Identify when a stmt could have been parsed as an expr

There are some expressions that can be parsed as a statement without
a trailing semicolon depending on the context, which can lead to
confusing errors due to the same looking code being accepted in some
places and not others. Identify these cases and suggest enclosing in
parenthesis making the parse non-ambiguous without changing the
accepted grammar.

src/test/ui/parser/expr-as-stmt.fixed

@@ -0,0 +1,34 @@
+// run-rustfix
+#![allow(unused_variables)]
+#![allow(dead_code)]
+#![allow(unused_must_use)]
+
+fn foo() -> i32 {
+    ({2}) + {2} //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn bar() -> i32 {
+    ({2}) + 2 //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn zul() -> u32 {
+    let foo = 3;
+    ({ 42 }) + foo; //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+    32
+}
+
+fn baz() -> i32 {
+    ({ 3 }) * 3 //~ ERROR type `{integer}` cannot be dereferenced
+    //~^ ERROR mismatched types
+}
+
+fn qux(a: Option<u32>, b: Option<u32>) -> bool {
+    (if let Some(x) = a { true } else { false })
+    && //~ ERROR ambiguous parse
+    if let Some(y) = a { true } else { false }
+}
+
+fn main() {}

src/test/ui/parser/expr-as-stmt.rs

@@ -0,0 +1,34 @@
+// run-rustfix
+#![allow(unused_variables)]
+#![allow(dead_code)]
+#![allow(unused_must_use)]
+
+fn foo() -> i32 {
+    {2} + {2} //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn bar() -> i32 {
+    {2} + 2 //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+fn zul() -> u32 {
+    let foo = 3;
+    { 42 } + foo; //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+    32
+}
+
+fn baz() -> i32 {
+    { 3 } * 3 //~ ERROR type `{integer}` cannot be dereferenced
+    //~^ ERROR mismatched types
+}
+
+fn qux(a: Option<u32>, b: Option<u32>) -> bool {
+    if let Some(x) = a { true } else { false }
+    && //~ ERROR ambiguous parse
+    if let Some(y) = a { true } else { false }
+}
+
+fn main() {}

src/test/ui/parser/expr-as-stmt.stderr

@@ -0,0 +1,80 @@
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:7:9
+   |
+LL |     {2} + {2}
+   |     --- ^ expected expression
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({2})`
+
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:12:9
+   |
+LL |     {2} + 2
+   |     --- ^ expected expression
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({2})`
+
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:18:12
+   |
+LL |     { 42 } + foo;
+   |     ------ ^ expected expression
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({ 42 })`
+
+error: ambiguous parse
+  --> $DIR/expr-as-stmt.rs:30:5
+   |
+LL |     if let Some(x) = a { true } else { false }
+   |     ------------------------------------------ help: parenthesis are required to parse this as an expression: `(if let Some(x) = a { true } else { false })`
+LL |     &&
+   |     ^^
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:7:6
+   |
+LL |     {2} + {2}
+   |      ^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:12:6
+   |
+LL |     {2} + 2
+   |      ^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:18:7
+   |
+LL |     { 42 } + foo;
+   |       ^^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:24:7
+   |
+LL |     { 3 } * 3
+   |       ^ expected (), found integer
+   |
+   = note: expected type `()`
+              found type `{integer}`
+
+error[E0614]: type `{integer}` cannot be dereferenced
+  --> $DIR/expr-as-stmt.rs:24:11
+   |
+LL |     { 3 } * 3
+   |     ----- ^^^
+   |     |
+   |     help: parenthesis are required to parse this as an expression: `({ 3 })`
+
+error: aborting due to 9 previous errors
+
+Some errors have detailed explanations: E0308, E0614.
+For more information about an error, try `rustc --explain E0308`.

src/test/ui/parser/match-arrows-block-then-binop.rs

@@ -1,7 +1,7 @@
 fn main() {
-
-    match 0 {
+    let _ = match 0 {
       0 => {
+        0
       } + 5 //~ ERROR expected pattern, found `+`
-    }
+    };
 }

src/test/ui/parser/match-arrows-block-then-binop.stderr

@@ -3,6 +3,12 @@ error: expected pattern, found `+`
    |
 LL |       } + 5
    |         ^ expected pattern
+help: parenthesis are required to parse this as an expression
+   |
+LL |       0 => ({
+LL |         0
+LL |       }) + 5
+   |
 
 error: aborting due to previous error