Handle more cases of value suggestions

Author: estebank

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -652,6 +652,55 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         err
     }
 
+    fn ty_kind_suggestion(&self, ty: Ty<'tcx>) -> Option<String> {
+        // Keep in sync with `rustc_hir_analysis/src/check/mod.rs:ty_kind_suggestion`.
+        // FIXME: deduplicate the above.
+        let implements_default = |ty| {
+            let Some(default_trait) = self.infcx.tcx.get_diagnostic_item(sym::Default) else {
+                return false;
+            };
+            self.infcx
+                .type_implements_trait(default_trait, [ty], self.param_env)
+                .must_apply_modulo_regions()
+        };
+
+        Some(match ty.kind() {
+            ty::Never | ty::Error(_) => return None,
+            ty::Bool => "false".to_string(),
+            ty::Char => "\'x\'".to_string(),
+            ty::Int(_) | ty::Uint(_) => "42".into(),
+            ty::Float(_) => "3.14159".into(),
+            ty::Slice(_) => "[]".to_string(),
+            ty::Adt(def, _) if Some(def.did()) == self.infcx.tcx.get_diagnostic_item(sym::Vec) => {
+                "vec![]".to_string()
+            }
+            ty::Adt(_, _) if implements_default(ty) => "Default::default()".to_string(),
+            ty::Ref(_, ty, mutability) => {
+                if let (ty::Str, hir::Mutability::Not) = (ty.kind(), mutability) {
+                    "\"\"".to_string()
+                } else {
+                    let Some(ty) = self.ty_kind_suggestion(*ty) else {
+                        return None;
+                    };
+                    format!("&{}{ty}", mutability.prefix_str())
+                }
+            }
+            ty::Array(ty, len) => format!(
+                "[{}; {}]",
+                self.ty_kind_suggestion(*ty)?,
+                len.eval_target_usize(self.infcx.tcx, ty::ParamEnv::reveal_all()),
+            ),
+            ty::Tuple(tys) => format!(
+                "({})",
+                tys.iter()
+                    .map(|ty| self.ty_kind_suggestion(ty))
+                    .collect::<Option<Vec<String>>>()?
+                    .join(", ")
+            ),
+            _ => "value".to_string(),
+        })
+    }
+
     fn suggest_assign_value(
         &self,
         err: &mut Diag<'_>,
@@ -661,24 +710,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let ty = moved_place.ty(self.body, self.infcx.tcx).ty;
         debug!("ty: {:?}, kind: {:?}", ty, ty.kind());
 
-        let tcx = self.infcx.tcx;
-        let implements_default = |ty, param_env| {
-            let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
-                return false;
-            };
-            self.infcx
-                .type_implements_trait(default_trait, [ty], param_env)
-                .must_apply_modulo_regions()
-        };
-
-        let assign_value = match ty.kind() {
-            ty::Never | ty::Error(_) => return,
-            ty::Bool => "false",
-            ty::Float(_) => "0.0",
-            ty::Int(_) | ty::Uint(_) => "0",
-            ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => "vec![]",
-            ty::Adt(_, _) if implements_default(ty, self.param_env) => "Default::default()",
-            _ => "value",
+        let Some(assign_value) = self.ty_kind_suggestion(ty) else {
+            return;
         };
 
         err.span_suggestion_verbose(

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -81,6 +81,7 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_errors::{pluralize, struct_span_code_err, Diag};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
+use rustc_hir::Mutability;
 use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::error_reporting::ObligationCauseExt as _;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
@@ -467,7 +468,9 @@ fn fn_sig_suggestion<'tcx>(
     )
 }
 
-pub fn ty_kind_suggestion<'tcx>(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Option<&'static str> {
+pub fn ty_kind_suggestion<'tcx>(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Option<String> {
+    // Keep in sync with `rustc_borrowck/src/diagnostics/conflict_errors.rs:ty_kind_suggestion`.
+    // FIXME: deduplicate the above.
     let implements_default = |ty| {
         let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
             return false;
@@ -478,14 +481,39 @@ pub fn ty_kind_suggestion<'tcx>(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Option<&'sta
             .must_apply_modulo_regions()
     };
     Some(match ty.kind() {
-        ty::Bool => "true",
-        ty::Char => "'a'",
-        ty::Int(_) | ty::Uint(_) => "42",
-        ty::Float(_) => "3.14159",
-        ty::Error(_) | ty::Never => return None,
-        ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => "vec![]",
-        ty::Adt(_, _) if implements_default(ty) => "Default::default()",
-        _ => "value",
+        ty::Never | ty::Error(_) => return None,
+        ty::Bool => "false".to_string(),
+        ty::Char => "\'x\'".to_string(),
+        ty::Int(_) | ty::Uint(_) => "42".into(),
+        ty::Float(_) => "3.14159".into(),
+        ty::Slice(_) => "[]".to_string(),
+        ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => {
+            "vec![]".to_string()
+        }
+        ty::Adt(_, _) if implements_default(ty) => "Default::default()".to_string(),
+        ty::Ref(_, ty, mutability) => {
+            if let (ty::Str, Mutability::Not) = (ty.kind(), mutability) {
+                "\"\"".to_string()
+            } else {
+                let Some(ty) = ty_kind_suggestion(*ty, tcx) else {
+                    return None;
+                };
+                format!("&{}{ty}", mutability.prefix_str())
+            }
+        }
+        ty::Array(ty, len) => format!(
+            "[{}; {}]",
+            ty_kind_suggestion(*ty, tcx)?,
+            len.eval_target_usize(tcx, ty::ParamEnv::reveal_all()),
+        ),
+        ty::Tuple(tys) => format!(
+            "({})",
+            tys.iter()
+                .map(|ty| ty_kind_suggestion(ty, tcx))
+                .collect::<Option<Vec<String>>>()?
+                .join(", ")
+        ),
+        _ => "value".to_string(),
     })
 }
 
@@ -523,7 +551,7 @@ fn suggestion_signature<'tcx>(
         }
         ty::AssocKind::Const => {
             let ty = tcx.type_of(assoc.def_id).instantiate_identity();
-            let val = ty_kind_suggestion(ty, tcx).unwrap_or("todo!()");
+            let val = ty_kind_suggestion(ty, tcx).unwrap_or_else(|| "value".to_string());
             format!("const {}: {} = {};", assoc.name, ty, val)
         }
     }