Rollup merge of #88553 - theo-lw:issue-88276, r=estebank

Improve diagnostics for unary plus operators (#88276)

This pull request improves the diagnostics emitted on parsing a unary plus operator. See #88276.

Before:

```
error: expected expression, found `+`
 --> src/main.rs:2:13
  |
2 |     let x = +1;
  |             ^ expected expression
```

After:

```
error: leading `+` is not supported
 --> main.rs:2:13
  |
2 |     let x = +1;
  |             ^
  |             |
  |             unexpected `+`
  |             help: try removing the `+`
```

compiler/rustc_ast/src/token.rs

@@ -586,6 +586,13 @@ impl Token {
         self.is_non_raw_ident_where(|id| id.name.is_bool_lit())
     }
 
+    pub fn is_numeric_lit(&self) -> bool {
+        matches!(
+            self.kind,
+            Literal(Lit { kind: LitKind::Integer, .. }) | Literal(Lit { kind: LitKind::Float, .. })
+        )
+    }
+
     /// Returns `true` if the token is a non-raw identifier for which `pred` holds.
     pub fn is_non_raw_ident_where(&self, pred: impl FnOnce(Ident) -> bool) -> bool {
         match self.ident() {

compiler/rustc_parse/src/parser/expr.rs

@@ -516,6 +516,26 @@ impl<'a> Parser<'a> {
             token::BinOp(token::And) | token::AndAnd => {
                 make_it!(this, attrs, |this, _| this.parse_borrow_expr(lo))
             }
+            token::BinOp(token::Plus) if this.look_ahead(1, |tok| tok.is_numeric_lit()) => {
+                let mut err = this.struct_span_err(lo, "leading `+` is not supported");
+                err.span_label(lo, "unexpected `+`");
+
+                // a block on the LHS might have been intended to be an expression instead
+                if let Some(sp) = this.sess.ambiguous_block_expr_parse.borrow().get(&lo) {
+                    this.sess.expr_parentheses_needed(&mut err, *sp);
+                } else {
+                    err.span_suggestion_verbose(
+                        lo,
+                        "try removing the `+`",
+                        "".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+                err.emit();
+
+                this.bump();
+                this.parse_prefix_expr(None)
+            } // `+expr`
             token::Ident(..) if this.token.is_keyword(kw::Box) => {
                 make_it!(this, attrs, |this, _| this.parse_box_expr(lo))
             }

src/test/ui/associated-types/issue-36499.stderr

@@ -1,8 +1,14 @@
-error: expected expression, found `+`
+error: leading `+` is not supported
   --> $DIR/issue-36499.rs:4:9
    |
 LL |     2 + +2;
-   |         ^ expected expression
+   |         ^ unexpected `+`
+   |
+help: try removing the `+`
+   |
+LL -     2 + +2;
+LL +     2 + 2;
+   | 
 
 error: aborting due to previous error
 

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

@@ -10,7 +10,7 @@ fn foo() -> i32 {
 }
 
 fn bar() -> i32 {
-    ({2}) + 2 //~ ERROR expected expression, found `+`
+    ({2}) + 2 //~ ERROR leading `+` is not supported
     //~^ ERROR mismatched types
 }
 

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

@@ -10,7 +10,7 @@ fn foo() -> i32 {
 }
 
 fn bar() -> i32 {
-    {2} + 2 //~ ERROR expected expression, found `+`
+    {2} + 2 //~ ERROR leading `+` is not supported
     //~^ ERROR mismatched types
 }
 

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

@@ -9,11 +9,11 @@ help: parentheses are required to parse this as an expression
 LL |     ({2}) + {2}
    |     +   +
 
-error: expected expression, found `+`
+error: leading `+` is not supported
   --> $DIR/expr-as-stmt.rs:13:9
    |
 LL |     {2} + 2
-   |         ^ expected expression
+   |         ^ unexpected `+`
    |
 help: parentheses are required to parse this as an expression
    |

src/test/ui/parser/issue-88276-unary-plus.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+#[allow(unused_parens)]
+fn main() {
+    let _ = 1; //~ ERROR leading `+` is not supported
+    let _ = (1.0 + 2.0) * 3.0; //~ ERROR leading `+` is not supported
+                           //~| ERROR leading `+` is not supported
+    let _ = [3, 4+6]; //~ ERROR leading `+` is not supported
+}

src/test/ui/parser/issue-88276-unary-plus.rs

@@ -0,0 +1,8 @@
+// run-rustfix
+#[allow(unused_parens)]
+fn main() {
+    let _ = +1; //~ ERROR leading `+` is not supported
+    let _ = (1.0 + +2.0) * +3.0; //~ ERROR leading `+` is not supported
+                           //~| ERROR leading `+` is not supported
+    let _ = [+3, 4+6]; //~ ERROR leading `+` is not supported
+}

src/test/ui/parser/issue-88276-unary-plus.stderr

@@ -0,0 +1,50 @@
+error: leading `+` is not supported
+  --> $DIR/issue-88276-unary-plus.rs:4:13
+   |
+LL |     let _ = +1;
+   |             ^ unexpected `+`
+   |
+help: try removing the `+`
+   |
+LL -     let _ = +1;
+LL +     let _ = 1;
+   | 
+
+error: leading `+` is not supported
+  --> $DIR/issue-88276-unary-plus.rs:5:20
+   |
+LL |     let _ = (1.0 + +2.0) * +3.0;
+   |                    ^ unexpected `+`
+   |
+help: try removing the `+`
+   |
+LL -     let _ = (1.0 + +2.0) * +3.0;
+LL +     let _ = (1.0 + 2.0) * +3.0;
+   | 
+
+error: leading `+` is not supported
+  --> $DIR/issue-88276-unary-plus.rs:5:28
+   |
+LL |     let _ = (1.0 + +2.0) * +3.0;
+   |                            ^ unexpected `+`
+   |
+help: try removing the `+`
+   |
+LL -     let _ = (1.0 + +2.0) * +3.0;
+LL +     let _ = (1.0 + +2.0) * 3.0;
+   | 
+
+error: leading `+` is not supported
+  --> $DIR/issue-88276-unary-plus.rs:7:14
+   |
+LL |     let _ = [+3, 4+6];
+   |              ^ unexpected `+`
+   |
+help: try removing the `+`
+   |
+LL -     let _ = [+3, 4+6];
+LL +     let _ = [3, 4+6];
+   | 
+
+error: aborting due to 4 previous errors
+