Write docs for lint / fix review nit

compiler/rustc_lint/src/methods.rs

@@ -6,6 +6,25 @@ use rustc_middle::ty;
 use rustc_span::{symbol::sym, ExpnKind, Span};
 
 declare_lint! {
+    /// The `temporary_cstring_as_ptr` lint detects getting the inner pointer of
+    /// a temporary `CString`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # #![allow(unused)]
+    /// let c_str = CString::new("foo").unwrap().as_ptr();
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The inner pointer of a `CString` lives only as long as the `CString` it
+    /// points to. Getting the inner pointer of a *temporary* `CString` allows the `CString`
+    /// to be dropped at the end of the statement, as it is not being referenced as far as the typesystem
+    /// is concerned. This means outside of the statement the pointer will point to freed memory, which
+    /// causes undefined behavior if the pointer is later dereferenced.
     pub TEMPORARY_CSTRING_AS_PTR,
     Warn,
     "detects getting the inner pointer of a temporary `CString`"
@@ -75,8 +94,7 @@ fn lint_cstring_as_ptr(
                             unwrap.span,
                             "this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime",
                         );
-                        diag.note("pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement...");
+                        diag.note("pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned");
-                        diag.note("...because nothing is referencing it as far as the type system is concerned");
                         diag.help("for more information, see https://doc.rust-lang.org/reference/destructors.html");
                         diag.emit();
                     });

src/test/ui/lint/lint-temporary-cstring-as-param.stderr

@@ -11,8 +11,7 @@ note: the lint level is defined here
    |
 LL | #![deny(temporary_cstring_as_ptr)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement...
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
-   = note: ...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
 
 error: aborting due to previous error

src/test/ui/lint/lint-temporary-cstring-as-ptr.rs

@@ -1,4 +1,5 @@
 // ignore-tidy-linelength
+// this program is not technically incorrect, but is an obscure enough style to be worth linting
 #![deny(temporary_cstring_as_ptr)]
 
 use std::ffi::CString;

src/test/ui/lint/lint-temporary-cstring-as-ptr.stderr

@@ -1,5 +1,5 @@
 error: getting the inner pointer of a temporary `CString`
-  --> $DIR/lint-temporary-cstring-as-ptr.rs:7:48
+  --> $DIR/lint-temporary-cstring-as-ptr.rs:8:48
    |
 LL |     let s = CString::new("some text").unwrap().as_ptr();
    |             ---------------------------------- ^^^^^^ this pointer will be invalid
@@ -7,12 +7,11 @@ LL |     let s = CString::new("some text").unwrap().as_ptr();
    |             this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
    |
 note: the lint level is defined here
-  --> $DIR/lint-temporary-cstring-as-ptr.rs:2:9
+  --> $DIR/lint-temporary-cstring-as-ptr.rs:3:9
    |
 LL | #![deny(temporary_cstring_as_ptr)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement...
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
-   = note: ...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
 
 error: aborting due to previous error