Skip some untranslatable code blocks (#1258)

This builds on the work of @dyoo in
https://github.com/google/mdbook-i18n-helpers/pull/69: by adding a
special `<!-- mdbook-xgettext: skip -->` comment, we can skip the
following code block.

I also modified a few code blocks to remove translatable text: variable
names are not expected to be translated, so it’s fine to have a line
with `println!("foo: {foo}")` in the code block.

This PR removes 36 messages from the POT file. The number of lines drop
by 633 (3%).

Part of #1257.

src/basic-syntax/compound-types.md

@@ -7,6 +7,7 @@
 
 Array assignment and access:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut a: [i8; 10] = [42; 10];
@@ -17,11 +18,12 @@ fn main() {
 
 Tuple assignment and access:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let t: (i8, bool) = (7, true);
-    println!("1st index: {}", t.0);
-    println!("2nd index: {}", t.1);
+    println!("t.0: {}", t.0);
+    println!("t.1: {}", t.1);
 }
 ```
 

src/basic-syntax/functions.md

@@ -2,6 +2,7 @@
 
 A Rust version of the famous [FizzBuzz](https://en.wikipedia.org/wiki/Fizz_buzz) interview question:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     print_fizzbuzz_to(20);

src/basic-syntax/references-dangling.md

@@ -2,6 +2,7 @@
 
 Rust will statically forbid dangling references:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable,compile_fail
 fn main() {
     let ref_x: &i32;

src/basic-syntax/references.md

@@ -2,6 +2,7 @@
 
 Like C++, Rust has references:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut x: i32 = 10;

src/basic-syntax/scalar-types.md

@@ -24,6 +24,7 @@ There are a few syntaxes which are not shown above:
   == "\\\\n"`. You can embed double-quotes by using an equal amount of `#` on
   either side of the quotes:
 
+  <!-- mdbook-xgettext: skip -->
   ```rust,editable
   fn main() {
       println!(r#"<a href="link.html">link</a>"#);
@@ -33,6 +34,7 @@ There are a few syntaxes which are not shown above:
 
 - Byte strings allow you to create a `&[u8]` value directly:
 
+  <!-- mdbook-xgettext: skip -->
   ```rust,editable
   fn main() {
       println!("{:?}", b"abc");

src/basic-syntax/scopes-shadowing.md

@@ -27,6 +27,7 @@ fn main() {
 * Shadowing looks obscure at first, but is convenient for holding on to values after `.unwrap()`.
 * The following code demonstrates why the compiler can't simply reuse memory locations when shadowing an immutable variable in a scope, even if the type does not change.
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let a = 1;

src/basic-syntax/slices.md

@@ -2,6 +2,7 @@
 
 A slice gives you a view into a larger collection:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut a: [i32; 6] = [10, 20, 30, 40, 50, 60];

src/basic-syntax/static-and-const.md

@@ -8,6 +8,7 @@ cannot be moved or reallocated during the execution of the program.
 Constant variables are evaluated at compile time and their values are inlined
 wherever they are used:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 const DIGEST_SIZE: usize = 3;
 const ZERO: Option<u8> = Some(42);
@@ -22,7 +23,7 @@ fn compute_digest(text: &str) -> [u8; DIGEST_SIZE] {
 
 fn main() {
     let digest = compute_digest("Hello");
-    println!("Digest: {digest:?}");
+    println!("digest: {digest:?}");
 }
 ```
 

src/basic-syntax/type-inference.md

@@ -2,6 +2,7 @@
 
 Rust will look at how the variable is _used_ to determine the type:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn takes_u32(x: u32) {
     println!("u32: {x}");
@@ -31,6 +32,7 @@ The compiler does the job for us and helps us write more concise code.
 
 The following code tells the compiler to copy into a certain generic container without the code ever explicitly specifying the contained type, using `_` as a placeholder:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut v = Vec::new();

src/basic-syntax/variables.md

@@ -3,6 +3,7 @@
 Rust provides type safety via static typing. Variable bindings are immutable by
 default:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let x: i32 = 10;

src/control-flow/blocks.md

@@ -4,6 +4,7 @@ A block in Rust contains a sequence of expressions.
 Each block has a value and a type,
 which are those of the last expression of the block:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let x = {
@@ -28,13 +29,14 @@ If the last expression ends with `;`, then the resulting value and type is `()`.
 The same rule is used for functions: the value of the function body is the
 return value:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn double(x: i32) -> i32 {
     x + x
 }
 
 fn main() {
-    println!("doubled: {}", double(7));
+    println!("double: {}", double(7));
 }
 ```
 

src/control-flow/break-continue.md

@@ -7,6 +7,7 @@ the next iteration use [`continue`](https://doc.rust-lang.org/reference/expressi
 Both `continue` and `break` can optionally take a label argument which is used
 to break out of nested loops:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let v = vec![10, 20, 30];

src/control-flow/for-expressions.md

@@ -4,6 +4,7 @@ The [`for` loop](https://doc.rust-lang.org/std/keyword.for.html) is closely
 related to the [`while let` loop](while-let-expressions.md). It will
 automatically call `into_iter()` on the expression and then iterate over it:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let v = vec![10, 20, 30];

src/control-flow/if-let-expressions.md

@@ -25,6 +25,7 @@ Rust.
 * Unlike `match`, `if let` does not support guard clauses for pattern matching.
 * Since 1.65, a similar [let-else](https://doc.rust-lang.org/rust-by-example/flow_control/let_else.html) construct allows to do a destructuring assignment, or if it fails, execute a block which is required to abort normal control flow (with `panic`/`return`/`break`/`continue`):
 
+   <!-- mdbook-xgettext: skip -->
    ```rust,editable
    fn main() {
        println!("{:?}", second_word_to_upper("foo bar"));

src/control-flow/loop-expressions.md

@@ -5,6 +5,7 @@ which creates an endless loop.
 
 Here you must either `break` or `return` to stop the loop:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut x = 10;
@@ -18,7 +19,7 @@ fn main() {
             break;
         }
     }
-    println!("Final x: {x}");
+    println!("x: {x}");
 }
 ```
 

src/control-flow/while-expressions.md

@@ -3,6 +3,7 @@
 The [`while` keyword](https://doc.rust-lang.org/reference/expressions/loop-expr.html#predicate-loops)
 works very similar to other languages:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut x = 10;
@@ -13,7 +14,7 @@ fn main() {
             3 * x + 1
         };
     }
-    println!("Final x: {x}");
+    println!("x: {x}");
 }
 ```
 

src/control-flow/while-let-expressions.md

@@ -3,6 +3,7 @@
 Like with `if let`, there is a [`while let`](https://doc.rust-lang.org/reference/expressions/loop-expr.html#predicate-pattern-loops)
 variant which repeatedly tests a value against a pattern:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let v = vec![10, 20, 30];

src/enums/sizes.md

@@ -31,6 +31,7 @@ Key Points:
 
  * You can control the discriminant if needed (e.g., for compatibility with C):
  
+     <!-- mdbook-xgettext: skip -->
      ```rust,editable
      #[repr(u32)]
      enum Bar {
@@ -67,6 +68,7 @@ Key Points:
      Example code if you want to show how the bitwise representation *may* look like in practice.
      It's important to note that the compiler provides no guarantees regarding this representation, therefore this is totally unsafe.
 
+     <!-- mdbook-xgettext: skip -->
      ```rust,editable
      use std::mem::transmute;
 
@@ -77,25 +79,23 @@ Key Points:
      }
 
      fn main() {
-         // TOTALLY UNSAFE. Rust provides no guarantees about the bitwise
-         // representation of types.
          unsafe {
-             println!("Bitwise representation of bool");
+             println!("bool:");
              dbg_bits!(false, u8);
              dbg_bits!(true, u8);
 
-             println!("Bitwise representation of Option<bool>");
+             println!("Option<bool>:");
              dbg_bits!(None::<bool>, u8);
              dbg_bits!(Some(false), u8);
              dbg_bits!(Some(true), u8);
 
-             println!("Bitwise representation of Option<Option<bool>>");
+             println!("Option<Option<bool>>:");
              dbg_bits!(Some(Some(false)), u8);
              dbg_bits!(Some(Some(true)), u8);
              dbg_bits!(Some(None::<bool>), u8);
              dbg_bits!(None::<Option<bool>>, u8);
 
-             println!("Bitwise representation of Option<&i32>");
+             println!("Option<&i32>:");
              dbg_bits!(None::<&i32>, usize);
              dbg_bits!(Some(&0i32), usize);
          }
@@ -104,6 +104,7 @@ Key Points:
 
      More complex example if you want to discuss what happens when we chain more than 256 `Option`s together.
 
+     <!-- mdbook-xgettext: skip -->
      ```rust,editable
      #![recursion_limit = "1000"]
 

src/exercises/day-1/for-loops.md

@@ -8,6 +8,7 @@ let array = [10, 20, 30];
 
 You can print such an array by asking for its debug representation with `{:?}`:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let array = [10, 20, 30];
@@ -38,6 +39,7 @@ fn main() {
 Use the above to write a function `pretty_print` which pretty-print a matrix and
 a function `transpose` which will transpose a matrix (turn rows into columns):
 
+<!-- mdbook-xgettext: skip -->
 ```bob
            ⎛⎡1 2 3⎤⎞      ⎡1 4 7⎤
 "transpose"⎜⎢4 5 6⎥⎟  "=="⎢2 5 8⎥

src/exercises/day-1/implicit-conversions.md

@@ -3,6 +3,7 @@
 Rust will not automatically apply _implicit conversions_ between types ([unlike
 C++][3]). You can see this in a program like this:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable,compile_fail
 fn multiply(x: i16, y: i16) -> i16 {
     x * y

src/memory-management/scope-based.md

@@ -24,6 +24,7 @@ void say_hello(std::unique_ptr<Person> person) {
 
 Special move constructors are used when passing ownership to a function:
 
+<!-- mdbook-xgettext: skip -->
 ```c++
 std::unique_ptr<Person> person = find_person("Carla");
 say_hello(std::move(person));

src/ownership.md

@@ -3,6 +3,7 @@
 All variable bindings have a _scope_ where they are valid and it is an error to
 use a variable outside its scope:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable,compile_fail
 struct Point(i32, i32);
 

src/ownership/borrowing.md

@@ -3,6 +3,7 @@
 Instead of transferring ownership when calling a function, you can let a
 function _borrow_ the value:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 #[derive(Debug)]
 struct Point(i32, i32);
@@ -27,6 +28,7 @@ fn main() {
 Notes on stack returns:
 * Demonstrate that the return from `add` is cheap because the compiler can eliminate the copy operation. Change the above code to print stack addresses and run it on the [Playground] or look at the assembly in [Godbolt](https://rust.godbolt.org/). In the "DEBUG" optimization level, the addresses should change, while they stay the same when changing to the "RELEASE" setting:
 
+  <!-- mdbook-xgettext: skip -->
   ```rust,editable
   #[derive(Debug)]
   struct Point(i32, i32);

src/ownership/copy-clone.md

@@ -2,6 +2,7 @@
 
 While move semantics are the default, certain types are copied by default:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let x = 42;
@@ -15,6 +16,7 @@ These types implement the `Copy` trait.
 
 You can opt-in your own types to use copy semantics:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 #[derive(Copy, Clone, Debug)]
 struct Point(i32, i32);

src/ownership/lifetimes-function-calls.md

@@ -2,6 +2,7 @@
 
 In addition to borrowing its arguments, a function can return a borrowed value:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 #[derive(Debug)]
 struct Point(i32, i32);
@@ -14,7 +15,7 @@ fn main() {
     let p1: Point = Point(10, 10);
     let p2: Point = Point(20, 20);
     let p3: &Point = left_most(&p1, &p2);
-    println!("left-most point: {:?}", p3);
+    println!("p3: {p3:?}");
 }
 ```
 
@@ -29,6 +30,7 @@ fn main() {
 In the above example, try the following:
 
 * Move the declaration of `p2` and `p3` into a new scope (`{ ... }`), resulting in the following code:
+  <!-- mdbook-xgettext: skip -->
   ```rust,ignore
   #[derive(Debug)]
   struct Point(i32, i32);
@@ -44,7 +46,7 @@ In the above example, try the following:
           let p2: Point = Point(20, 20);
           p3 = left_most(&p1, &p2);
       }
-      println!("left-most point: {:?}", p3);
+      println!("p3: {p3:?}");
   }
   ```
   Note how this does not compile since `p3` outlives `p2`.

src/ownership/moved-strings-rust.md

@@ -1,5 +1,6 @@
 # Moved Strings in Rust
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let s1: String = String::from("Rust");

src/ownership/shared-unique-borrows.md

@@ -5,6 +5,7 @@ Rust puts constraints on the ways you can borrow values:
 * You can have one or more `&T` values at any given time, _or_
 * You can have exactly one `&mut T` value.
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable,compile_fail
 fn main() {
     let mut a: i32 = 10;