Rollup merge of rust-lang#50539 - clarcharr:log_const, r=dtolnay

Add more logarithm constants

Right now, we have `ln(2)` and `ln(10)`, but only `log2(e)` and `log10(e)`. This also adds `log2(10)` and `log10(2)` for consistency.

Author: kennytm

src/libcore/num/f32.rs

@@ -128,10 +128,18 @@ pub mod consts {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const LOG2_E: f32 = 1.44269504088896340735992468100189214_f32;
 
+    /// log<sub>2</sub>(10)
+    #[unstable(feature = "extra_log_consts", issue = "50540")]
+    pub const LOG2_10: f32 = 3.32192809488736234787031942948939018_f32;
+
     /// log<sub>10</sub>(e)
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const LOG10_E: f32 = 0.434294481903251827651128918916605082_f32;
 
+    /// log<sub>10</sub>(2)
+    #[unstable(feature = "extra_log_consts", issue = "50540")]
+    pub const LOG10_2: f32 = 0.301029995663981195213738894724493027_f32;
+
     /// ln(2)
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const LN_2: f32 = 0.693147180559945309417232121458176568_f32;

src/libcore/num/f64.rs

@@ -124,10 +124,18 @@ pub mod consts {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const E: f64 = 2.71828182845904523536028747135266250_f64;
 
+    /// log<sub>2</sub>(10)
+    #[unstable(feature = "extra_log_consts", issue = "50540")]
+    pub const LOG2_10: f64 = 3.32192809488736234787031942948939018_f64;
+
     /// log<sub>2</sub>(e)
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const LOG2_E: f64 = 1.44269504088896340735992468100189214_f64;
 
+    /// log<sub>10</sub>(2)
+    #[unstable(feature = "extra_log_consts", issue = "50540")]
+    pub const LOG10_2: f64 = 0.301029995663981195213738894724493027_f64;
+
     /// log<sub>10</sub>(e)
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const LOG10_E: f64 = 0.434294481903251827651128918916605082_f64;