Suggest `let` or `==` on typo'd let-chain

When encountering a bare assignment in a let-chain, suggest turning the
assignment into a `let` expression or an equality check.

```
error: expected expression, found `let` statement
  --> $DIR/bad-if-let-suggestion.rs:5:8
   |
LL |     if let x = 1 && i = 2 {}
   |        ^^^^^^^^^
   |
   = note: only supported directly in conditions of `if` and `while` expressions
help: you might have meant to continue the let-chain
   |
LL |     if let x = 1 && let i = 2 {}
   |                     +++
help: you might have meant to compare for equality
   |
LL |     if let x = 1 && i == 2 {}
   |                        +
```

2 files changed, 15 insertions, 10 deletions

diff --git a/tests/ui/expr/if/bad-if-let-suggestion.rs b/tests/ui/expr/if/bad-if-let-suggestion.rs
index 99d584ac7a5..b0d0676e1ea 100644
--- a/tests/ui/expr/if/bad-if-let-suggestion.rs
+++ b/tests/ui/expr/if/bad-if-let-suggestion.rs
@@ -1,6 +1,3 @@
-// FIXME(compiler-errors): This really should suggest `let` on the RHS of the
-// `&&` operator, but that's kinda hard to do because of precedence.
-// Instead, for now we just make sure not to suggest `if let let`.
 fn a() {
     if let x = 1 && i = 2 {}
     //~^ ERROR cannot find value `i` in this scope
diff --git a/tests/ui/expr/if/bad-if-let-suggestion.stderr b/tests/ui/expr/if/bad-if-let-suggestion.stderr
index 20ac9ca76ba..0d1f895bd82 100644
--- a/tests/ui/expr/if/bad-if-let-suggestion.stderr
+++ b/tests/ui/expr/if/bad-if-let-suggestion.stderr
@@ -1,19 +1,27 @@
 error: expected expression, found `let` statement
-  --> $DIR/bad-if-let-suggestion.rs:5:8
+  --> $DIR/bad-if-let-suggestion.rs:2:8
    |
 LL |     if let x = 1 && i = 2 {}
    |        ^^^^^^^^^
    |
    = note: only supported directly in conditions of `if` and `while` expressions
+help: you might have meant to continue the let-chain
+   |
+LL |     if let x = 1 && let i = 2 {}
+   |                     +++
+help: you might have meant to compare for equality
+   |
+LL |     if let x = 1 && i == 2 {}
+   |                        +
 
 error[E0425]: cannot find value `i` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:5:21
+  --> $DIR/bad-if-let-suggestion.rs:2:21
    |
 LL |     if let x = 1 && i = 2 {}
    |                     ^ not found in this scope
 
 error[E0425]: cannot find value `i` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:12:9
+  --> $DIR/bad-if-let-suggestion.rs:9:9
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -22,7 +30,7 @@ LL |     if (i + j) = i {}
    |         ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `j` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:12:13
+  --> $DIR/bad-if-let-suggestion.rs:9:13
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -31,7 +39,7 @@ LL |     if (i + j) = i {}
    |             ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `i` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:12:18
+  --> $DIR/bad-if-let-suggestion.rs:9:18
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -40,7 +48,7 @@ LL |     if (i + j) = i {}
    |                  ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `x` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:19:8
+  --> $DIR/bad-if-let-suggestion.rs:16:8
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -49,7 +57,7 @@ LL |     if x[0] = 1 {}
    |        ^ help: a function with a similar name exists: `a`
 
 error[E0308]: mismatched types
-  --> $DIR/bad-if-let-suggestion.rs:5:8
+  --> $DIR/bad-if-let-suggestion.rs:2:8
    |
 LL |     if let x = 1 && i = 2 {}
    |        ^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`