Minimal implementation of implicit deref patterns for Strings

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

@@ -302,6 +302,8 @@ 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

@@ -817,10 +817,10 @@ 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

@@ -1405,6 +1405,7 @@ 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

@@ -1705,7 +1705,7 @@ 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(..)

library/alloc/src/string.rs

@@ -362,8 +362,8 @@ use crate::vec::Vec;
 /// [`Deref`]: core::ops::Deref "ops::Deref"
 /// [`as_str()`]: String::as_str
 #[derive(PartialOrd, Eq, Ord)]
-#[cfg_attr(not(test), rustc_diagnostic_item = "String")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(all(not(bootstrap), not(test)), lang = "String")]
 pub struct String {
     vec: Vec<u8>,
 }

src/test/mir-opt/deref-patterns/string.foo.PreCodegen.after.mir

@@ -0,0 +1,74 @@
+// MIR for `foo` after PreCodegen
+
+fn foo(_1: Option<String>) -> i32 {
+    debug s => _1;                       // in scope 0 at $DIR/string.rs:+0:12: +0:13
+    let mut _0: i32;                     // return place in scope 0 at $DIR/string.rs:+0:34: +0:37
+    let mut _2: &std::string::String;    // in scope 0 at $DIR/string.rs:+2:14: +2:17
+    let mut _3: &str;                    // in scope 0 at $DIR/string.rs:+2:14: +2:17
+    let mut _4: bool;                    // in scope 0 at $DIR/string.rs:+2:14: +2:17
+    let mut _5: isize;                   // in scope 0 at $DIR/string.rs:+2:9: +2:18
+    let _6: std::option::Option<std::string::String>; // in scope 0 at $DIR/string.rs:+3:9: +3:10
+    let mut _7: bool;                    // in scope 0 at $DIR/string.rs:+5:1: +5:2
+    scope 1 {
+        debug s => _6;                   // in scope 1 at $DIR/string.rs:+3:9: +3:10
+    }
+
+    bb0: {
+        _7 = const false;                // scope 0 at $DIR/string.rs:+1:11: +1:12
+        _7 = const true;                 // scope 0 at $DIR/string.rs:+1:11: +1:12
+        _5 = discriminant(_1);           // scope 0 at $DIR/string.rs:+1:11: +1:12
+        switchInt(move _5) -> [1_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/string.rs:+1:5: +1:12
+    }
+
+    bb1: {
+        StorageLive(_6);                 // scope 0 at $DIR/string.rs:+3:9: +3:10
+        _7 = const false;                // scope 0 at $DIR/string.rs:+3:9: +3:10
+        _6 = move _1;                    // scope 0 at $DIR/string.rs:+3:9: +3:10
+        _0 = const 4321_i32;             // scope 1 at $DIR/string.rs:+3:14: +3:18
+        drop(_6) -> bb6;                 // scope 0 at $DIR/string.rs:+3:17: +3:18
+    }
+
+    bb2: {
+        _2 = &((_1 as Some).0: std::string::String); // scope 0 at $DIR/string.rs:+2:14: +2:17
+        _3 = <String as Deref>::deref(move _2) -> bb3; // scope 0 at $DIR/string.rs:+2:14: +2:17
+                                         // mir::Constant
+                                         // + span: $DIR/string.rs:9:14: 9:17
+                                         // + literal: Const { ty: for<'a> fn(&'a String) -> &'a <String as Deref>::Target {<String as Deref>::deref}, val: Value(<ZST>) }
+    }
+
+    bb3: {
+        _4 = <str as PartialEq>::eq(_3, const "a") -> bb4; // scope 0 at $DIR/string.rs:+2:14: +2:17
+                                         // mir::Constant
+                                         // + span: $DIR/string.rs:9:14: 9:17
+                                         // + literal: Const { ty: for<'a, 'b> fn(&'a str, &'b str) -> bool {<str as PartialEq>::eq}, val: Value(<ZST>) }
+                                         // mir::Constant
+                                         // + span: $DIR/string.rs:9:14: 9:17
+                                         // + literal: Const { ty: &str, val: Value(Slice(..)) }
+    }
+
+    bb4: {
+        switchInt(move _4) -> [false: bb1, otherwise: bb5]; // scope 0 at $DIR/string.rs:+2:14: +2:17
+    }
+
+    bb5: {
+        _0 = const 1234_i32;             // scope 0 at $DIR/string.rs:+2:22: +2:26
+        goto -> bb9;                     // scope 0 at $DIR/string.rs:+2:22: +2:26
+    }
+
+    bb6: {
+        StorageDead(_6);                 // scope 0 at $DIR/string.rs:+3:17: +3:18
+        goto -> bb9;                     // scope 0 at $DIR/string.rs:+3:17: +3:18
+    }
+
+    bb7: {
+        return;                          // scope 0 at $DIR/string.rs:+5:2: +5:2
+    }
+
+    bb8: {
+        drop(_1) -> bb7;                 // scope 0 at $DIR/string.rs:+5:1: +5:2
+    }
+
+    bb9: {
+        switchInt(_7) -> [false: bb7, otherwise: bb8]; // scope 0 at $DIR/string.rs:+5:1: +5:2
+    }
+}

src/test/mir-opt/deref-patterns/string.rs

@@ -0,0 +1,12 @@
+// compile-flags: -Z mir-opt-level=0 -C panic=abort
+
+#![feature(string_deref_patterns)]
+#![crate_type = "lib"]
+
+// EMIT_MIR string.foo.PreCodegen.after.mir
+pub fn foo(s: Option<String>) -> i32 {
+    match s {
+        Some("a") => 1234,
+        s => 4321,
+    }
+}

src/test/ui/deref-patterns/basic.rs

@@ -0,0 +1,17 @@
+// run-pass
+// check-run-results
+#![feature(string_deref_patterns)]
+
+fn main() {
+    test(Some(String::from("42")));
+    test(Some(String::new()));
+    test(None);
+}
+
+fn test(o: Option<String>) {
+    match o {
+        Some("42") => println!("the answer"),
+        Some(_) => println!("something else?"),
+        None => println!("nil"),
+    }
+}

src/test/ui/deref-patterns/basic.run.stdout

@@ -0,0 +1,3 @@
+the answer
+something else?
+nil

src/test/ui/deref-patterns/default-infer.rs

@@ -0,0 +1,9 @@
+// check-pass
+#![feature(string_deref_patterns)]
+
+fn main() {
+    match <_ as Default>::default() {
+        "" => (),
+        _ => unreachable!(),
+    }
+}

src/test/ui/deref-patterns/gate.rs

@@ -0,0 +1,7 @@
+// gate-test-string_deref_patterns
+fn main() {
+    match String::new() {
+        "" | _ => {}
+        //~^ mismatched types
+    }
+}

src/test/ui/deref-patterns/gate.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/gate.rs:4:9
+   |
+LL |     match String::new() {
+   |           ------------- this expression has type `String`
+LL |         "" | _ => {}
+   |         ^^ expected struct `String`, found `&str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/deref-patterns/refs.rs

@@ -0,0 +1,18 @@
+// check-pass
+#![feature(string_deref_patterns)]
+
+fn foo(s: &String) -> i32 {
+    match *s {
+        "a" => 42,
+        _ => -1,
+    }
+}
+
+fn bar(s: Option<&&&&String>) -> i32 {
+    match s {
+        Some(&&&&"&&&&") => 1,
+        _ => -1,
+    }
+}
+
+fn main() {}

src/tools/clippy/clippy_lints/src/format.rs

@@ -73,7 +73,7 @@ impl<'tcx> LateLintPass<'tcx> for UselessFormat {
                 if format_args.format_string.parts == [kw::Empty];
                 if arg.format.is_default();
                 if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
-                    ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::String, adt.did()),
+                    ty::Adt(adt, _) => Some(adt.did()) == cx.tcx.lang_items().string(),
                     ty::Str => true,
                     _ => false,
                 };

src/tools/clippy/clippy_lints/src/format_push_string.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_help;
-use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::ty::is_type_lang_item;
 use clippy_utils::{match_def_path, paths, peel_hir_expr_refs};
-use rustc_hir::{BinOpKind, Expr, ExprKind};
+use rustc_hir::{BinOpKind, Expr, ExprKind, LangItem};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::sym;
@@ -41,7 +41,7 @@ declare_clippy_lint! {
 declare_lint_pass!(FormatPushString => [FORMAT_PUSH_STRING]);
 
 fn is_string(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
-    is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(e).peel_refs(), sym::String)
+    is_type_lang_item(cx, cx.typeck_results().expr_ty(e).peel_refs(), LangItem::String)
 }
 fn is_format(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
     if let Some(macro_def_id) = e.span.ctxt().outer_expn_data().macro_def_id {

src/tools/clippy/clippy_lints/src/from_str_radix_10.rs

@@ -1,10 +1,10 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::is_integer_literal;
 use clippy_utils::sugg::Sugg;
-use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
-use rustc_hir::{def, Expr, ExprKind, PrimTy, QPath, TyKind};
+use rustc_hir::{def, Expr, ExprKind, LangItem, PrimTy, QPath, TyKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::Ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -98,5 +98,5 @@ impl<'tcx> LateLintPass<'tcx> for FromStrRadix10 {
 
 /// Checks if a Ty is `String` or `&str`
 fn is_ty_stringish(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
-    is_type_diagnostic_item(cx, ty, sym::String) || is_type_diagnostic_item(cx, ty, sym::str)
+    is_type_lang_item(cx, ty, LangItem::String) || is_type_diagnostic_item(cx, ty, sym::str)
 }

src/tools/clippy/clippy_lints/src/inherent_to_string.rs

@@ -1,8 +1,8 @@
 use clippy_utils::diagnostics::span_lint_and_help;
-use clippy_utils::ty::{implements_trait, is_type_diagnostic_item};
+use clippy_utils::ty::{implements_trait, is_type_lang_item};
 use clippy_utils::{return_ty, trait_ref_of_method};
 use if_chain::if_chain;
-use rustc_hir::{GenericParamKind, ImplItem, ImplItemKind};
+use rustc_hir::{GenericParamKind, ImplItem, ImplItemKind, LangItem};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::sym;
@@ -105,7 +105,7 @@ impl<'tcx> LateLintPass<'tcx> for InherentToString {
             if impl_item.generics.params.iter().all(|p| matches!(p.kind, GenericParamKind::Lifetime { .. }));
 
             // Check if return type is String
-            if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id()), sym::String);
+            if is_type_lang_item(cx, return_ty(cx, impl_item.hir_id()), LangItem::String);
 
             // Filters instances of to_string which are required by a trait
             if trait_ref_of_method(cx, impl_item.owner_id.def_id).is_none();

src/tools/clippy/clippy_lints/src/manual_retain.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::snippet;
-use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::ty::{is_type_diagnostic_item, is_type_lang_item};
 use clippy_utils::{get_parent_expr, match_def_path, paths, SpanlessEq};
 use clippy_utils::{meets_msrv, msrvs};
 use rustc_errors::Applicability;
@@ -140,7 +140,7 @@ fn check_to_owned(
         && let Some(chars_expr_def_id) = cx.typeck_results().type_dependent_def_id(chars_expr.hir_id)
         && match_def_path(cx, chars_expr_def_id, &paths::STR_CHARS)
         && let ty = cx.typeck_results().expr_ty(str_expr).peel_refs()
-        && is_type_diagnostic_item(cx, ty, sym::String)
+        && is_type_lang_item(cx, ty, hir::LangItem::String)
         && SpanlessEq::new(cx).eq_expr(left_expr, str_expr) {
         suggest(cx, parent_expr, left_expr, filter_expr);
     }

src/tools/clippy/clippy_lints/src/manual_string_new.rs

@@ -44,7 +44,7 @@ impl LateLintPass<'_> for ManualStringNew {
         let ty = cx.typeck_results().expr_ty(expr);
         match ty.kind() {
             ty::Adt(adt_def, _) if adt_def.is_struct() => {
-                if !cx.tcx.is_diagnostic_item(sym::String, adt_def.did()) {
+                if cx.tcx.lang_items().string() != Some(adt_def.did()) {
                     return;
                 }
             },