Improve error message for assert!() macro in functions returning bool

When assert!() is used in a function that returns a value, the compiler
emits a confusing error message suggesting the if expression is missing
an else clause. This commit detects when the if expression comes from
an assert!() or debug_assert!() macro expansion and provides a clearer
error message explaining that assert!() does not return a value.

Author: enthropy7

compiler/rustc_hir_typeck/src/_match.rs

@@ -4,7 +4,7 @@ use rustc_hir::def_id::LocalDefId;
 use rustc_hir::{self as hir, ExprKind, HirId, PatKind};
 use rustc_hir_pretty::ty_to_string;
 use rustc_middle::ty::{self, Ty};
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 use rustc_trait_selection::traits::{
     MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
 };
@@ -302,15 +302,41 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         then_expr: &'tcx hir::Expr<'tcx>,
         error: &mut bool,
     ) {
+        // Check if this `if` expression comes from an `assert!()` or `debug_assert!()` macro expansion
+        // Walk the macro expansion chain to find the root assert macro, similar to how panic_call does it
+        let mut is_assert_macro = false;
+        let mut expn = if_span.ctxt().outer_expn_data();
+        loop {
+            if let Some(def_id) = expn.macro_def_id {
+                if self.tcx.is_diagnostic_item(sym::assert_macro, def_id)
+                    || self.tcx.is_diagnostic_item(sym::debug_assert_macro, def_id)
+                {
+                    is_assert_macro = true;
+                    break;
+                }
+            }
+            let parent = expn.call_site.ctxt().outer_expn_data();
+            if parent.macro_def_id.is_none() {
+                break;
+            }
+            expn = parent;
+        }
+
         if let Some((if_span, msg)) = ret_reason {
             err.span_label(if_span, msg);
         } else if let ExprKind::Block(block, _) = then_expr.kind
             && let Some(expr) = block.expr
         {
             err.span_label(expr.span, "found here");
         }
-        err.note("`if` expressions without `else` evaluate to `()`");
-        err.help("consider adding an `else` block that evaluates to the expected type");
+
+        if is_assert_macro {
+            err.note("`assert!()` macro expands to an `if` expression without an `else` clause");
+            err.help("`assert!()` does not return a value; consider using a different approach or adding a return statement");
+        } else {
+            err.note("`if` expressions without `else` evaluate to `()`");
+            err.help("consider adding an `else` block that evaluates to the expected type");
+        }
         *error = true;
         if let ExprKind::Let(hir::LetExpr { span, pat, init, .. }) = cond_expr.kind
             && let ExprKind::Block(block, _) = then_expr.kind

tests/ui/expr/if/assert-macro-without-else.rs

@@ -0,0 +1,9 @@
+//@ dont-require-annotations: NOTE
+
+fn f() -> bool {
+    assert!(1 < 2)
+    //~^ ERROR `if` may be missing an `else` clause [E0317]
+    //~| NOTE expected `bool`, found `()`
+}
+
+fn main() {}

tests/ui/expr/if/assert-macro-without-else.stderr

@@ -0,0 +1,14 @@
+error[E0317]: `if` may be missing an `else` clause
+  --> $DIR/assert-macro-without-else.rs:4:5
+   |
+LL | fn f() -> bool {
+   |           ---- expected `bool` because of this return type
+LL |     assert!(1 < 2)
+   |     ^^^^^^^^^^^^^^ expected `bool`, found `()`
+   |
+   = note: `assert!()` macro expands to an `if` expression without an `else` clause
+   = help: `assert!()` does not return a value; consider using a different approach or adding a return statement
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0317`.