From 6e5637983c155b2914647bb5e437337a44a63a7d Mon Sep 17 00:00:00 2001
From: Florian Diebold <flodiebold@gmail.com>
Date: Fri, 21 May 2021 17:48:15 +0200
Subject: Record type mismatches for failed coercions in match etc.

---
 crates/hir_ty/src/infer/coerce.rs | 14 +++++++++-----
 crates/hir_ty/src/infer/expr.rs   | 22 ++++++++++++----------
 2 files changed, 21 insertions(+), 15 deletions(-)

diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
index 03dd6ae76..40b4db926 100644
--- a/crates/hir_ty/src/infer/coerce.rs
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -6,11 +6,11 @@
 //! librustc_typeck/check/coercion.rs.
 
 use chalk_ir::{cast::Cast, Mutability, TyVariableKind};
-use hir_def::lang_item::LangItemTarget;
+use hir_def::{expr::ExprId, lang_item::LangItemTarget};
 
 use crate::{
-    autoderef, static_lifetime, Canonical, DomainGoal, FnPointer, FnSig, Interner, Solution,
-    Substitution, Ty, TyBuilder, TyExt, TyKind,
+    autoderef, infer::TypeMismatch, static_lifetime, Canonical, DomainGoal, FnPointer, FnSig,
+    Interner, Solution, Substitution, Ty, TyBuilder, TyExt, TyKind,
 };
 
 use super::{InEnvironment, InferOk, InferResult, InferenceContext, TypeError};
@@ -40,7 +40,7 @@ impl<'a> InferenceContext<'a> {
     ///    coerce both to function pointers;
     ///  - if we were concerned with lifetime subtyping, we'd need to look for a
     ///    least upper bound.
-    pub(super) fn coerce_merge_branch(&mut self, ty1: &Ty, ty2: &Ty) -> Ty {
+    pub(super) fn coerce_merge_branch(&mut self, id: Option<ExprId>, ty1: &Ty, ty2: &Ty) -> Ty {
         let ty1 = self.resolve_ty_shallow(ty1);
         let ty2 = self.resolve_ty_shallow(ty2);
         // Special case: two function types. Try to coerce both to
@@ -80,7 +80,11 @@ impl<'a> InferenceContext<'a> {
         } else if self.coerce(&ty1, &ty2) {
             ty2.clone()
         } else {
-            // TODO record a type mismatch
+            if let Some(id) = id {
+                self.result
+                    .type_mismatches
+                    .insert(id.into(), TypeMismatch { expected: ty1.clone(), actual: ty2.clone() });
+            }
             cov_mark::hit!(coerce_merge_fail_fallback);
             ty1.clone()
         }
diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index 4ef847d3a..08c05c67c 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -138,13 +138,14 @@ impl<'a> InferenceContext<'a> {
                 let mut result_ty = self.table.new_type_var();
                 let then_ty = self.infer_expr_inner(*then_branch, &expected);
                 both_arms_diverge &= mem::replace(&mut self.diverges, Diverges::Maybe);
-                result_ty = self.coerce_merge_branch(&result_ty, &then_ty);
+                result_ty = self.coerce_merge_branch(Some(*then_branch), &result_ty, &then_ty);
                 let else_ty = match else_branch {
                     Some(else_branch) => self.infer_expr_inner(*else_branch, &expected),
                     None => TyBuilder::unit(),
                 };
                 both_arms_diverge &= self.diverges;
-                result_ty = self.coerce_merge_branch(&result_ty, &else_ty);
+                // FIXME: create a synthetic `else {}` so we have something to refer to here instead of None?
+                result_ty = self.coerce_merge_branch(*else_branch, &result_ty, &else_ty);
 
                 self.diverges = condition_diverges | both_arms_diverge;
 
@@ -358,7 +359,7 @@ impl<'a> InferenceContext<'a> {
 
                     let arm_ty = self.infer_expr_inner(arm.expr, &expected);
                     all_arms_diverge &= self.diverges;
-                    result_ty = self.coerce_merge_branch(&result_ty, &arm_ty);
+                    result_ty = self.coerce_merge_branch(Some(arm.expr), &result_ty, &arm_ty);
                 }
 
                 self.diverges = matchee_diverges | all_arms_diverge;
@@ -372,12 +373,6 @@ impl<'a> InferenceContext<'a> {
             }
             Expr::Continue { .. } => TyKind::Never.intern(&Interner),
             Expr::Break { expr, label } => {
-                let val_ty = if let Some(expr) = expr {
-                    self.infer_expr(*expr, &Expectation::none())
-                } else {
-                    TyBuilder::unit()
-                };
-
                 let last_ty =
                     if let Some(ctxt) = find_breakable(&mut self.breakables, label.as_ref()) {
                         ctxt.break_ty.clone()
@@ -385,7 +380,14 @@ impl<'a> InferenceContext<'a> {
                         self.err_ty()
                     };
 
-                let merged_type = self.coerce_merge_branch(&last_ty, &val_ty);
+                let val_ty = if let Some(expr) = expr {
+                    self.infer_expr(*expr, &Expectation::none())
+                } else {
+                    TyBuilder::unit()
+                };
+
+                // FIXME: create a synthetic `()` during lowering so we have something to refer to here?
+                let merged_type = self.coerce_merge_branch(*expr, &last_ty, &val_ty);
 
                 if let Some(ctxt) = find_breakable(&mut self.breakables, label.as_ref()) {
                     ctxt.break_ty = merged_type;
-- 
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