Auto merge of #120131 - oli-obk:pattern_types_syntax, r=compiler-errors

Implement minimal, internal-only pattern types in the type system

rebase of https://github.com/rust-lang/rust/pull/107606

You can create pattern types with `std::pat::pattern_type!(ty is pat)`. The feature is incomplete and will panic on you if you use any pattern other than integral range patterns. The only way to create or deconstruct a pattern type is via `transmute`.

This PR's implementation differs from the MCP's text. Specifically

> This means you could implement different traits for different pattern types with the same base type. Thus, we just forbid implementing any traits for pattern types.

is violated in this PR. The reason is that we do need impls after all in order to make them usable as fields. constants of type `std::time::Nanoseconds` struct are used in patterns, so the type must be structural-eq, which it only can be if you derive several traits on it. It doesn't need to be structural-eq recursively, so we can just manually implement the relevant traits on the pattern type and use the pattern type as a private field.

Waiting on:

* [x] move all unrelated commits into their own PRs.
* [x] fix niche computation (see 2db07f94f44f078daffe5823680d07d4fded883f)
* [x] add lots more tests
* [x] T-types MCP https://github.com/rust-lang/types-team/issues/126 to finish
* [x] some commit cleanup
* [x] full self-review
* [x] remove 61bd325da19a918cc3e02bbbdce97281a389c648, it's not necessary anymore I think.
* [ ] ~~make sure we never accidentally leak pattern types to user code (add stability checks or feature gate checks and appopriate tests)~~ we don't even do this for the new float primitives
* [x] get approval that [the scope expansion to trait impls](https://rust-lang.zulipchat.com/#narrow/stream/326866-t-types.2Fnominated/topic/Pattern.20types.20types-team.23126/near/427670099) is ok

r? `@BoxyUwU`

6 files changed, 104 insertions, 2 deletions

diff --git a/compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs b/compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs
index 067878091a7..4a85e9983f4 100644
--- a/compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs
@@ -144,7 +144,12 @@ impl<'tcx> InherentCollect<'tcx> {
         let id = id.owner_id.def_id;
         let item_span = self.tcx.def_span(id);
         let self_ty = self.tcx.type_of(id).instantiate_identity();
-        let self_ty = self.tcx.peel_off_weak_alias_tys(self_ty);
+        let mut self_ty = self.tcx.peel_off_weak_alias_tys(self_ty);
+        // We allow impls on pattern types exactly when we allow impls on the base type.
+        // FIXME(pattern_types): Figure out the exact coherence rules we want here.
+        while let ty::Pat(base, _) = *self_ty.kind() {
+            self_ty = base;
+        }
         match *self_ty.kind() {
             ty::Adt(def, _) => self.check_def_id(id, self_ty, def.did()),
             ty::Foreign(did) => self.check_def_id(id, self_ty, did),
@@ -154,6 +159,7 @@ impl<'tcx> InherentCollect<'tcx> {
             ty::Dynamic(..) => {
                 Err(self.tcx.dcx().emit_err(errors::InherentDyn { span: item_span }))
             }
+            ty::Pat(_, _) => unreachable!(),
             ty::Bool
             | ty::Char
             | ty::Int(_)
diff --git a/compiler/rustc_hir_analysis/src/coherence/orphan.rs b/compiler/rustc_hir_analysis/src/coherence/orphan.rs
index 1770f7b4e91..5585d2e069c 100644
--- a/compiler/rustc_hir_analysis/src/coherence/orphan.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/orphan.rs
@@ -206,6 +206,11 @@ pub(crate) fn orphan_check_impl(
                 (LocalImpl::Disallow { problematic_kind }, NonlocalImpl::DisallowOther)
             }
 
+            ty::Pat(..) => (
+                LocalImpl::Disallow { problematic_kind: "pattern type" },
+                NonlocalImpl::DisallowOther,
+            ),
+
             ty::Bool
             | ty::Char
             | ty::Int(..)
diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index 2d4742fa1dc..d129614e0e1 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -7,6 +7,8 @@ use rustc_errors::{
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::Ty;
 use rustc_span::{symbol::Ident, Span, Symbol};
+mod pattern_types;
+pub use pattern_types::*;
 
 #[derive(Diagnostic)]
 #[diag(hir_analysis_ambiguous_assoc_item)]
@@ -1629,3 +1631,10 @@ pub struct OpaqueCapturesHigherRankedLifetime {
     pub decl_span: Span,
     pub bad_place: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_pattern_type_non_const_range)]
+pub struct NonConstRange {
+    #[primary_span]
+    pub span: Span,
+}
diff --git a/compiler/rustc_hir_analysis/src/errors/pattern_types.rs b/compiler/rustc_hir_analysis/src/errors/pattern_types.rs
new file mode 100644
index 00000000000..008d2698989
--- /dev/null
+++ b/compiler/rustc_hir_analysis/src/errors/pattern_types.rs
@@ -0,0 +1,9 @@
+use rustc_macros::Diagnostic;
+use rustc_span::Span;
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_pattern_type_wild_pat)]
+pub struct WildPatTy {
+    #[primary_span]
+    pub span: Span,
+}
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index f726f2a7b89..ebfccd27d17 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -21,7 +21,7 @@ mod object_safety;
 
 use crate::bounds::Bounds;
 use crate::collect::HirPlaceholderCollector;
-use crate::errors::AmbiguousLifetimeBound;
+use crate::errors::{AmbiguousLifetimeBound, WildPatTy};
 use crate::hir_ty_lowering::errors::{prohibit_assoc_item_binding, GenericsArgsErrExtend};
 use crate::hir_ty_lowering::generics::{check_generic_arg_count, lower_generic_args};
 use crate::middle::resolve_bound_vars as rbv;
@@ -39,6 +39,7 @@ use rustc_hir::{GenericArg, GenericArgs};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::ObligationCause;
 use rustc_middle::middle::stability::AllowUnstable;
+use rustc_middle::mir::interpret::{LitToConstError, LitToConstInput};
 use rustc_middle::ty::{
     self, Const, GenericArgKind, GenericArgsRef, GenericParamDefKind, ParamEnv, Ty, TyCtxt,
     TypeVisitableExt,
@@ -2195,6 +2196,64 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 // handled specially and will not descend into this routine.
                 self.ty_infer(None, hir_ty.span)
             }
+            hir::TyKind::Pat(ty, pat) => {
+                let ty = self.lower_ty(ty);
+                let pat_ty = match pat.kind {
+                    hir::PatKind::Wild => {
+                        let err = tcx.dcx().emit_err(WildPatTy { span: pat.span });
+                        Ty::new_error(tcx, err)
+                    }
+                    hir::PatKind::Range(start, end, include_end) => {
+                        let expr_to_const = |expr: &'tcx hir::Expr<'tcx>| -> ty::Const<'tcx> {
+                            let (expr, neg) = match expr.kind {
+                                hir::ExprKind::Unary(hir::UnOp::Neg, negated) => {
+                                    (negated, Some((expr.hir_id, expr.span)))
+                                }
+                                _ => (expr, None),
+                            };
+                            let c = match &expr.kind {
+                                hir::ExprKind::Lit(lit) => {
+                                    let lit_input =
+                                        LitToConstInput { lit: &lit.node, ty, neg: neg.is_some() };
+                                    match tcx.lit_to_const(lit_input) {
+                                        Ok(c) => c,
+                                        Err(LitToConstError::Reported(err)) => {
+                                            ty::Const::new_error(tcx, err, ty)
+                                        }
+                                        Err(LitToConstError::TypeError) => todo!(),
+                                    }
+                                }
+                                _ => {
+                                    let err = tcx
+                                        .dcx()
+                                        .emit_err(crate::errors::NonConstRange { span: expr.span });
+                                    ty::Const::new_error(tcx, err, ty)
+                                }
+                            };
+                            self.record_ty(expr.hir_id, c.ty(), expr.span);
+                            if let Some((id, span)) = neg {
+                                self.record_ty(id, c.ty(), span);
+                            }
+                            c
+                        };
+
+                        let start = start.map(expr_to_const);
+                        let end = end.map(expr_to_const);
+
+                        let include_end = match include_end {
+                            hir::RangeEnd::Included => true,
+                            hir::RangeEnd::Excluded => false,
+                        };
+
+                        let pat = tcx.mk_pat(ty::PatternKind::Range { start, end, include_end });
+                        Ty::new_pat(tcx, ty, pat)
+                    }
+                    hir::PatKind::Err(e) => Ty::new_error(tcx, e),
+                    _ => span_bug!(pat.span, "unsupported pattern for pattern type: {pat:#?}"),
+                };
+                self.record_ty(pat.hir_id, ty, pat.span);
+                pat_ty
+            }
             hir::TyKind::Err(guar) => Ty::new_error(tcx, *guar),
         };
 
diff --git a/compiler/rustc_hir_analysis/src/variance/constraints.rs b/compiler/rustc_hir_analysis/src/variance/constraints.rs
index 28c86d8019e..20e4110e137 100644
--- a/compiler/rustc_hir_analysis/src/variance/constraints.rs
+++ b/compiler/rustc_hir_analysis/src/variance/constraints.rs
@@ -249,6 +249,20 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
                 self.add_constraints_from_ty(current, typ, variance);
             }
 
+            ty::Pat(typ, pat) => {
+                match *pat {
+                    ty::PatternKind::Range { start, end, include_end: _ } => {
+                        if let Some(start) = start {
+                            self.add_constraints_from_const(current, start, variance);
+                        }
+                        if let Some(end) = end {
+                            self.add_constraints_from_const(current, end, variance);
+                        }
+                    }
+                }
+                self.add_constraints_from_ty(current, typ, variance);
+            }
+
             ty::Slice(typ) => {
                 self.add_constraints_from_ty(current, typ, variance);
             }