Rollup merge of #120221 - compiler-errors:statements-are-not-patterns, r=nnethercote

Don't make statement nonterminals match pattern nonterminals

Right now, the heuristic we use to check if a token may begin a pattern nonterminal falls back to `may_be_ident`:
https://github.com/rust-lang/rust/blob/ef71f1047e04438181d7cb925a833e2ada6ab390/compiler/rustc_parse/src/parser/nonterminal.rs#L21-L37

This has the unfortunate side effect that a `stmt` nonterminal eagerly matches against a `pat` nonterminal, leading to a parse error:
```rust
macro_rules! m {
    ($pat:pat) => {};
    ($stmt:stmt) => {};
}

macro_rules! m2 {
    ($stmt:stmt) => {
        m! { $stmt }
    };
}

m2! { let x = 1 }
```

This PR fixes it by more accurately reflecting the set of nonterminals that may begin a pattern nonterminal.

As a side-effect, I modified `Token::can_begin_pattern` to work correctly and used that in `Parser::nonterminal_may_begin_with`.

3 files changed, 10 insertions, 21 deletions

diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
index 999f6f0eeb0..e66d0df012b 100644
--- a/compiler/rustc_parse/src/parser/nonterminal.rs
+++ b/compiler/rustc_parse/src/parser/nonterminal.rs
@@ -86,25 +86,7 @@ impl<'a> Parser<'a> {
                 token::Interpolated(nt) => may_be_ident(nt),
                 _ => false,
             },
-            NonterminalKind::Pat(pat_kind) => match &token.kind {
-                // box, ref, mut, and other identifiers (can stricten)
-                token::Ident(..) | token::NtIdent(..) |
-                token::OpenDelim(Delimiter::Parenthesis) |  // tuple pattern
-                token::OpenDelim(Delimiter::Bracket) |      // slice pattern
-                token::BinOp(token::And) |                  // reference
-                token::BinOp(token::Minus) |                // negative literal
-                token::AndAnd |                             // double reference
-                token::Literal(_) |                         // literal
-                token::DotDot |                             // range pattern (future compat)
-                token::DotDotDot |                          // range pattern (future compat)
-                token::PathSep |                             // path
-                token::Lt |                                 // path (UFCS constant)
-                token::BinOp(token::Shl) => true,           // path (double UFCS)
-                // leading vert `|` or-pattern
-                token::BinOp(token::Or) => matches!(pat_kind, PatWithOr),
-                token::Interpolated(nt) => may_be_ident(nt),
-                _ => false,
-            },
+            NonterminalKind::Pat(pat_kind) => token.can_begin_pattern(pat_kind),
             NonterminalKind::Lifetime => match &token.kind {
                 token::Lifetime(_) | token::NtLifetime(..) => true,
                 _ => false,
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index cc68ae237ba..8233f9a7943 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -444,7 +444,11 @@ impl<'a> Parser<'a> {
 
         let mut lo = self.token.span;
 
-        if self.token.is_keyword(kw::Let) && self.look_ahead(1, |tok| tok.can_begin_pattern()) {
+        if self.token.is_keyword(kw::Let)
+            && self.look_ahead(1, |tok| {
+                tok.can_begin_pattern(token::NtPatKind::PatParam { inferred: false })
+            })
+        {
             self.bump();
             self.dcx().emit_err(RemoveLet { span: lo });
             lo = self.token.span;
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index d8bf10e6021..8ee40ecd77e 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -378,7 +378,10 @@ impl<'a> Parser<'a> {
                     if self.may_recover()
                         && prev_token_before_parsing == token::PathSep
                         && (style == PathStyle::Expr && self.token.can_begin_expr()
-                            || style == PathStyle::Pat && self.token.can_begin_pattern())
+                            || style == PathStyle::Pat
+                                && self.token.can_begin_pattern(token::NtPatKind::PatParam {
+                                    inferred: false,
+                                }))
                     {
                         snapshot = Some(self.create_snapshot_for_diagnostic());
                     }