image sensor, and this is an important value that characterizes a lens.

We get an Ident struct instance containing the name (identifier) of the annotated type using ast.ident. The struct in Listing 19-32 shows that when we run the impl_hello_macro function on the code in Listing 19-30, the ident we get will have the ident field with a value of "Pancakes". Thus, the name variable in Listing 19-33 will contain an Ident struct instance that, when printed, will be the string "Pancakes", the name of the struct in Listing 19-30.

We’ve introduced three new crates: proc_macro, syn, and quote. The proc_macro crate comes with Rust, so we didn’t need to add that to the dependencies in Cargo.toml. The proc_macro crate is the compiler’s API that allows us to read and manipulate Rust code from our code.

The next step is to define the procedural macro. At the time of this writing, procedural macros need to be in their own crate. Eventually, this restriction might be lifted. The convention for structuring crates and macro crates is as follows: for a crate named foo, a custom derive procedural macro crate is called foo_derive. Let’s start a new crate called hello_macro_derive inside our hello_macro project:

We could also use the vec! macro to make a vector of two integers or a vector of five string slices. We wouldn’t be able to use a function to do the same because we wouldn’t know the number or type of values up front.

Attribute-like macros are similar to custom derive macros, but instead of generating code for the derive attribute, they allow you to create new attributes. They’re also more flexible: derive only works for structs and enums; attributes can be applied to other items as well, such as functions. Here’s an example of using an attribute-like macro: say you have an attribute named route that annotates functions when using a web application framework:

To start defining the procedural macro, place the code in Listing 19-31 into your src/lib.rs file for the hello_macro_derive crate. Note that this code won’t compile until we add a definition for the impl_hello_macro function.

Depth of field

The 150-500mm F/5-6.7 Di III VC VXD (Model A057) is compact enough to be handheld while maintaining a focal length of 500mm on the telephoto end. It allows users to easily enjoy the world of the 500mm ultra-telephoto lens while maintaining its high image quality. The high-speed, high-precision AF with excellent tracking performance and the VC mechanism support handheld shooting in the ultra-telephoto range.

The 17-50mm F/4 Di III VXD (Model A068)It's the world’s first lens covering from ultra wide-angle 17mm to the standard 50mm focal length. The highly-compact TAMRON 17-50mm F/4 Di III VXD (model a068) for Sony E-mount full-frame mirrorless cameras offers maximum versatility for still and video creators. From landscapes to living rooms, this lens captures all that you see.

The 50-400mm F/4.5-6.3 Di III VC VXD (Model A067) is an ultra-telephoto zoom lens with an 8x zoom starting at 50mm at the wide-angle end and compatible with full-frame mirrorless cameras. The lens delivers uncompromised high image quality over the entire 50-400mm focal length range, yet is as compact and lightweight as a 100-400mm class lens. Equipped with the VXD mechanism and the VC mechanism, the lens can quickly focus on the subject's movement when shooting sports and wild birds. The 50-400mm F4.5-6.3 VC is a new ultra-telephoto zoom lens that combines unparalleled image quality and mobility.

First, we use a set of parentheses to encompass the whole pattern. We use a dollar sign ($) to declare a variable in the macro system that will contain the Rust code matching the pattern. The dollar sign makes it clear this is a macro variable as opposed to a regular Rust variable. Next comes a set of parentheses that captures values that match the pattern within the parentheses for use in the replacement code. Within $() is $x:expr, which matches any Rust expression and gives the expression the name $x.

This definition is similar to the custom derive macro’s signature: we receive the tokens that are inside the parentheses and return the code we wanted to generate.

At this point, cargo build should complete successfully in both hello_macro and hello_macro_derive. Let’s hook up these crates to the code in Listing 19-30 to see the procedural macro in action! Create a new binary project in your projects directory using cargo new pancakes. We need to add hello_macro and hello_macro_derive as dependencies in the pancakes crate’s Cargo.toml. If you’re publishing your versions of hello_macro and hello_macro_derive to crates.io, they would be regular dependencies; if not, you can specify them as path dependencies as follows:

This #[route] attribute would be defined by the framework as a procedural macro. The signature of the macro definition function would look like this:

Notice that we’ve split the code into the hello_macro_derive function, which is responsible for parsing the TokenStream, and the impl_hello_macro function, which is responsible for transforming the syntax tree: this makes writing a procedural macro more convenient. The code in the outer function (hello_macro_derive in this case) will be the same for almost every procedural macro crate you see or create. The code you specify in the body of the inner function (impl_hello_macro in this case) will be different depending on your procedural macro’s purpose.

The focal length of a lens is defined as the distance from the center of the lens (principal point) to the image sensor of the camera. Focal length is an important figure that determines the angle of view. A lens with a short focal length is a wide-angle lens, a lens with a long focal length is a telephoto lens and a standard lens is somewhere in between. When choosing a lens, pay attention to the focal length and select a lens that suits the subject or scene you want to photograph.

70-180mm F/2.8 Di III VC VXD G2 (Model A065) has evolved to G2 level.This is the world’s smallest and lightest, fast-aperture telephoto zoom lens for Sony E-mount with astounding portability and superb image quality.

The 50-400mm F/4.5-6.3 Di III VC VXD (Model A067) is an ultra-telephoto zoom lens with an 8x zoom starting at 50mm at the wide-angle end and compatible with full-frame mirrorless cameras. The lens delivers uncompromised high image quality over the entire 50-400mm focal length range, yet is as compact and lightweight as a 100-400mm class lens. Equipped with the VXD mechanism and the VC mechanism, the lens can quickly focus on the subject's movement when shooting sports and wild birds. The 50-400mm F4.5-6.3 is a new ultra-telephoto zoom lens that combines unparalleled image quality and mobility.

This code will print Hello, Macro! My name is Pancakes! when we’re done. The first step is to make a new library crate, like this:

Let’s create a crate named hello_macro that defines a trait named HelloMacro with one associated function named hello_macro. Rather than making our users implement the HelloMacro trait for each of their types, we’ll provide a procedural macro so users can annotate their type with #[derive(HelloMacro)] to get a default implementation of the hello_macro function. The default implementation will print Hello, Macro! My name is TypeName! where TypeName is the name of the type on which this trait has been defined. In other words, we’ll write a crate that enables another programmer to write code like Listing 19-30 using our crate.

Wide-angle zoom lenses cover the range from ultra wide-angle to standard range. Ultra wide-angle lenses allow you to get closer to buildings and vehicles and still capture a wide range of subjects, resulting in more dynamic shots. It also allows for unique portrait and pet photography, making the most of the perspective. Lightweight and compact TAMRON lenses are particularly useful when shooting landscapes, as the time and distance spent carrying them around can be long.

The downside to implementing a macro instead of a function is that macro definitions are more complex than function definitions because you’re writing Rust code that writes Rust code. Due to this indirection, macro definitions are generally more difficult to read, understand, and maintain than function definitions.

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This macro would parse the SQL statement inside it and check that it’s syntactically correct, which is much more complex processing than a macro_rules! macro can do. The sql! macro would be defined like this:

Now that we have the code to turn the annotated Rust code from a TokenStream into a DeriveInput instance, let’s generate the code that implements the HelloMacro trait on the annotated type, as shown in Listing 19-33.

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Depth of fieldgithub

The 17-28mm F/2.8 Di III RXD (Model A046) achieves a filter diameter of ø67mm, which is surprising for a large aperture ultra wide-angle zoom lens for full-frame cameras. It’s small and light weight with a good camera balance. It's a dedicated lens for mirrorless interchangeable -lens cameras that can be carried easily and can be used in various situations.

Aperture

Put the code in Listing 19-30 into src/main.rs, and run cargo run: it should print Hello, Macro! My name is Pancakes! The implementation of the HelloMacro trait from the procedural macro was included without the pancakes crate needing to implement it; the #[derive(HelloMacro)] added the trait implementation.

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Telephotolens

The 35-150mm F/2-2.8 Di III VXD (Model A058) is a high resolution travel zoom lens that covers everything from the 35mm wide angle to the 150mm telephoto focal length, the first zoom lens achieving an aperture of F2 at the wide angle end. It has a groundbreaking fast-aperture and utilizes the linear motor focus mechanism VXD (Voice-coil eXtreme-torque Drive), thereby achieving high speed, high precision autofocusing. The innovative lens design enabled us to greatly improve the lens's grip and functionality. The software, developed in-house, enables to easily customize functions and to update firmware.

Another important difference between macros and functions is that you must define macros or bring them into scope before you call them in a file, as opposed to functions you can define anywhere and call anywhere.

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Whew! Now you have some Rust features in your toolbox that you likely won’t use often, but you’ll know they’re available in very particular circumstances. We’ve introduced several complex topics so that when you encounter them in error message suggestions or in other peoples’ code, you’ll be able to recognize these concepts and syntax. Use this chapter as a reference to guide you to solutions.

A standard zoom lens covers wide-angle to telephoto focal lengths around 50mm. In addition to snap shots and portraits, these lenses prove their worth in a wide variety of shooting scenarios such as landscapes at the wide end and shots of animals and buildings at the telephoto end.

Telephoto zoom lenses enable photographers bring the subject closer, making them ideal for events such as sports events, as well as for birding and wildlife. They are also useful in natural landscapes photographed in conjunction with trekking, hiking and mountaineering.They are also recommended for impressive portrait photography with naturally blurred backgrounds and compression effects.

Valid pattern syntax in macro definitions is different than the pattern syntax covered in Chapter 18 because macro patterns are matched against Rust code structure rather than values. Let’s walk through what the pattern pieces in Listing 19-28 mean; for the full macro pattern syntax, see the Rust Reference.

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Depth of fieldsimulator

The structure in the vec! body is similar to the structure of a match expression. Here we have one arm with the pattern ( $( $x:expr ),* ), followed by => and the block of code associated with this pattern. If the pattern matches, the associated block of code will be emitted. Given that this is the only pattern in this macro, there is only one valid way to match; any other pattern will result in an error. More complex macros will have more than one arm.

To learn more about how to write macros, consult the online documentation or other resources, such as “The Little Book of Rust Macros” started by Daniel Keep and continued by Lukas Wirth.

Additionally, we can’t yet provide the hello_macro function with default implementation that will print the name of the type the trait is implemented on: Rust doesn’t have reflection capabilities, so it can’t look up the type’s name at runtime. We need a macro to generate code at compile time.

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Soon we’ll define the impl_hello_macro function, which is where we’ll build the new Rust code we want to include. But before we do, note that the output for our derive macro is also a TokenStream. The returned TokenStream is added to the code that our crate users write, so when they compile their crate, they’ll get the extra functionality that we provide in the modified TokenStream.

Here are TAMRON's recommended lenses by lens type. TAMRON zoom lenses cover a wide range of focal lengths with a single lens yet are of a size that makes them easy to carry. On top of this, they are characterized by their high descriptive performance and close-up shooting ability. We hope you will find the lens that suits you best.

Another characteristic of focal length is depth of field. Depth of field refers to the range in which the image looks it is in focus, and a deep depth of field means that the image looks in focus through wide range, such as from a subject to background. And if the depth of field is shallow, only the area around the subject will be in focus and the background will have a smooth bokeh. The shorter the focal length of the lens and the larger the aperture (F-number), the deeper the depth of field. This is why wide-angle lenses are suitable for landscape photography, as the short focal length increases the depth of field and allows the photographer to capture details in the background clearly. On the other hand, as telephoto lenses have a long focal length and shallow depth of field, the bokeh in the background of a subject is more pronounced. By utilizing these bokeh effects, you can capture portraits and other images where the subject stands out against the background.

The 35-150mm F/2-2.8 Di III VXD (Model A058) is a high resolution travel zoom lens that covers everything from the 35mm wide angle to the 150mm telephoto focal length, the first zoom lens achieving an aperture of F2 at the wide angle end. It has a groundbreaking fast-aperture and utilizes the linear motor focus mechanism VXD (Voice-coil eXtreme-torque Drive), thereby achieving high speed, high precision autofocusing. The innovative lens design enabled us to greatly improve the lens's grip and functionality. The software, developed in-house, enables to easily customize functions and to update firmware.

The hello_macro_derive function first converts the input from a TokenStream to a data structure that we can then interpret and perform operations on. This is where syn comes into play. The parse function in syn takes a TokenStream and returns a DeriveInput struct representing the parsed Rust code. Listing 19-32 shows the relevant parts of the DeriveInput struct we get from parsing the struct Pancakes; string:

The fields of this struct show that the Rust code we’ve parsed is a unit struct with the ident (identifier, meaning the name) of Pancakes. There are more fields on this struct for describing all sorts of Rust code; check the syn documentation for DeriveInput for more information.

The 20-40mm F/2.8 Di III VXD (Model A062) is a new large-aperture standard zoom lens that thoroughly pursues portability. While covering the range from the ultra-wide angle of 20mm to the standard range of 40mm, it is the smallest and lightest in its class. It also offers high image quality throughout the entire zoom range, making it useful not only for still image shooting but also for video recording such as vlogging. The VXD, which is quiet and agile, achieves high-speed, high-precision autofocusing. It is a new, unprecedented large-aperture standard zoom lens that allows users to easily enjoy taking out and shooting both still and video.