Address review comments

compiler/rustc_lint/src/methods.rs

@@ -3,10 +3,7 @@ use crate::LateLintPass;
 use crate::LintContext;
 use rustc_hir::{Expr, ExprKind, PathSegment};
 use rustc_middle::ty;
-use rustc_span::{
+use rustc_span::{symbol::sym, ExpnKind, Span};
-    symbol::{sym, Symbol},
-    ExpnKind, Span,
-};
 
 declare_lint! {
     pub TEMPORARY_CSTRING_AS_PTR,
@@ -59,8 +56,6 @@ impl<'tcx> LateLintPass<'tcx> for TemporaryCStringAsPtr {
     }
 }
 
-const CSTRING_PATH: [Symbol; 4] = [sym::std, sym::ffi, sym::c_str, sym::CString];
-
 fn lint_cstring_as_ptr(
     cx: &LateContext<'_>,
     as_ptr_span: Span,
@@ -68,19 +63,21 @@ fn lint_cstring_as_ptr(
     unwrap: &rustc_hir::Expr<'_>,
 ) {
     let source_type = cx.typeck_results().expr_ty(source);
-    if let ty::Adt(def, substs) = source_type.kind {
+    if let ty::Adt(def, substs) = source_type.kind() {
         if cx.tcx.is_diagnostic_item(sym::result_type, def.did) {
-            if let ty::Adt(adt, _) = substs.type_at(0).kind {
+            if let ty::Adt(adt, _) = substs.type_at(0).kind() {
-                if cx.match_def_path(adt.did, &CSTRING_PATH) {
+                if cx.tcx.is_diagnostic_item(sym::cstring_type, adt.did) {
                     cx.struct_span_lint(TEMPORARY_CSTRING_AS_PTR, as_ptr_span, |diag| {
                         let mut diag = diag
                             .build("getting the inner pointer of a temporary `CString`");
                         diag.span_label(as_ptr_span, "this pointer will be invalid");
                         diag.span_label(
                             unwrap.span,
-                            "this `CString` is deallocated at the end of the expression, bind it to a variable to extend its lifetime",
+                            "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` is deallocated because nothing is referencing it as far as the type system is concerned");
+                        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("...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();
                     });
                 }

compiler/rustc_span/src/symbol.rs

@@ -400,6 +400,7 @@ symbols! {
         crate_type,
         crate_visibility_modifier,
         crt_dash_static: "crt-static",
+        cstring_type,
         ctlz,
         ctlz_nonzero,
         ctpop,

library/std/src/ffi/c_str.rs

@@ -109,7 +109,9 @@ use crate::sys;
 /// documentation of `CString` before use, as improper ownership management
 /// of `CString` instances can lead to invalid memory accesses, memory leaks,
 /// and other memory errors.
+
 #[derive(PartialEq, PartialOrd, Eq, Ord, Hash, Clone)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "cstring_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct CString {
     // Invariant 1: the slice ends with a zero byte and has a length of at least one.

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

@@ -0,0 +1,10 @@
+// ignore-tidy-linelength
+#![deny(temporary_cstring_as_ptr)]
+
+use std::ffi::CString;
+
+fn some_function(data: *const i8) {}
+
+fn main() {
+    some_function(CString::new("").unwrap().as_ptr()); //~ ERROR getting the inner pointer of a temporary `CString`
+}

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

@@ -0,0 +1,19 @@
+error: getting the inner pointer of a temporary `CString`
+  --> $DIR/lint-temporary-cstring-as-param.rs:9:45
+   |
+LL |     some_function(CString::new("").unwrap().as_ptr());
+   |                   ------------------------- ^^^^^^ this pointer will be invalid
+   |                   |
+   |                   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-param.rs:2:9
+   |
+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: ...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.stderr

@@ -4,14 +4,16 @@ error: getting the inner pointer of a temporary `CString`
 LL |     let s = CString::new("some text").unwrap().as_ptr();
    |             ---------------------------------- ^^^^^^ this pointer will be invalid
    |             |
-   |             this `CString` is deallocated at the end of the expression, bind it to a variable to extend its lifetime
+   |             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
    |
 LL | #![deny(temporary_cstring_as_ptr)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` is deallocated because nothing is referencing it as far as the type system is concerned
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement...
+   = 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