Improve parsing errors and suggestions for bad if statements

compiler/rustc_parse/src/parser/expr.rs

@@ -2248,36 +2248,59 @@ impl<'a> Parser<'a> {
         &mut self,
         attrs: AttrVec,
         lo: Span,
-        cond: P<Expr>,
+        mut cond: P<Expr>,
     ) -> PResult<'a, P<Expr>> {
-        let missing_then_block_binop_span = || {
+        let cond_span = cond.span;
-            match cond.kind {
+        // Tries to interpret `cond` as either a missing expression if it's a block,
-                ExprKind::Binary(Spanned { span: binop_span, .. }, _, ref right)
+        // or as an unfinished expression if it's a binop and the RHS is a block.
-                    if let ExprKind::Block(..) = right.kind => Some(binop_span),
+        // We could probably add more recoveries here too...
-                _ => None
+        let mut recover_block_from_condition = |this: &mut Self| {
+            let block = match &mut cond.kind {
+                ExprKind::Binary(Spanned { span: binop_span, .. }, _, right)
+                    if let ExprKind::Block(_, None) = right.kind => {
+                        this.error_missing_if_then_block(lo, cond_span.shrink_to_lo().to(*binop_span), true).emit();
+                        std::mem::replace(right, this.mk_expr_err(binop_span.shrink_to_hi()))
+                    },
+                ExprKind::Block(_, None) => {
+                    this.error_missing_if_cond(lo, cond_span).emit();
+                    std::mem::replace(&mut cond, this.mk_expr_err(cond_span.shrink_to_hi()))
+                }
+                _ => {
+                    return None;
+                }
+            };
+            if let ExprKind::Block(block, _) = &block.kind {
+                Some(block.clone())
+            } else {
+                unreachable!()
             }
         };
-        // Verify that the parsed `if` condition makes sense as a condition. If it is a block, then
+        // Parse then block
-        // verify that the last statement is either an implicit return (no `;`) or an explicit
+        let thn = if self.token.is_keyword(kw::Else) {
-        // return. This won't catch blocks with an explicit `return`, but that would be caught by
+            if let Some(block) = recover_block_from_condition(self) {
-        // the dead code lint.
+                block
-        let thn = if self.token.is_keyword(kw::Else) || !cond.returns() {
-            if let Some(binop_span) = missing_then_block_binop_span() {
-                self.error_missing_if_then_block(lo, None, Some(binop_span)).emit();
-                self.mk_block_err(cond.span)
             } else {
-                self.error_missing_if_cond(lo, cond.span)
+                self.error_missing_if_then_block(lo, cond_span, false).emit();
+                self.mk_block_err(cond_span.shrink_to_hi())
             }
         } else {
             let attrs = self.parse_outer_attributes()?.take_for_recovery(); // For recovery.
-            let not_block = self.token != token::OpenDelim(Delimiter::Brace);
+            let block = if self.check(&token::OpenDelim(Delimiter::Brace)) {
-            let block = self.parse_block().map_err(|err| {
+                self.parse_block()?
-                if not_block {
+            } else {
-                    self.error_missing_if_then_block(lo, Some(err), missing_then_block_binop_span())
+                if let Some(block) = recover_block_from_condition(self) {
+                    block
                 } else {
-                    err
+                    // Parse block, which will always fail, but we can add a nice note to the error
+                    self.parse_block().map_err(|mut err| {
+                        err.span_note(
+                            cond_span,
+                            "the `if` expression is missing a block after this condition",
+                        );
+                        err
+                    })?
                 }
-            })?;
+            };
             self.error_on_if_block_attrs(lo, false, block.span, &attrs);
             block
         };
@@ -2288,31 +2311,34 @@ impl<'a> Parser<'a> {
     fn error_missing_if_then_block(
         &self,
         if_span: Span,
-        err: Option<DiagnosticBuilder<'a, ErrorGuaranteed>>,
+        cond_span: Span,
-        binop_span: Option<Span>,
+        is_unfinished: bool,
     ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
-        let msg = "this `if` expression has a condition, but no block";
+        let mut err = self.struct_span_err(
-
+            if_span,
-        let mut err = if let Some(mut err) = err {
+            "this `if` expression is missing a block after the condition",
-            err.span_label(if_span, msg);
+        );
-            err
+        if is_unfinished {
+            err.span_help(cond_span, "this binary operation is possibly unfinished");
         } else {
-            self.struct_span_err(if_span, msg)
+            err.span_help(cond_span.shrink_to_hi(), "add a block here");
-        };
-
-        if let Some(binop_span) = binop_span {
-            err.span_help(binop_span, "maybe you forgot the right operand of the condition?");
         }
-
         err
     }
 
-    fn error_missing_if_cond(&self, lo: Span, span: Span) -> P<ast::Block> {
+    fn error_missing_if_cond(
-        let sp = self.sess.source_map().next_point(lo);
+        &self,
-        self.struct_span_err(sp, "missing condition for `if` expression")
+        lo: Span,
-            .span_label(sp, "expected if condition here")
+        span: Span,
-            .emit();
+    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
-        self.mk_block_err(span)
+        let next_span = self.sess.source_map().next_point(lo);
+        let mut err = self.struct_span_err(next_span, "missing condition for `if` expression");
+        err.span_label(next_span, "expected condition here");
+        err.span_label(
+            self.sess.source_map().start_point(span),
+            "if this block is the condition of the `if` expression, then it must be followed by another block"
+        );
+        err
     }
 
     /// Parses the condition of a `if` or `while` expression.

compiler/rustc_parse/src/parser/stmt.rs

@@ -432,10 +432,23 @@ impl<'a> Parser<'a> {
         //
         // which is valid in other languages, but not Rust.
         match self.parse_stmt_without_recovery(false, ForceCollect::No) {
-            // If the next token is an open brace (e.g., `if a b {`), the place-
+            // If the next token is an open brace, e.g., we have:
-            // inside-a-block suggestion would be more likely wrong than right.
+            //
+            //     if expr other_expr {
+            //        ^    ^          ^- lookahead(1) is a brace
+            //        |    |- current token is not "else"
+            //        |- (statement we just parsed)
+            //
+            // the place-inside-a-block suggestion would be more likely wrong than right.
+            //
+            // FIXME(compiler-errors): this should probably parse an arbitrary expr and not
+            // just lookahead one token, so we can see if there's a brace after _that_,
+            // since we want to protect against:
+            //     `if 1 1 + 1 {` being suggested as  `if { 1 } 1 + 1 {`
+            //                                            +   +
             Ok(Some(_))
-                if self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::Brace))
+                if (!self.token.is_keyword(kw::Else)
+                    && self.look_ahead(1, |t| t == &token::OpenDelim(Delimiter::Brace)))
                     || do_not_suggest_help => {}
             // Do not suggest `if foo println!("") {;}` (as would be seen in test for #46836).
             Ok(Some(Stmt { kind: StmtKind::Empty, .. })) => {}

src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.stderr

@@ -25,10 +25,16 @@ LL |         println!("Then when?");
 error: expected `{`, found `;`
   --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:20:31
    |
-LL |     if not  // lack of braces is [sic]
-   |     -- this `if` expression has a condition, but no block
 LL |         println!("Then when?");
    |                               ^ expected `{`
+   |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:19:8
+   |
+LL |       if not  // lack of braces is [sic]
+   |  ________^
+LL | |         println!("Then when?");
+   | |______________________________^
 
 error: unexpected `2` after identifier
   --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:26:24

src/test/ui/expr/if/if-without-block.rs

@@ -1,9 +1,7 @@
 fn main() {
     let n = 1;
     if 5 == {
-    //~^ NOTE this `if` expression has a condition, but no block
+    //~^ ERROR this `if` expression is missing a block after the condition
         println!("five");
     }
 }
-//~^ ERROR expected `{`, found `}`
-//~| NOTE expected `{`

src/test/ui/expr/if/if-without-block.stderr

@@ -1,17 +1,14 @@
-error: expected `{`, found `}`
+error: this `if` expression is missing a block after the condition
-  --> $DIR/if-without-block.rs:7:1
+  --> $DIR/if-without-block.rs:3:5
    |
 LL |     if 5 == {
-   |     -- this `if` expression has a condition, but no block
+   |     ^^
-...
-LL | }
-   | ^ expected `{`
    |
-help: maybe you forgot the right operand of the condition?
+help: this binary operation is possibly unfinished
-  --> $DIR/if-without-block.rs:3:10
+  --> $DIR/if-without-block.rs:3:8
    |
 LL |     if 5 == {
-   |          ^^
+   |        ^^^^
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-39848.stderr

@@ -2,13 +2,19 @@ error: expected `{`, found `foo`
   --> $DIR/issue-39848.rs:3:21
    |
 LL |         if $tgt.has_$field() {}
-   |         --          ^^^^^^ expected `{`
+   |                     ^^^^^^ expected `{`
-   |         |
-   |         this `if` expression has a condition, but no block
 ...
 LL |     get_opt!(bar, foo);
    |     ------------------ in this macro invocation
    |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/issue-39848.rs:3:12
+   |
+LL |         if $tgt.has_$field() {}
+   |            ^^^^^^^^^
+...
+LL |     get_opt!(bar, foo);
+   |     ------------------ in this macro invocation
    = note: this error originates in the macro `get_opt` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: try placing this code inside a block
    |

src/test/ui/missing/missing-block-hint.stderr

@@ -2,18 +2,25 @@ error: expected `{`, found `=>`
   --> $DIR/missing-block-hint.rs:3:18
    |
 LL |         if (foo) => {}
-   |         --       ^^ expected `{`
+   |                  ^^ expected `{`
-   |         |
+   |
-   |         this `if` expression has a condition, but no block
+note: the `if` expression is missing a block after this condition
+  --> $DIR/missing-block-hint.rs:3:12
+   |
+LL |         if (foo) => {}
+   |            ^^^^^
 
 error: expected `{`, found `bar`
   --> $DIR/missing-block-hint.rs:7:13
    |
-LL |         if (foo)
-   |         -- this `if` expression has a condition, but no block
 LL |             bar;
    |             ^^^ expected `{`
    |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/missing-block-hint.rs:6:12
+   |
+LL |         if (foo)
+   |            ^^^^^
 help: try placing this code inside a block
    |
 LL |             { bar; }

src/test/ui/parser/bad-if-statements.rs

@@ -0,0 +1,38 @@
+fn a() {
+    if {}
+    //~^ ERROR missing condition for `if` expression
+}
+
+fn b() {
+    if true && {}
+    //~^ ERROR this `if` expression is missing a block after the condition
+}
+
+fn c() {
+    let x = {};
+    if true x
+    //~^ ERROR expected `{`, found `x`
+}
+
+fn a2() {
+    if {} else {}
+    //~^ ERROR missing condition for `if` expression
+}
+
+fn b2() {
+    if true && {} else {}
+    //~^ ERROR this `if` expression is missing a block after the condition
+}
+
+fn c2() {
+    let x = {};
+    if true x else {}
+    //~^ ERROR expected `{`, found `x`
+}
+
+fn d() {
+    if true else {}
+    //~^ ERROR this `if` expression is missing a block after the condition
+}
+
+fn main() {}

src/test/ui/parser/bad-if-statements.stderr

@@ -0,0 +1,86 @@
+error: missing condition for `if` expression
+  --> $DIR/bad-if-statements.rs:2:7
+   |
+LL |     if {}
+   |       ^- if this block is the condition of the `if` expression, then it must be followed by another block
+   |       |
+   |       expected condition here
+
+error: this `if` expression is missing a block after the condition
+  --> $DIR/bad-if-statements.rs:7:5
+   |
+LL |     if true && {}
+   |     ^^
+   |
+help: this binary operation is possibly unfinished
+  --> $DIR/bad-if-statements.rs:7:8
+   |
+LL |     if true && {}
+   |        ^^^^^^^
+
+error: expected `{`, found `x`
+  --> $DIR/bad-if-statements.rs:13:13
+   |
+LL |     if true x
+   |             ^ expected `{`
+   |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/bad-if-statements.rs:13:8
+   |
+LL |     if true x
+   |        ^^^^
+help: try placing this code inside a block
+   |
+LL |     if true { x }
+   |             +   +
+
+error: missing condition for `if` expression
+  --> $DIR/bad-if-statements.rs:18:7
+   |
+LL |     if {} else {}
+   |       ^- if this block is the condition of the `if` expression, then it must be followed by another block
+   |       |
+   |       expected condition here
+
+error: this `if` expression is missing a block after the condition
+  --> $DIR/bad-if-statements.rs:23:5
+   |
+LL |     if true && {} else {}
+   |     ^^
+   |
+help: this binary operation is possibly unfinished
+  --> $DIR/bad-if-statements.rs:23:8
+   |
+LL |     if true && {} else {}
+   |        ^^^^^^^
+
+error: expected `{`, found `x`
+  --> $DIR/bad-if-statements.rs:29:13
+   |
+LL |     if true x else {}
+   |             ^ expected `{`
+   |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/bad-if-statements.rs:29:8
+   |
+LL |     if true x else {}
+   |        ^^^^
+help: try placing this code inside a block
+   |
+LL |     if true { x } else {}
+   |             +   +
+
+error: this `if` expression is missing a block after the condition
+  --> $DIR/bad-if-statements.rs:34:5
+   |
+LL |     if true else {}
+   |     ^^
+   |
+help: add a block here
+  --> $DIR/bad-if-statements.rs:34:12
+   |
+LL |     if true else {}
+   |            ^
+
+error: aborting due to 7 previous errors
+

src/test/ui/parser/if-block-unreachable-expr.rs

@@ -0,0 +1,8 @@
+// check-pass
+
+// This regressed from 1.20 -> 1.21 -- the condition is unreachable,
+// but it's still an expression, and should parse fine.
+
+fn main() {
+    if { if true { return; } else { return; }; } {}
+}

src/test/ui/parser/issue-61858.stderr

@@ -2,9 +2,13 @@ error: expected `{`, found `)`
   --> $DIR/issue-61858.rs:2:15
    |
 LL |     (if foobar)
-   |      --       ^ expected `{`
+   |               ^ expected `{`
-   |      |
+   |
-   |      this `if` expression has a condition, but no block
+note: the `if` expression is missing a block after this condition
+  --> $DIR/issue-61858.rs:2:9
+   |
+LL |     (if foobar)
+   |         ^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/parser/issue-68091-unicode-ident-after-if.rs

@@ -1,7 +1,8 @@
 macro_rules! x {
     ($($c:tt)*) => {
-        $($c)ö* {} //~ ERROR missing condition for `if` expression
+        $($c)ö* {}
-    };             //~| ERROR mismatched types
+        //~^ ERROR missing condition for `if` expression
+    };
 }
 
 fn main() {

src/test/ui/parser/issue-68091-unicode-ident-after-if.stderr

@@ -2,19 +2,9 @@ error: missing condition for `if` expression
   --> $DIR/issue-68091-unicode-ident-after-if.rs:3:14
    |
 LL |         $($c)ö* {}
-   |              ^ expected if condition here
+   |              ^  - if this block is the condition of the `if` expression, then it must be followed by another block
+   |              |
+   |              expected condition here
 
-error[E0308]: mismatched types
+error: aborting due to previous error
-  --> $DIR/issue-68091-unicode-ident-after-if.rs:3:17
-   |
-LL |         $($c)ö* {}
-   |                 ^^ expected `bool`, found `()`
-...
-LL |     x!(if);
-   |     ------ in this macro invocation
-   |
-   = note: this error originates in the macro `x` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/parser/issue-91421.rs

@@ -2,9 +2,8 @@
 
 fn main() {
     let value = if true && {
-    //~^ ERROR: this `if` expression has a condition, but no block
+    //~^ ERROR: this `if` expression is missing a block after the condition
-    //~| HELP: maybe you forgot the right operand of the condition?
+    //~| HELP: this binary operation is possibly unfinished
         3
-        //~^ ERROR: mismatched types [E0308]
     } else { 4 };
 }

src/test/ui/parser/issue-91421.stderr

@@ -1,21 +1,14 @@
-error: this `if` expression has a condition, but no block
+error: this `if` expression is missing a block after the condition
   --> $DIR/issue-91421.rs:4:17
    |
 LL |     let value = if true && {
    |                 ^^
    |
-help: maybe you forgot the right operand of the condition?
+help: this binary operation is possibly unfinished
-  --> $DIR/issue-91421.rs:4:25
+  --> $DIR/issue-91421.rs:4:20
    |
 LL |     let value = if true && {
-   |                         ^^
+   |                    ^^^^^^^
 
-error[E0308]: mismatched types
+error: aborting due to previous error
-  --> $DIR/issue-91421.rs:7:9
-   |
-LL |         3
-   |         ^ expected `bool`, found integer
 
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/parser/issues/issue-13483.rs

@@ -1,15 +1,15 @@
 fn main() {
     if true {
-    } else if { //~ ERROR missing condition
+    } else if {
-    //~^ ERROR mismatched types
+    //~^ ERROR missing condition for `if` expression
     } else {
     }
 }
 
 fn foo() {
     if true {
-    } else if { //~ ERROR missing condition
+    } else if {
-    //~^ ERROR mismatched types
+    //~^ ERROR missing condition for `if` expression
     }
     bar();
 }

src/test/ui/parser/issues/issue-13483.stderr

@@ -2,32 +2,17 @@ error: missing condition for `if` expression
   --> $DIR/issue-13483.rs:3:14
    |
 LL |     } else if {
-   |              ^ expected if condition here
+   |              ^- if this block is the condition of the `if` expression, then it must be followed by another block
+   |              |
+   |              expected condition here
 
 error: missing condition for `if` expression
   --> $DIR/issue-13483.rs:11:14
    |
 LL |     } else if {
-   |              ^ expected if condition here
+   |              ^- if this block is the condition of the `if` expression, then it must be followed by another block
+   |              |
+   |              expected condition here
 
-error[E0308]: mismatched types
+error: aborting due to 2 previous errors
-  --> $DIR/issue-13483.rs:3:15
-   |
-LL |       } else if {
-   |  _______________^
-LL | |
-LL | |     } else {
-   | |_____^ expected `bool`, found `()`
 
-error[E0308]: mismatched types
-  --> $DIR/issue-13483.rs:11:15
-   |
-LL |       } else if {
-   |  _______________^
-LL | |
-LL | |     }
-   | |_____^ expected `bool`, found `()`
-
-error: aborting due to 4 previous errors
-
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/parser/issues/issue-51602.stderr

@@ -2,9 +2,13 @@ error: expected `{`, found keyword `in`
   --> $DIR/issue-51602.rs:2:10
    |
 LL |     if i in 1..10 {
-   |     --   ^^ expected `{`
+   |          ^^ expected `{`
-   |     |
+   |
-   |     this `if` expression has a condition, but no block
+note: the `if` expression is missing a block after this condition
+  --> $DIR/issue-51602.rs:2:8
+   |
+LL |     if i in 1..10 {
+   |        ^
 
 error: aborting due to previous error
 

src/test/ui/parser/issues/issue-62554.stderr

@@ -57,10 +57,13 @@ error: expected `{`, found `macro_rules`
   --> $DIR/issue-62554.rs:6:23
    |
 LL | fn foo(u: u8) { if u8 macro_rules! u8 { (u6) => { fn uuuuuuuuuuu() { use s loo mod u8 {
-   |                 --    ^^^^^^^^^^^ expected `{`
+   |                       ^^^^^^^^^^^ expected `{`
-   |                 |
-   |                 this `if` expression has a condition, but no block
    |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/issue-62554.rs:6:20
+   |
+LL | fn foo(u: u8) { if u8 macro_rules! u8 { (u6) => { fn uuuuuuuuuuu() { use s loo mod u8 {
+   |                    ^^
 help: try placing this code inside a block
    |
 LL | fn foo(u: u8) { if u8 { macro_rules! u8 { (u6) => { fn uuuuuuuuuuu() { use s loo mod u8 { }

src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.rs

@@ -21,15 +21,15 @@ fn main() {
     };
 
     if let Some(n) = opt else {
-    //~^ ERROR missing condition for `if` expression
+    //~^ ERROR this `if` expression is missing a block after the condition
         return;
     };
     if let Some(n) = opt && n == 1 else {
-    //~^ ERROR missing condition for `if` expression
+    //~^ ERROR this `if` expression is missing a block after the condition
         return;
     };
     if let Some(n) = opt && let another = n else {
-    //~^ ERROR missing condition for `if` expression
+    //~^ ERROR this `if` expression is missing a block after the condition
         return;
     };
 

src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr

@@ -20,23 +20,41 @@ help: wrap the expression in parentheses
 LL |     let Some(n) = (opt && let another = n) else {
    |                   +                      +
 
-error: missing condition for `if` expression
+error: this `if` expression is missing a block after the condition
-  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:23:7
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:23:5
    |
 LL |     if let Some(n) = opt else {
-   |       ^ expected if condition here
+   |     ^^
+   |
+help: add a block here
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:23:25
+   |
+LL |     if let Some(n) = opt else {
+   |                         ^
 
-error: missing condition for `if` expression
+error: this `if` expression is missing a block after the condition
-  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:27:7
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:27:5
    |
 LL |     if let Some(n) = opt && n == 1 else {
-   |       ^ expected if condition here
+   |     ^^
+   |
+help: add a block here
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:27:35
+   |
+LL |     if let Some(n) = opt && n == 1 else {
+   |                                   ^
 
-error: missing condition for `if` expression
+error: this `if` expression is missing a block after the condition
-  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:31:7
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:31:5
    |
 LL |     if let Some(n) = opt && let another = n else {
-   |       ^ expected if condition here
+   |     ^^
+   |
+help: add a block here
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:31:44
+   |
+LL |     if let Some(n) = opt && let another = n else {
+   |                                            ^
 
 error: expected `{`, found keyword `else`
   --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:37:33