rework ptr-to-ref conversion suggestion for method calls

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -529,16 +529,44 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Applicability::MachineApplicable,
             );
         }
-        if let ty::RawPtr(_, _) = &rcvr_ty.kind() {
-            err.note(
-                "try using `<*const T>::as_ref()` to get a reference to the \
-                 type behind the pointer: https://doc.rust-lang.org/std/\
-                 primitive.pointer.html#method.as_ref",
-            );
-            err.note(
-                "using `<*const T>::as_ref()` on a pointer which is unaligned or points \
-                 to invalid or uninitialized memory is undefined behavior",
+
+        // on pointers, check if the method would exist on a reference
+        if let SelfSource::MethodCall(rcvr_expr) = source
+            && let ty::RawPtr(ty, ptr_mutbl) = *rcvr_ty.kind()
+            && let Ok(pick) = self.lookup_probe_for_diagnostic(
+                item_name,
+                Ty::new_ref(tcx, ty::Region::new_error_misc(tcx), ty, ptr_mutbl),
+                self.tcx.hir().expect_expr(self.tcx.parent_hir_id(rcvr_expr.hir_id)),
+                ProbeScope::TraitsInScope,
+                None,
+            )
+            && let ty::Ref(_, _, sugg_mutbl) = *pick.self_ty.kind()
+            && (sugg_mutbl.is_not() || ptr_mutbl.is_mut())
+        {
+            let (method, method_anchor) = match sugg_mutbl {
+                Mutability::Not => {
+                    let method_anchor = match ptr_mutbl {
+                        Mutability::Not => "as_ref",
+                        Mutability::Mut => "as_ref-1",
+                    };
+                    ("as_ref", method_anchor)
+                }
+                Mutability::Mut => ("as_mut", "as_mut"),
+            };
+            err.span_note(
+                tcx.def_span(pick.item.def_id),
+                format!("the method `{item_name}` exists on the type `{ty}`", ty = pick.self_ty),
             );
+            let mut_str = ptr_mutbl.ptr_str();
+            err.note(format!(
+                "you might want to use the unsafe method `<*{mut_str} T>::{method}` to get \
+                an optional reference to the value behind the pointer"
+            ));
+            err.note(format!(
+                "read the documentation for `<*{mut_str} T>::{method}` and ensure you satisfy its \
+                safety preconditions before calling it to avoid undefined behavior: \
+                https://doc.rust-lang.org/std/primitive.pointer.html#method.{method_anchor}"
+            ));
         }
 
         let mut ty_span = match rcvr_ty.kind() {

tests/ui/methods/suggest-convert-ptr-to-ref.rs

@@ -1,5 +1,17 @@
 fn main() {
-    let x = 8u8;
+    let mut x = 8u8;
     let z: *const u8 = &x;
-    println!("{}", z.to_string());  //~ ERROR E0599
+    // issue #21596
+    println!("{}", z.to_string()); //~ ERROR E0599
+
+    let t: *mut u8 = &mut x;
+    println!("{}", t.to_string()); //~ ERROR E0599
+    t.make_ascii_lowercase(); //~ ERROR E0599
+
+    // suggest `as_mut` simply because the name is similar
+    let _ = t.as_mut_ref(); //~ ERROR E0599
+    let _ = t.as_ref_mut(); //~ ERROR E0599
+
+    // no ptr-to-ref suggestion
+    z.make_ascii_lowercase(); //~ ERROR E0599
 }

tests/ui/methods/suggest-convert-ptr-to-ref.stderr

@@ -1,15 +1,70 @@
 error[E0599]: `*const u8` doesn't implement `std::fmt::Display`
-  --> $DIR/issue-21596.rs:4:22
+  --> $DIR/suggest-convert-ptr-to-ref.rs:5:22
    |
 LL |     println!("{}", z.to_string());
    |                      ^^^^^^^^^ `*const u8` cannot be formatted with the default formatter
    |
-   = note: try using `<*const T>::as_ref()` to get a reference to the type behind the pointer: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref
-   = note: using `<*const T>::as_ref()` on a pointer which is unaligned or points to invalid or uninitialized memory is undefined behavior
+note: the method `to_string` exists on the type `&u8`
+  --> $SRC_DIR/alloc/src/string.rs:LL:COL
+   = note: you might want to use the unsafe method `<*const T>::as_ref` to get an optional reference to the value behind the pointer
+   = note: read the documentation for `<*const T>::as_ref` and ensure you satisfy its safety preconditions before calling it to avoid undefined behavior: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref
    = note: the following trait bounds were not satisfied:
            `*const u8: std::fmt::Display`
            which is required by `*const u8: ToString`
 
-error: aborting due to 1 previous error
+error[E0599]: `*mut u8` doesn't implement `std::fmt::Display`
+  --> $DIR/suggest-convert-ptr-to-ref.rs:8:22
+   |
+LL |     println!("{}", t.to_string());
+   |                      ^^^^^^^^^ `*mut u8` cannot be formatted with the default formatter
+   |
+note: the method `to_string` exists on the type `&&mut u8`
+  --> $SRC_DIR/alloc/src/string.rs:LL:COL
+   = note: you might want to use the unsafe method `<*mut T>::as_ref` to get an optional reference to the value behind the pointer
+   = note: read the documentation for `<*mut T>::as_ref` and ensure you satisfy its safety preconditions before calling it to avoid undefined behavior: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref-1
+   = note: the following trait bounds were not satisfied:
+           `*mut u8: std::fmt::Display`
+           which is required by `*mut u8: ToString`
+
+error[E0599]: no method named `make_ascii_lowercase` found for raw pointer `*mut u8` in the current scope
+  --> $DIR/suggest-convert-ptr-to-ref.rs:9:7
+   |
+LL |     t.make_ascii_lowercase();
+   |       ^^^^^^^^^^^^^^^^^^^^ method not found in `*mut u8`
+   |
+note: the method `make_ascii_lowercase` exists on the type `&mut u8`
+  --> $SRC_DIR/core/src/num/mod.rs:LL:COL
+   = note: you might want to use the unsafe method `<*mut T>::as_mut` to get an optional reference to the value behind the pointer
+   = note: read the documentation for `<*mut T>::as_mut` and ensure you satisfy its safety preconditions before calling it to avoid undefined behavior: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_mut
+
+error[E0599]: no method named `as_mut_ref` found for raw pointer `*mut u8` in the current scope
+  --> $DIR/suggest-convert-ptr-to-ref.rs:12:15
+   |
+LL |     let _ = t.as_mut_ref();
+   |               ^^^^^^^^^^
+   |
+help: there is a method `as_mut` with a similar name
+   |
+LL |     let _ = t.as_mut();
+   |               ~~~~~~
+
+error[E0599]: no method named `as_ref_mut` found for raw pointer `*mut u8` in the current scope
+  --> $DIR/suggest-convert-ptr-to-ref.rs:13:15
+   |
+LL |     let _ = t.as_ref_mut();
+   |               ^^^^^^^^^^
+   |
+help: there is a method `as_mut` with a similar name
+   |
+LL |     let _ = t.as_mut();
+   |               ~~~~~~
+
+error[E0599]: no method named `make_ascii_lowercase` found for raw pointer `*const u8` in the current scope
+  --> $DIR/suggest-convert-ptr-to-ref.rs:16:7
+   |
+LL |     z.make_ascii_lowercase();
+   |       ^^^^^^^^^^^^^^^^^^^^ method not found in `*const u8`
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0599`.