Auto merge of #123877 - ShE3py:expr-in-pats-2, r=fmease

Further improve diagnostics for expressions in pattern position

Follow-up of #118625, see #121697.

```rs
fn main() {
    match 'b' {
        y.0.0.1.z().f()? as u32 => {},
    }
}
```
Before:
```
error: expected one of `=>`, ``@`,` `if`, or `|`, found `.`
 --> src/main.rs:3:10
  |
3 |         y.0.0.1.z().f()? as u32 => {},
  |          ^ expected one of `=>`, ``@`,` `if`, or `|`
```
After:
```
error: expected a pattern, found an expression
 --> src/main.rs:3:9
  |
3 |         y.0.0.1.z().f()? as u32 => {},
  |         ^^^^^^^^^^^^^^^^^^^^^^^ arbitrary expressions are not allowed in patterns
  |
help: consider moving the expression to a match arm guard
  |
3 |         val if val == y.0.0.1.z().f()? as u32 => {},
  |         ~~~ +++++++++++++++++++++++++++++++++
help: consider extracting the expression into a `const`
  |
2 +     const VAL: /* Type */ = y.0.0.1.z().f()? as u32;
3 ~     match 'b' {
4 ~         VAL => {},
  |
help: consider wrapping the expression in an inline `const` (requires `#![feature(inline_const_pat)]`)
  |
3 |         const { y.0.0.1.z().f()? as u32 } => {},
  |         +++++++                         +
```

---

r? fmease
`@rustbot` label +A-diagnostics +A-parser +A-patterns +C-enhancement

1 files changed, 77 insertions, 2 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index da1103a4fe5..e3e7fcebaaa 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -1,3 +1,5 @@
+// ignore-tidy-filelength
+
 use std::borrow::Cow;
 
 use rustc_ast::token::Token;
@@ -2592,13 +2594,86 @@ pub(crate) struct ExpectedCommaAfterPatternField {
 #[derive(Diagnostic)]
 #[diag(parse_unexpected_expr_in_pat)]
 pub(crate) struct UnexpectedExpressionInPattern {
+    /// The unexpected expr's span.
     #[primary_span]
     #[label]
     pub span: Span,
     /// Was a `RangePatternBound` expected?
     pub is_bound: bool,
-    /// Was the unexpected expression a `MethodCallExpression`?
-    pub is_method_call: bool,
+    /// The unexpected expr's precedence (used in match arm guard suggestions).
+    pub expr_precedence: i8,
+}
+
+#[derive(Subdiagnostic)]
+pub(crate) enum UnexpectedExpressionInPatternSugg {
+    #[multipart_suggestion(
+        parse_unexpected_expr_in_pat_create_guard_sugg,
+        applicability = "maybe-incorrect"
+    )]
+    CreateGuard {
+        /// Where to put the suggested identifier.
+        #[suggestion_part(code = "{ident}")]
+        ident_span: Span,
+        /// Where to put the match arm.
+        #[suggestion_part(code = " if {ident} == {expr}")]
+        pat_hi: Span,
+        /// The suggested identifier.
+        ident: String,
+        /// The unexpected expression.
+        expr: String,
+    },
+
+    #[multipart_suggestion(
+        parse_unexpected_expr_in_pat_update_guard_sugg,
+        applicability = "maybe-incorrect"
+    )]
+    UpdateGuard {
+        /// Where to put the suggested identifier.
+        #[suggestion_part(code = "{ident}")]
+        ident_span: Span,
+        /// The beginning of the match arm guard's expression (insert a `(` if `Some`).
+        #[suggestion_part(code = "(")]
+        guard_lo: Option<Span>,
+        /// The end of the match arm guard's expression.
+        #[suggestion_part(code = "{guard_hi_paren} && {ident} == {expr}")]
+        guard_hi: Span,
+        /// Either `")"` or `""`.
+        guard_hi_paren: &'static str,
+        /// The suggested identifier.
+        ident: String,
+        /// The unexpected expression.
+        expr: String,
+    },
+
+    #[multipart_suggestion(
+        parse_unexpected_expr_in_pat_const_sugg,
+        applicability = "has-placeholders"
+    )]
+    Const {
+        /// Where to put the extracted constant declaration.
+        #[suggestion_part(code = "{indentation}const {ident}: /* Type */ = {expr};\n")]
+        stmt_lo: Span,
+        /// Where to put the suggested identifier.
+        #[suggestion_part(code = "{ident}")]
+        ident_span: Span,
+        /// The suggested identifier.
+        ident: String,
+        /// The unexpected expression.
+        expr: String,
+        /// The statement's block's indentation.
+        indentation: String,
+    },
+
+    #[multipart_suggestion(
+        parse_unexpected_expr_in_pat_inline_const_sugg,
+        applicability = "maybe-incorrect"
+    )]
+    InlineConst {
+        #[suggestion_part(code = "const {{ ")]
+        start_span: Span,
+        #[suggestion_part(code = " }}")]
+        end_span: Span,
+    },
 }
 
 #[derive(Diagnostic)]