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

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

rebase of #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 2db07f9)
* [x] add lots more tests
* [x] T-types MCP rust-lang/types-team#126 to finish
* [x] some commit cleanup
* [x] full self-review
* [x] remove 61bd325, 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`

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -2152,6 +2152,9 @@ pub enum TyKind {
     MacCall(P<MacCall>),
     /// Placeholder for a `va_list`.
     CVarArgs,
+    /// Pattern types like `pattern_type!(u32 is 1..=)`, which is the same as `NonZeroU32`,
+    /// just as part of the type system.
+    Pat(P<Ty>, P<Pat>),
     /// Sometimes we need a dummy value when no error has occurred.
     Dummy,
     /// Placeholder for a kind that has failed to be defined.

compiler/rustc_ast/src/mut_visit.rs

@@ -502,6 +502,10 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
         }
         TyKind::Tup(tys) => visit_thin_vec(tys, |ty| vis.visit_ty(ty)),
         TyKind::Paren(ty) => vis.visit_ty(ty),
+        TyKind::Pat(ty, pat) => {
+            vis.visit_ty(ty);
+            vis.visit_pat(pat);
+        }
         TyKind::Path(qself, path) => {
             vis.visit_qself(qself);
             vis.visit_path(path);

compiler/rustc_ast/src/visit.rs

@@ -446,6 +446,10 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) -> V::Result {
             }
             try_visit!(visitor.visit_path(path, typ.id));
         }
+        TyKind::Pat(ty, pat) => {
+            try_visit!(visitor.visit_ty(ty));
+            try_visit!(visitor.visit_pat(pat));
+        }
         TyKind::Array(ty, length) => {
             try_visit!(visitor.visit_ty(ty));
             try_visit!(visitor.visit_anon_const(length));

compiler/rustc_ast_lowering/src/index.rs

@@ -381,4 +381,8 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
             ArrayLen::Body(..) => intravisit::walk_array_len(self, len),
         }
     }
+
+    fn visit_pattern_type_pattern(&mut self, p: &'hir hir::Pat<'hir>) {
+        self.visit_pat(p)
+    }
 }

compiler/rustc_ast_lowering/src/lib.rs

@@ -1463,7 +1463,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     }
                 }
             }
-            TyKind::MacCall(_) => panic!("`TyKind::MacCall` should have been expanded by now"),
+            TyKind::Pat(ty, pat) => hir::TyKind::Pat(self.lower_ty(ty, itctx), self.lower_pat(pat)),
+            TyKind::MacCall(_) => {
+                span_bug!(t.span, "`TyKind::MacCall` should have been expanded by now")
+            }
             TyKind::CVarArgs => {
                 let guar = self.dcx().span_delayed_bug(
                     t.span,

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -332,6 +332,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             ast::TyKind::Never => {
                 gate!(&self, never_type, ty.span, "the `!` type is experimental");
             }
+            ast::TyKind::Pat(..) => {
+                gate!(&self, pattern_types, ty.span, "pattern types are unstable");
+            }
             _ => {}
         }
         visit::walk_ty(self, ty)

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1188,6 +1188,11 @@ impl<'a> State<'a> {
             ast::TyKind::CVarArgs => {
                 self.word("...");
             }
+            ast::TyKind::Pat(ty, pat) => {
+                self.print_type(ty);
+                self.word(" is ");
+                self.print_pat(pat);
+            }
         }
         self.end();
     }

compiler/rustc_borrowck/src/lib.rs

@@ -1606,6 +1606,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     | ty::Foreign(_)
                     | ty::Str
                     | ty::Array(_, _)
+                    | ty::Pat(_, _)
                     | ty::Slice(_)
                     | ty::FnDef(_, _)
                     | ty::FnPtr(_)
@@ -1648,6 +1649,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     | ty::Foreign(_)
                     | ty::Str
                     | ty::Array(_, _)
+                    | ty::Pat(_, _)
                     | ty::Slice(_)
                     | ty::RawPtr(_, _)
                     | ty::Ref(_, _, _)

compiler/rustc_builtin_macros/src/lib.rs

@@ -46,6 +46,7 @@ mod format;
 mod format_foreign;
 mod global_allocator;
 mod log_syntax;
+mod pattern_type;
 mod source_util;
 mod test;
 mod trace_macros;
@@ -95,6 +96,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         log_syntax: log_syntax::expand_log_syntax,
         module_path: source_util::expand_mod,
         option_env: env::expand_option_env,
+        pattern_type: pattern_type::expand,
         std_panic: edition_panic::expand_panic,
         stringify: source_util::expand_stringify,
         trace_macros: trace_macros::expand_trace_macros,

compiler/rustc_builtin_macros/src/pattern_type.rs

@@ -0,0 +1,29 @@
+use rustc_ast::{ast, ptr::P, tokenstream::TokenStream, Pat, Ty};
+use rustc_errors::PResult;
+use rustc_expand::base::{self, DummyResult, ExpandResult, ExtCtxt, MacroExpanderResult};
+use rustc_span::{sym, Span};
+
+pub fn expand<'cx>(
+    cx: &'cx mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+) -> MacroExpanderResult<'cx> {
+    let (ty, pat) = match parse_pat_ty(cx, tts) {
+        Ok(parsed) => parsed,
+        Err(err) => {
+            return ExpandResult::Ready(DummyResult::any(sp, err.emit()));
+        }
+    };
+
+    ExpandResult::Ready(base::MacEager::ty(cx.ty(sp, ast::TyKind::Pat(ty, pat))))
+}
+
+fn parse_pat_ty<'a>(cx: &mut ExtCtxt<'a>, stream: TokenStream) -> PResult<'a, (P<Ty>, P<Pat>)> {
+    let mut parser = cx.new_parser_from_tts(stream);
+
+    let ty = parser.parse_ty()?;
+    parser.eat_keyword(sym::is);
+    let pat = parser.parse_pat_no_top_alt(None, None)?;
+
+    Ok((ty, pat))
+}