Improve clippy_utils function docs

Author: xFrednet

clippy_utils/src/source.rs

@@ -155,14 +155,22 @@ fn reindent_multiline_inner(s: &str, ignore_first: bool, indent: Option<usize>,
         .join("\n")
 }
 
-/// Converts a span to a code snippet if available, otherwise use default.
+/// Converts a span to a code snippet if available, otherwise returns the default.
 ///
 /// This is useful if you want to provide suggestions for your lint or more generally, if you want
-/// to convert a given `Span` to a `str`.
+/// to convert a given `Span` to a `str`. To create suggestions consider using
+/// [`snippet_with_applicability`] to ensure that the applicability stays correct.
 ///
 /// # Example
 /// ```rust,ignore
-/// snippet(cx, expr.span, "..")
+/// // Given two spans one for `value` and one for the `init` expression.
+/// let value = Vec::new();
+/// //  ^^^^^   ^^^^^^^^^^
+/// //  span1   span2
+///
+/// // The snipped call would return the corresponding code snippet
+/// snippet(cx, span1, "..") // -> "value"
+/// snippet(cx, span2, "..") // -> "Vec::new()"
 /// ```
 pub fn snippet<'a, T: LintContext>(cx: &T, span: Span, default: &'a str) -> Cow<'a, str> {
     snippet_opt(cx, span).map_or_else(|| Cow::Borrowed(default), From::from)

clippy_utils/src/ty.rs

@@ -114,7 +114,12 @@ pub fn has_iter_method(cx: &LateContext<'_>, probably_ref_ty: Ty<'_>) -> Option<
 
 /// Checks whether a type implements a trait.
 /// The function returns false in case the type contains an inference variable.
-/// See also [`get_trait_def_id`](super::get_trait_def_id).
+///
+/// See:
+/// * [`get_trait_def_id`](super::get_trait_def_id) to get a trait [`DefId`].
+/// * [Common tools for writing lints] for an example how to use this function and other options.
+///
+/// [Common tools for writing lints]: https://github.com/rust-lang/rust-clippy/blob/master/doc/common_tools_writing_lints.md#checking-if-a-type-implements-a-specific-trait
 pub fn implements_trait<'tcx>(
     cx: &LateContext<'tcx>,
     ty: Ty<'tcx>,
@@ -254,9 +259,17 @@ pub fn is_type_ref_to_diagnostic_item(cx: &LateContext<'_>, ty: Ty<'_>, diag_ite
     }
 }
 
-/// Checks if the type is equal to a diagnostic item
+/// Checks if the type is equal to a diagnostic item. To check if a type implements a
+/// trait marked with a diagnostic item use [`implements_trait`].
+///
+/// For a further exploitation what diagnostic items are see [diagnostic items] in
+/// rustc-dev-guide.
+///
+/// ---
 ///
 /// If you change the signature, remember to update the internal lint `MatchTypeOnDiagItem`
+///
+/// [Diagnostic Items]: https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-items.html
 pub fn is_type_diagnostic_item(cx: &LateContext<'_>, ty: Ty<'_>, diag_item: Symbol) -> bool {
     match ty.kind() {
         ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(diag_item, adt.did),