Start work on dangling pointers lint

compiler/rustc_lint/messages.ftl

@@ -209,7 +209,10 @@ lint_dangling_pointers_from_temporaries = a dangling pointer will be produced be
     .label_ptr = this pointer will immediately be invalid
     .label_temporary = this `{$ty}` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
     .note = pointers do not have a lifetime; when calling `{$callee}` the `{$ty}` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
-    .help = for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+    .help_info = you must make sure that the variable you bind the `{$typ}` to lives at least as long as the pointer returned by the call to `{$callee}`
+    .help_info = in particular, if this pointer is returned from the current function, binding the `{$typ}` inside the function will not suffice
+    .help_visit = for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
 
 lint_default_hash_types = prefer `{$preferred}` over `{$used}`, it has better performance
     .note = a `use rustc_data_structures::fx::{$preferred}` may be necessary

compiler/rustc_lint/src/lints.rs

@@ -1148,6 +1148,7 @@ pub(crate) struct IgnoredUnlessCrateSpecified<'a> {
 #[diag(lint_dangling_pointers_from_temporaries)]
 #[note]
 #[help]
+#[help(lint_info)]
 // FIXME: put #[primary_span] on `ptr_span` once it does not cause conflicts
 pub(crate) struct DanglingPointersFromTemporaries<'tcx> {
     pub callee: Symbol,