entry: link to Entry API if can't make a suggestion (#15546)

For #11598

changelog: [`entry`] link to `Entry` API if can't make a suggestion

Author: samueltardieu

clippy_lints/src/entry.rs

@@ -1,4 +1,4 @@
-use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
+use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
 use clippy_utils::source::{reindent_multiline, snippet_indent, snippet_with_applicability, snippet_with_context};
 use clippy_utils::ty::is_copy;
 use clippy_utils::visitors::for_each_expr;
@@ -194,7 +194,17 @@ impl<'tcx> LateLintPass<'tcx> for HashMapPass {
         if let Some(sugg) = sugg {
             span_lint_and_sugg(cx, MAP_ENTRY, expr.span, lint_msg, "try", sugg, app);
         } else {
-            span_lint(cx, MAP_ENTRY, expr.span, lint_msg);
+            span_lint_and_help(
+                cx,
+                MAP_ENTRY,
+                expr.span,
+                lint_msg,
+                None,
+                format!(
+                    "consider using the `Entry` API: https://doc.rust-lang.org/std/collections/struct.{}.html#entry-api",
+                    map_ty.name()
+                ),
+            );
         }
     }
 }

tests/ui/entry_unfixable.stderr

@@ -10,6 +10,7 @@ LL | |                 false
 LL | |             }
    | |_____________^
    |
+   = help: consider using the `Entry` API: https://doc.rust-lang.org/std/collections/struct.HashMap.html#entry-api
    = note: `-D clippy::map-entry` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::map_entry)]`
 
@@ -24,6 +25,8 @@ LL | |     } else {
 LL | |         hm.insert(key, true);
 LL | |     }
    | |_____^
+   |
+   = help: consider using the `Entry` API: https://doc.rust-lang.org/std/collections/struct.HashMap.html#entry-api
 
 error: usage of `contains_key` followed by `insert` on a `HashMap`
   --> tests/ui/entry_unfixable.rs:80:13
@@ -36,6 +39,8 @@ LL | |                 let interner = INTERNER.lock().unwrap();
 LL | |                 return Err(interner.resolve(name).unwrap().to_owned());
 LL | |             }
    | |_____________^
+   |
+   = help: consider using the `Entry` API: https://doc.rust-lang.org/std/collections/struct.HashMap.html#entry-api
 
 error: aborting due to 3 previous errors