Rollup merge of rust-lang#156087 - P8L1:improve-pin-pattern-suggestions, r=Kivooeo

Improve `&pin` reference-pattern suggestions

This fills in the `pin_ergonomics` FIXME in `borrow_pat_suggestion` by making the diagnostic prefix account for both `Pinnedness` and `Mutability`.

Previously, the type-position suggestion path used ordinary reference spelling, which can spell `&` and `&mut` but cannot correctly spell pinned reference-pattern suggestions such as `&pin const` and `&pin mut`.

This is a diagnostic-only change. It adds focused UI coverage for both pinned const and pinned mut reference-pattern suggestions.

Author: GuillaumeGomez

compiler/rustc_hir_typeck/src/pat.rs

@@ -1352,7 +1352,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     /// Precondition: pat is a `Ref(_)` pattern
-    // FIXME(pin_ergonomics): add suggestions for `&pin mut` or `&pin const` patterns
     fn borrow_pat_suggestion(&self, err: &mut Diag<'_>, pat: &Pat<'_>) {
         let tcx = self.tcx;
         if let PatKind::Ref(inner, pinned, mutbl) = pat.kind
@@ -1407,14 +1406,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             };
 
             match binding_parent {
+                hir::Node::Param(hir::Param { ty_span, pat, .. })
+                    if pat.span != *ty_span
+                        && pinned.is_pinned()
+                        && !tcx.features().pin_ergonomics() =>
+                {
+                    // FIXME(pin_ergonomics): Once `pin_ergonomics` is stabilized, remove this
+                    // gate and allow the pinned reference type-position suggestion unconditionally.
+                }
                 // Check that there is explicit type (ie this is not a closure param with inferred type)
                 // so we don't suggest moving something to the type that does not exist
                 hir::Node::Param(hir::Param { ty_span, pat, .. }) if pat.span != *ty_span => {
                     err.multipart_suggestion(
                         format!("to take parameter `{binding}` by reference, move `&{pin_and_mut}` to the type"),
                         vec![
                             (pat.span.until(inner.span), "".to_owned()),
-                            (ty_span.shrink_to_lo(), mutbl.ref_prefix_str().to_owned()),
+                            (ty_span.shrink_to_lo(), format!("&{}", pinned.prefix_str(mutbl))),
                         ],
                         Applicability::MachineApplicable
                     );

tests/ui/pin-ergonomics/ref-pat-suggestions.fixed

@@ -0,0 +1,12 @@
+//@ run-rustfix
+
+#![feature(pin_ergonomics)]
+#![allow(unused)]
+
+fn pin_mut_param(x: &pin mut i32) {}
+//~^ ERROR mismatched types
+
+fn pin_const_param(x: &pin const i32) {}
+//~^ ERROR mismatched types
+
+fn main() {}

tests/ui/pin-ergonomics/ref-pat-suggestions.rs

@@ -0,0 +1,12 @@
+//@ run-rustfix
+
+#![feature(pin_ergonomics)]
+#![allow(unused)]
+
+fn pin_mut_param(&pin mut x: i32) {}
+//~^ ERROR mismatched types
+
+fn pin_const_param(&pin const x: i32) {}
+//~^ ERROR mismatched types
+
+fn main() {}

tests/ui/pin-ergonomics/ref-pat-suggestions.stderr

@@ -0,0 +1,40 @@
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:6:18
+   |
+LL | fn pin_mut_param(&pin mut x: i32) {}
+   |                  ^^^^^^^^^^  --- expected due to this
+   |                  |
+   |                  expected `i32`, found `Pin<&mut _>`
+   |
+   = note: expected type `i32`
+            found struct `Pin<&mut _>`
+note: to declare a mutable parameter use: `mut x`
+  --> $DIR/ref-pat-suggestions.rs:6:18
+   |
+LL | fn pin_mut_param(&pin mut x: i32) {}
+   |                  ^^^^^^^^^^
+help: to take parameter `x` by reference, move `&pin mut` to the type
+   |
+LL - fn pin_mut_param(&pin mut x: i32) {}
+LL + fn pin_mut_param(x: &pin mut i32) {}
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/ref-pat-suggestions.rs:9:20
+   |
+LL | fn pin_const_param(&pin const x: i32) {}
+   |                    ^^^^^^^^^^^^  --- expected due to this
+   |                    |
+   |                    expected `i32`, found `Pin<&_>`
+   |
+   = note: expected type `i32`
+            found struct `Pin<&_>`
+help: to take parameter `x` by reference, move `&pin const` to the type
+   |
+LL - fn pin_const_param(&pin const x: i32) {}
+LL + fn pin_const_param(x: &pin const i32) {}
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.