Add a note to the ArbitraryExpressionInPattern error

compiler/rustc_ast_lowering/messages.ftl

@@ -5,6 +5,7 @@ ast_lowering_abi_specified_multiple_times =
 
 ast_lowering_arbitrary_expression_in_pattern =
     arbitrary expressions aren't allowed in patterns
+    .pattern_from_macro_note = the :expr fragment specifier forces the metavariable's content to be an expression
 
 ast_lowering_argument = argument
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -368,6 +368,8 @@ pub struct NeverPatternWithGuard {
 pub struct ArbitraryExpressionInPattern {
     #[primary_span]
     pub span: Span,
+    #[note(ast_lowering_pattern_from_macro_note)]
+    pub pattern_from_macro_note: bool,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_ast_lowering/src/pat.rs

@@ -339,7 +339,11 @@ fn lower_expr_within_pat(&mut self, expr: &Expr, allow_paths: bool) -> &'hir hir
             ExprKind::Path(..) if allow_paths => {}
             ExprKind::Unary(UnOp::Neg, inner) if matches!(inner.kind, ExprKind::Lit(_)) => {}
             _ => {
-                let guar = self.dcx().emit_err(ArbitraryExpressionInPattern { span: expr.span });
+                let pattern_from_macro = expr.is_approximately_pattern();
+                let guar = self.dcx().emit_err(ArbitraryExpressionInPattern {
+                    span: expr.span,
+                    pattern_from_macro_note: pattern_from_macro,
+                });
                 return self.arena.alloc(self.expr_err(expr.span, guar));
             }
         }

tests/ui/issues/issue-43250.stderr

@@ -3,12 +3,16 @@ error: arbitrary expressions aren't allowed in patterns
    |
 LL |     m!(y);
    |        ^
+   |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
 
 error: arbitrary expressions aren't allowed in patterns
   --> $DIR/issue-43250.rs:11:8
    |
 LL |     m!(C);
    |        ^
+   |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
 
 error: aborting due to 2 previous errors
 

tests/ui/lowering/expr-in-pat-issue-99380.rs

@@ -0,0 +1,11 @@
+macro_rules! foo {
+    ($p:expr) => {
+        if let $p = Some(42) {
+            return;
+        }
+    };
+}
+
+fn main() {
+    foo!(Some(3)); //~ ERROR arbitrary expressions aren't allowed in patterns
+}

tests/ui/lowering/expr-in-pat-issue-99380.stderr

@@ -0,0 +1,10 @@
+error: arbitrary expressions aren't allowed in patterns
+  --> $DIR/expr-in-pat-issue-99380.rs:10:10
+   |
+LL |     foo!(Some(3));
+   |          ^^^^^^^
+   |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
+
+error: aborting due to 1 previous error
+

tests/ui/macros/vec-macro-in-pattern.stderr

@@ -4,6 +4,7 @@ error: arbitrary expressions aren't allowed in patterns
 LL |         Some(vec![43]) => {}
    |              ^^^^^^^^
    |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
    = note: this error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error

tests/ui/match/expr_before_ident_pat.stderr

@@ -9,6 +9,8 @@ error: arbitrary expressions aren't allowed in patterns
    |
 LL |     funny!(a, a);
    |            ^
+   |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
 
 error: aborting due to 2 previous errors
 

tests/ui/pattern/issue-92074-macro-ice.stderr

@@ -7,6 +7,7 @@ LL |     () => { force_expr!(Vec::new()) }
 LL |     assert!(matches!(x, En::A(make_vec!())));
    |                               ----------- in this macro invocation
    |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
    = note: this error originates in the macro `make_vec` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: arbitrary expressions aren't allowed in patterns
@@ -18,6 +19,7 @@ LL |     () => { force_pat!(get_usize(), get_usize()) }
 LL |     assert!(matches!(5, make_pat!()));
    |                         ----------- in this macro invocation
    |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
    = note: this error originates in the macro `make_pat` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: arbitrary expressions aren't allowed in patterns
@@ -29,6 +31,7 @@ LL |     () => { force_pat!(get_usize(), get_usize()) }
 LL |     assert!(matches!(5, make_pat!()));
    |                         ----------- in this macro invocation
    |
+   = note: the :expr fragment specifier forces the metavariable's content to be an expression
    = note: this error originates in the macro `make_pat` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 3 previous errors