On function and method calls in patterns, link to the book

```
error: expected a pattern, found an expression
 --> f889.rs:3:13
  |
3 |     let (x, y.drop()) = (1, 2); //~ ERROR
  |             ^^^^^^^^ not a pattern
  |
  = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>

error[E0532]: expected a pattern, found a function call
 --> f889.rs:2:13
  |
2 |     let (x, drop(y)) = (1, 2); //~ ERROR
  |             ^^^^ not a tuple struct or tuple variant
  |
  = note: function calls are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
```

Fix #97200.

3 files changed, 16 insertions, 1 deletions

diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
index 5d1c300b453..948199fd55c 100644
--- a/compiler/rustc_parse/messages.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -812,7 +812,8 @@ parse_unexpected_expr_in_pat =
        *[false] a pattern
     }, found an expression
 
-    .label = arbitrary expressions are not allowed in patterns
+    .label = not a pattern
+    .note = arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 
 parse_unexpected_expr_in_pat_const_sugg = consider extracting the expression into a `const`
 
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 40502158469..dade3912751 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -2608,6 +2608,7 @@ pub(crate) struct ExpectedCommaAfterPatternField {
 
 #[derive(Diagnostic)]
 #[diag(parse_unexpected_expr_in_pat)]
+#[note]
 pub(crate) struct UnexpectedExpressionInPattern {
     /// The unexpected expr's span.
     #[primary_span]
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index 35d166e8b4a..fce5ec36c66 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -443,6 +443,7 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
 
         self.suggest_bare_struct_literal(&mut err);
         self.suggest_changing_type_to_const_param(&mut err, res, source, span);
+        self.explain_functions_in_pattern(&mut err, res, source);
 
         if self.suggest_pattern_match_with_let(&mut err, source, span) {
             // Fallback label.
@@ -1166,6 +1167,18 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         }
     }
 
+    fn explain_functions_in_pattern(
+        &mut self,
+        err: &mut Diag<'_>,
+        res: Option<Res>,
+        source: PathSource<'_>,
+    ) {
+        let PathSource::TupleStruct(_, _) = source else { return };
+        let Some(Res::Def(DefKind::Fn, _)) = res else { return };
+        err.primary_message("expected a pattern, found a function call");
+        err.note("function calls are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>");
+    }
+
     fn suggest_changing_type_to_const_param(
         &mut self,
         err: &mut Diag<'_>,