Don't suggest unwrap for Result in const

Author: lapla-cogito

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2127,6 +2127,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 "?",
             )
         } else {
+            // Do not suggest `.expect()` in const context where it's not available.
+            if self.tcx.hir_is_inside_const_context(expr.hir_id) {
+                return false;
+            }
+
             (
                 format!(
                     "consider using `{kind}::expect` to unwrap the `{found}` value, \

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -3060,15 +3060,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 Applicability::MachineApplicable,
                             );
                         } else {
-                            err.span_suggestion_verbose(
-                                expr.span.shrink_to_hi(),
-                                format!(
-                                    "consider using `{kind}::expect` to unwrap the `{self_ty}` value, \
-                                    panicking if the value is {article} `{kind}::{variant}`"
-                                ),
-                                ".expect(\"REASON\")",
-                                Applicability::HasPlaceholders,
-                            );
+                            // Do not suggest `.expect()` in const context where it's not available.
+                            if !tcx.hir_is_inside_const_context(expr.hir_id) {
+                                err.span_suggestion_verbose(
+                                    expr.span.shrink_to_hi(),
+                                    format!(
+                                        "consider using `{kind}::expect` to unwrap the `{self_ty}` value, \
+                                        panicking if the value is {article} `{kind}::{variant}`"
+                                    ),
+                                    ".expect(\"REASON\")",
+                                    Applicability::HasPlaceholders,
+                                );
+                            }
                         }
                     }
                     // FIXME(compiler-errors): Support suggestions for other matching enum variants

tests/ui/consts/const-fn-unwrap.rs

@@ -0,0 +1,15 @@
+const fn f(value: u32) -> Result<u32, ()> {
+    Ok(value)
+}
+
+const TEST: u32 = f(2);
+//~^ ERROR: mismatched types
+
+const fn g() -> Result<String, ()> {
+    Ok(String::new())
+}
+
+const TEST2: usize = g().len();
+//~^ ERROR: no method named `len` found for enum `Result<T, E>`
+
+fn main() {}

tests/ui/consts/const-fn-unwrap.stderr

@@ -0,0 +1,24 @@
+error[E0308]: mismatched types
+  --> $DIR/const-fn-unwrap.rs:5:19
+   |
+LL | const TEST: u32 = f(2);
+   |                   ^^^^ expected `u32`, found `Result<u32, ()>`
+   |
+   = note: expected type `u32`
+              found enum `Result<u32, ()>`
+
+error[E0599]: no method named `len` found for enum `Result<T, E>` in the current scope
+  --> $DIR/const-fn-unwrap.rs:12:26
+   |
+LL | const TEST2: usize = g().len();
+   |                          ^^^
+   |
+note: the method `len` exists on the type `String`
+  --> $SRC_DIR/alloc/src/string.rs:LL:COL
+help: there is a method `le` with a similar name, but with different arguments
+  --> $SRC_DIR/core/src/cmp.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0599.
+For more information about an error, try `rustc --explain E0308`.