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

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`
 

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]

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<'_>,

tests/ui/half-open-range-patterns/range_pat_interactions1.stderr

@@ -2,8 +2,9 @@ error: expected a pattern range bound, found an expression
   --> $DIR/range_pat_interactions1.rs:17:16
    |
 LL |             0..5+1 => errors_only.push(x),
-   |                ^^^ arbitrary expressions are not allowed in patterns
+   |                ^^^ not a pattern
    |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 help: consider extracting the expression into a `const`
    |
 LL +         const VAL: /* Type */ = 5+1;

tests/ui/half-open-range-patterns/range_pat_interactions2.stderr

@@ -14,8 +14,9 @@ error: expected a pattern range bound, found an expression
   --> $DIR/range_pat_interactions2.rs:10:18
    |
 LL |             0..=(5+1) => errors_only.push(x),
-   |                  ^^^ arbitrary expressions are not allowed in patterns
+   |                  ^^^ not a pattern
    |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 help: consider extracting the expression into a `const`
    |
 LL +         const VAL: /* Type */ = 5+1;

tests/ui/parser/bad-name.stderr

@@ -8,7 +8,9 @@ error: expected a pattern, found an expression
   --> $DIR/bad-name.rs:2:7
    |
 LL |   let x.y::<isize>.z foo;
-   |       ^^^^^^^^^^^^^^ arbitrary expressions are not allowed in patterns
+   |       ^^^^^^^^^^^^^^ not a pattern
+   |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 
 error: expected one of `(`, `.`, `::`, `:`, `;`, `=`, `?`, `|`, or an operator, found `foo`
   --> $DIR/bad-name.rs:2:22

tests/ui/parser/issues/issue-24197.stderr

@@ -2,7 +2,9 @@ error: expected a pattern, found an expression
   --> $DIR/issue-24197.rs:2:9
    |
 LL |     let buf[0] = 0;
-   |         ^^^^^^ arbitrary expressions are not allowed in patterns
+   |         ^^^^^^ not a pattern
+   |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 
 error: aborting due to 1 previous error
 

tests/ui/parser/issues/issue-24375.stderr

@@ -2,8 +2,9 @@ error: expected a pattern, found an expression
   --> $DIR/issue-24375.rs:6:9
    |
 LL |         tmp[0] => {}
-   |         ^^^^^^ arbitrary expressions are not allowed in patterns
+   |         ^^^^^^ not a pattern
    |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 help: consider moving the expression to a match arm guard
    |
 LL |         val if val == tmp[0] => {}

tests/ui/parser/pat-lt-bracket-5.stderr

@@ -2,7 +2,9 @@ error: expected a pattern, found an expression
   --> $DIR/pat-lt-bracket-5.rs:2:9
    |
 LL |     let v[0] = v[1];
-   |         ^^^^ arbitrary expressions are not allowed in patterns
+   |         ^^^^ not a pattern
+   |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 
 error[E0425]: cannot find value `v` in this scope
   --> $DIR/pat-lt-bracket-5.rs:2:16

tests/ui/parser/pat-lt-bracket-6.stderr

@@ -2,7 +2,9 @@ error: expected a pattern, found an expression
   --> $DIR/pat-lt-bracket-6.rs:5:14
    |
 LL |     let Test(&desc[..]) = x;
-   |              ^^^^^^^^^ arbitrary expressions are not allowed in patterns
+   |              ^^^^^^^^^ not a pattern
+   |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 
 error[E0308]: mismatched types
   --> $DIR/pat-lt-bracket-6.rs:10:30

tests/ui/parser/pat-ranges-3.stderr

@@ -2,13 +2,17 @@ error: expected a pattern range bound, found an expression
   --> $DIR/pat-ranges-3.rs:4:16
    |
 LL |     let 10 ..= 10 + 3 = 12;
-   |                ^^^^^^ arbitrary expressions are not allowed in patterns
+   |                ^^^^^^ not a pattern
+   |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 
 error: expected a pattern range bound, found an expression
   --> $DIR/pat-ranges-3.rs:7:9
    |
 LL |     let 10 - 3 ..= 10 = 8;
-   |         ^^^^^^ arbitrary expressions are not allowed in patterns
+   |         ^^^^^^ not a pattern
+   |
+   = note: arbitrary expressions are not allowed in patterns: <https://doc.rust-lang.org/book/ch18-00-patterns.html>
 
 error: aborting due to 2 previous errors