Rollup merge of #141874 - usamoi:eps, r=tgross35

add f16_epsilon and f128_epsilon diagnostic items

cc https://github.com/rust-lang/rust/issues/116909
r? ``@tgross35``

compiler/rustc_span/src/symbol.rs

@@ -937,8 +937,10 @@ symbols! {
         external_doc,
         f,
         f128,
+        f128_epsilon,
         f128_nan,
         f16,
+        f16_epsilon,
         f16_nan,
         f16c_target_feature,
         f32,

library/core/src/num/f128.rs

@@ -171,6 +171,7 @@ impl f128 {
     /// [Machine epsilon]: https://en.wikipedia.org/wiki/Machine_epsilon
     /// [`MANTISSA_DIGITS`]: f128::MANTISSA_DIGITS
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_diagnostic_item = "f128_epsilon"]
     pub const EPSILON: f128 = 1.92592994438723585305597794258492732e-34_f128;
 
     /// Smallest finite `f128` value.

library/core/src/num/f16.rs

@@ -168,6 +168,7 @@ impl f16 {
     /// [Machine epsilon]: https://en.wikipedia.org/wiki/Machine_epsilon
     /// [`MANTISSA_DIGITS`]: f16::MANTISSA_DIGITS
     #[unstable(feature = "f16", issue = "116909")]
+    #[rustc_diagnostic_item = "f16_epsilon"]
     pub const EPSILON: f16 = 9.7656e-4_f16;
 
     /// Smallest finite `f16` value.