Auto merge of #98914 - fee1-dead-contrib:min-deref-patterns, r=compiler-errors

Minimal implementation of implicit deref patterns for Strings

cc `@compiler-errors` `@BoxyUwU` rust-lang/lang-team#88 #87121

~~I forgot to add a feature gate, will do so in a minute~~ Done

Author: bors

compiler/rustc_feature/src/active.rs

@@ -506,6 +506,8 @@ declare_features! (
     (active, stmt_expr_attributes, "1.6.0", Some(15701), None),
     /// Allows lints part of the strict provenance effort.
     (active, strict_provenance, "1.61.0", Some(95228), None),
+    /// Allows string patterns to dereference values to match them.
+    (active, string_deref_patterns, "CURRENT_RUSTC_VERSION", Some(87121), None),
     /// Allows the use of `#[target_feature]` on safe functions.
     (active, target_feature_11, "1.45.0", Some(69098), None),
     /// Allows using `#[thread_local]` on `static` items.

compiler/rustc_hir/src/lang_items.rs

@@ -312,6 +312,8 @@ language_item_table! {
     Range,                   sym::Range,               range_struct,               Target::Struct,         GenericRequirement::None;
     RangeToInclusive,        sym::RangeToInclusive,    range_to_inclusive_struct,  Target::Struct,         GenericRequirement::None;
     RangeTo,                 sym::RangeTo,             range_to_struct,            Target::Struct,         GenericRequirement::None;
+
+    String,                  sym::String,              string,                     Target::Struct,         GenericRequirement::None;
 }
 
 pub enum GenericRequirement {

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -464,7 +464,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     ref_cnt += 1;
                 }
                 if let ty::Adt(adt, _) = peeled.kind()
-                    && self.tcx.is_diagnostic_item(sym::String, adt.did())
+                    && Some(adt.did()) == self.tcx.lang_items().string()
                 {
                     err.span_suggestion_verbose(
                         expr.span.shrink_to_hi(),

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -821,10 +821,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 ty.is_str()
                                     || matches!(
                                         ty.kind(),
-                                        ty::Adt(adt, _) if self.tcx.is_diagnostic_item(sym::String, adt.did())
+                                        ty::Adt(adt, _) if Some(adt.did()) == self.tcx.lang_items().string()
                                     )
                             }
-                            ty::Adt(adt, _) => self.tcx.is_diagnostic_item(sym::String, adt.did()),
+                            ty::Adt(adt, _) => Some(adt.did()) == self.tcx.lang_items().string(),
                             _ => false,
                         };
                         if is_string_or_ref_str && item_name.name == sym::iter {

compiler/rustc_hir_typeck/src/op.rs

@@ -556,9 +556,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let rm_borrow_msg = "remove the borrow to obtain an owned `String`";
         let to_owned_msg = "create an owned `String` from a string reference";
 
+        let string_type = self.tcx.lang_items().string();
         let is_std_string = |ty: Ty<'tcx>| {
-            ty.ty_adt_def()
-                .map_or(false, |ty_def| self.tcx.is_diagnostic_item(sym::String, ty_def.did()))
+            ty.ty_adt_def().map_or(false, |ty_def| Some(ty_def.did()) == string_type)
         };
 
         match (lhs_ty.kind(), rhs_ty.kind()) {

compiler/rustc_hir_typeck/src/pat.rs

@@ -401,6 +401,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
+        if self.tcx.features().string_deref_patterns && let hir::ExprKind::Lit(Spanned { node: ast::LitKind::Str(..), .. }) = lt.kind {
+            let tcx = self.tcx;
+            let expected = self.resolve_vars_if_possible(expected);
+            pat_ty = match expected.kind() {
+                ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => expected,
+                ty::Str => tcx.mk_static_str(),
+                _ => pat_ty,
+            };
+        }
+
         // Somewhat surprising: in this case, the subtyping relation goes the
         // opposite way as the other cases. Actually what we really want is not
         // a subtyping relation at all but rather that there exists a LUB

compiler/rustc_lint/src/non_fmt_panic.rs

@@ -148,7 +148,7 @@ fn check_panic<'tcx>(cx: &LateContext<'tcx>, f: &'tcx hir::Expr<'tcx>, arg: &'tc
                 ty::Ref(_, r, _) if *r.kind() == ty::Str,
             ) || matches!(
                 ty.ty_adt_def(),
-                Some(ty_def) if cx.tcx.is_diagnostic_item(sym::String, ty_def.did()),
+                Some(ty_def) if Some(ty_def.did()) == cx.tcx.lang_items().string(),
             );
 
             let infcx = cx.tcx.infer_ctxt().build();

compiler/rustc_mir_build/src/build/matches/test.rs

@@ -240,6 +240,39 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
 
             TestKind::Eq { value, ty } => {
+                let tcx = self.tcx;
+                if let ty::Adt(def, _) = ty.kind() && Some(def.did()) == tcx.lang_items().string() {
+                    if !tcx.features().string_deref_patterns {
+                        bug!("matching on `String` went through without enabling string_deref_patterns");
+                    }
+                    let re_erased = tcx.lifetimes.re_erased;
+                    let ref_string = self.temp(tcx.mk_imm_ref(re_erased, ty), test.span);
+                    let ref_str_ty = tcx.mk_imm_ref(re_erased, tcx.types.str_);
+                    let ref_str = self.temp(ref_str_ty, test.span);
+                    let deref = tcx.require_lang_item(LangItem::Deref, None);
+                    let method = trait_method(tcx, deref, sym::deref, ty, &[]);
+                    let eq_block = self.cfg.start_new_block();
+                    self.cfg.push_assign(block, source_info, ref_string, Rvalue::Ref(re_erased, BorrowKind::Shared, place));
+                    self.cfg.terminate(
+                        block,
+                        source_info,
+                        TerminatorKind::Call {
+                            func: Operand::Constant(Box::new(Constant {
+                                span: test.span,
+                                user_ty: None,
+                                literal: method,
+                            })),
+                            args: vec![Operand::Move(ref_string)],
+                            destination: ref_str,
+                            target: Some(eq_block),
+                            cleanup: None,
+                            from_hir_call: false,
+                            fn_span: source_info.span
+                        }
+                    );
+                    self.non_scalar_compare(eq_block, make_target_blocks, source_info, value, ref_str, ref_str_ty);
+                    return;
+                }
                 if !ty.is_scalar() {
                     // Use `PartialEq::eq` instead of `BinOp::Eq`
                     // (the binop can only handle primitives)

compiler/rustc_span/src/symbol.rs

@@ -1407,6 +1407,7 @@ symbols! {
         str_trim_end,
         str_trim_start,
         strict_provenance,
+        string_deref_patterns,
         stringify,
         struct_field_attributes,
         struct_inherit,

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1729,7 +1729,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 ty::Bool => Some(0),
                 ty::Char => Some(1),
                 ty::Str => Some(2),
-                ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
+                ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => Some(2),
                 ty::Int(..)
                 | ty::Uint(..)
                 | ty::Float(..)