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Require DerefMut if deref pattern has nested ref mut binding

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Original file line number	Diff line number	Diff line change
`@@ -2017,6 +2017,30 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {`
`2017`	`2017`	`let ty = self.normalize(span, ty);`
`2018`	`2018`	`let ty = self.try_structurally_resolve_type(span, ty);`
`2019`	`2019`	`self.check_pat(inner, ty, pat_info);`
	`2020`	`+`
	`2021`	+ // Check if the pattern has any `ref mut` bindings, which would require
	`2022`	+ // `DerefMut` to be emitted in MIR building instead of just `Deref`.
	`2023`	`+ let mut needs_mut = false;`
	`2024`	`+ inner.walk(\|pat\| {`
	`2025`	`+ if let hir::PatKind::Binding(_, id, _, _) = pat.kind`
	`2026`	`+ && let Some(ty::BindByReference(ty::Mutability::Mut)) =`
	`2027`	`+ self.typeck_results.borrow().pat_binding_modes().get(id)`
	`2028`	`+ {`
	`2029`	`+ needs_mut = true;`
	`2030`	`+ // No need to continue recursing`
	`2031`	`+ false`
	`2032`	`+ } else {`
	`2033`	`+ true`
	`2034`	`+ }`
	`2035`	`+ });`
	`2036`	`+ if needs_mut {`
	`2037`	`+ self.register_bound(`
	`2038`	`+ expected,`
	`2039`	`+ tcx.require_lang_item(hir::LangItem::DerefMut, Some(span)),`
	`2040`	`+ self.misc(span),`
	`2041`	`+ );`
	`2042`	`+ }`
	`2043`	`+`
`2020`	`2044`	`expected`
`2021`	`2045`	`}`
`2022`	`2046`

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Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,17 @@`
	`1`	`+#![feature(deref_patterns)]`
	`2`	+//~^ WARN the feature `deref_patterns` is incomplete
	`3`	`+`
	`4`	`+use std::rc::Rc;`
	`5`	`+`
	`6`	`+fn main() {`
	`7`	`+ match &mut vec![1] {`
	`8`	`+ deref!(x) => {}`
	`9`	`+ _ => {}`
	`10`	`+ }`
	`11`	`+`
	`12`	`+ match &mut Rc::new(1) {`
	`13`	`+ deref!(x) => {}`
	`14`	+ //~^ ERROR the trait bound `Rc<{integer}>: DerefMut` is not satisfied
	`15`	`+ _ => {}`
	`16`	`+ }`
	`17`	`+}`

Lines changed: 20 additions & 0 deletions

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,20 @@`
	`1`	+warning: the feature `deref_patterns` is incomplete and may not be safe to use and/or cause compiler crashes
	`2`	`+ --> $DIR/ref-mut.rs:1:12`
	`3`	`+ \|`
	`4`	`+LL \| #![feature(deref_patterns)]`
	`5`	`+ \| ^^^^^^^^^^^^^^`
	`6`	`+ \|`
	`7`	`+ = note: see issue #87121 <https://github.com/rust-lang/rust/issues/87121> for more information`
	`8`	+ = note: `#[warn(incomplete_features)]` on by default
	`9`	`+`
	`10`	+error[E0277]: the trait bound `Rc<{integer}>: DerefMut` is not satisfied
	`11`	`+ --> $DIR/ref-mut.rs:13:9`
	`12`	`+ \|`
	`13`	`+LL \| deref!(x) => {}`
	`14`	+ \| ^^^^^^^^^ the trait `DerefMut` is not implemented for `Rc<{integer}>`
	`15`	`+ \|`
	`16`	+ = note: this error originates in the macro `deref` (in Nightly builds, run with -Z macro-backtrace for more info)
	`17`	`+`
	`18`	`+error: aborting due to 1 previous error; 1 warning emitted`
	`19`	`+`
	`20`	+For more information about this error, try `rustc --explain E0277`.

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