Rollup merge of #71845 - steveklabnik:add-const-examples, r=dtolnay

Add const examples

I only added them to `std::f32` to get feedback on this approach before adding the other constants.

When looking at #68952, I found the docs a little confusing. Unless you're intimately aware of what's going on here, I don't think it's super clear what is deprecated and what you're supposed to do instead. I think short examples really clarify what's meant here, so that's what I did.

src/libcore/num/f32.rs

@@ -18,15 +18,46 @@ use crate::num::FpCategory;
 
 /// The radix or base of the internal representation of `f32`.
 /// Use [`f32::RADIX`](../../std/primitive.f32.html#associatedconstant.RADIX) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let r = std::f32::RADIX;
+///
+/// // intended way
+/// let r = f32::RADIX;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const RADIX: u32 = f32::RADIX;
 
 /// Number of significant digits in base 2.
 /// Use [`f32::MANTISSA_DIGITS`](../../std/primitive.f32.html#associatedconstant.MANTISSA_DIGITS) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let d = std::f32::MANTISSA_DIGITS;
+///
+/// // intended way
+/// let d = f32::MANTISSA_DIGITS;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MANTISSA_DIGITS: u32 = f32::MANTISSA_DIGITS;
+
 /// Approximate number of significant digits in base 10.
 /// Use [`f32::DIGITS`](../../std/primitive.f32.html#associatedconstant.DIGITS) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let d = std::f32::DIGITS;
+///
+/// // intended way
+/// let d = f32::DIGITS;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const DIGITS: u32 = f32::DIGITS;
 
@@ -36,50 +67,166 @@ pub const DIGITS: u32 = f32::DIGITS;
 /// This is the difference between `1.0` and the next larger representable number.
 ///
 /// [Machine epsilon]: https://en.wikipedia.org/wiki/Machine_epsilon
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let e = std::f32::EPSILON;
+///
+/// // intended way
+/// let e = f32::EPSILON;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const EPSILON: f32 = f32::EPSILON;
 
 /// Smallest finite `f32` value.
 /// Use [`f32::MIN`](../../std/primitive.f32.html#associatedconstant.MIN) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let min = std::f32::MIN;
+///
+/// // intended way
+/// let min = f32::MIN;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MIN: f32 = f32::MIN;
+
 /// Smallest positive normal `f32` value.
 /// Use [`f32::MIN_POSITIVE`](../../std/primitive.f32.html#associatedconstant.MIN_POSITIVE) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let min = std::f32::MIN_POSITIVE;
+///
+/// // intended way
+/// let min = f32::MIN_POSITIVE;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MIN_POSITIVE: f32 = f32::MIN_POSITIVE;
+
 /// Largest finite `f32` value.
 /// Use [`f32::MAX`](../../std/primitive.f32.html#associatedconstant.MAX) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let max = std::f32::MAX;
+///
+/// // intended way
+/// let max = f32::MAX;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MAX: f32 = f32::MAX;
 
 /// One greater than the minimum possible normal power of 2 exponent.
 /// Use [`f32::MIN_EXP`](../../std/primitive.f32.html#associatedconstant.MIN_EXP) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let min = std::f32::MIN_EXP;
+///
+/// // intended way
+/// let min = f32::MIN_EXP;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MIN_EXP: i32 = f32::MIN_EXP;
+
 /// Maximum possible power of 2 exponent.
 /// Use [`f32::MAX_EXP`](../../std/primitive.f32.html#associatedconstant.MAX_EXP) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let max = std::f32::MAX_EXP;
+///
+/// // intended way
+/// let max = f32::MAX_EXP;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MAX_EXP: i32 = f32::MAX_EXP;
 
 /// Minimum possible normal power of 10 exponent.
 /// Use [`f32::MIN_10_EXP`](../../std/primitive.f32.html#associatedconstant.MIN_10_EXP) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let min = std::f32::MIN_10_EXP;
+///
+/// // intended way
+/// let min = f32::MIN_10_EXP;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MIN_10_EXP: i32 = f32::MIN_10_EXP;
+
 /// Maximum possible power of 10 exponent.
 /// Use [`f32::MAX_10_EXP`](../../std/primitive.f32.html#associatedconstant.MAX_10_EXP) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let max = std::f32::MAX_10_EXP;
+///
+/// // intended way
+/// let max = f32::MAX_10_EXP;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MAX_10_EXP: i32 = f32::MAX_10_EXP;
 
 /// Not a Number (NaN).
 /// Use [`f32::NAN`](../../std/primitive.f32.html#associatedconstant.NAN) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let nan = std::f32::NAN;
+///
+/// // intended way
+/// let nan = f32::NAN;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const NAN: f32 = f32::NAN;
+
 /// Infinity (∞).
 /// Use [`f32::INFINITY`](../../std/primitive.f32.html#associatedconstant.INFINITY) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let inf = std::f32::INFINITY;
+///
+/// // intended way
+/// let inf = f32::INFINITY;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const INFINITY: f32 = f32::INFINITY;
+
 /// Negative infinity (−∞).
 /// Use [`f32::NEG_INFINITY`](../../std/primitive.f32.html#associatedconstant.NEG_INFINITY) instead.
+///
+/// # Examples
+///
+/// ```rust
+/// // deprecated way
+/// let ninf = std::f32::NEG_INFINITY;
+///
+/// // intended way
+/// let ninf = f32::NEG_INFINITY;
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const NEG_INFINITY: f32 = f32::NEG_INFINITY;