Rollup merge of #4832 - dario23:i4829, r=phansch

Add some positive examples to lint docs

fixes #4829
changelog: Add some positive examples to lint docs

Author: phansch

clippy_lints/src/approx_const.rs

@@ -23,7 +23,13 @@ declare_clippy_lint! {
     ///
     /// **Example:**
     /// ```rust
+    /// // Bad
     /// let x = 3.14;
+    /// let y = 1_f64 / x;
+    ///
+    /// // Good
+    /// let x = std::f32::consts::PI;
+    /// let y = std::f64::consts::FRAC_1_PI;
     /// ```
     pub APPROX_CONSTANT,
     correctness,

clippy_lints/src/lifetimes.rs

@@ -24,9 +24,15 @@ declare_clippy_lint! {
     ///
     /// **Example:**
     /// ```rust
+    /// // Bad: unnecessary lifetime annotations
     /// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
     ///     x
     /// }
+    ///
+    /// // Good
+    /// fn elided(x: &u8, y: u8) -> &u8 {
+    ///     x
+    /// }
     /// ```
     pub NEEDLESS_LIFETIMES,
     complexity,
@@ -46,9 +52,15 @@ declare_clippy_lint! {
     ///
     /// **Example:**
     /// ```rust
+    /// // Bad: unnecessary lifetimes
     /// fn unused_lifetime<'a>(x: u8) {
     ///     // ..
     /// }
+    ///
+    /// // Good
+    /// fn no_lifetime(x: u8) {
+    ///     // ...
+    /// }
     /// ```
     pub EXTRA_UNUSED_LIFETIMES,
     complexity,