Auto merge of #97474 - compiler-errors:if-cond-and-block, r=oli-obk

Improve parsing errors and suggestions for bad `if` statements

1. Parses `if {}` as `if <err> {}` (block-like conditions that are missing a "then" block), and `if true && {}` as `if true && <err> {}` (unfinished binary operation), which is a more faithful recovery and leads to better typeck errors later on.
1. Points out the span of the condition if we don't see a "then" block after it, to help the user understand what is being parsed as a condition (and by elimination, what isn't).
1. Allow `if cond token else { }` to be fixed properly to `if cond { token } else { }`.
1. Fudge with the error messages a bit. This is somewhat arbitrary and I can revert my rewordings if they're useless.

----

Also this PR addresses a strange parsing regression (1.20 -> 1.21) where we chose to reject this piece of code somewhat arbitrarily, even though we should parse it fine:

```rust
fn main() {
    if { if true { return } else { return }; } {}
}
```

For context, all of these other expressions parse correctly:

```rust
fn main() {
    if { if true { return } else { return } } {}
    if { return; } {}
    if { return } {}
    if { return if true { } else { }; } {}
}
```

The parser used a heuristic to determine if the "the parsed `if` condition makes sense as a condition" that did like a one-expr-deep reachability analysis. This should not be handled by the parser though.

19 files changed, 243 insertions, 100 deletions

diff --git a/src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.stderr b/src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.stderr
index 609a5efd46f..3ccc14bba54 100644
--- a/src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.stderr
+++ b/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
diff --git a/src/test/ui/expr/if/if-without-block.rs b/src/test/ui/expr/if/if-without-block.rs
index 8a4c59f3261..5add9dfda4b 100644
--- a/src/test/ui/expr/if/if-without-block.rs
+++ b/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 `{`
diff --git a/src/test/ui/expr/if/if-without-block.stderr b/src/test/ui/expr/if/if-without-block.stderr
index d3f6ca07617..2d1ee04ce09 100644
--- a/src/test/ui/expr/if/if-without-block.stderr
+++ b/src/test/ui/expr/if/if-without-block.stderr
@@ -1,17 +1,14 @@
-error: expected `{`, found `}`
-  --> $DIR/if-without-block.rs:7:1
+error: this `if` expression is missing a block after the condition
+  --> $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?
-  --> $DIR/if-without-block.rs:3:10
+help: this binary operation is possibly unfinished
+  --> $DIR/if-without-block.rs:3:8
    |
 LL |     if 5 == {
-   |          ^^
+   |        ^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-39848.stderr b/src/test/ui/issues/issue-39848.stderr
index feabe3814dc..387ef0776ff 100644
--- a/src/test/ui/issues/issue-39848.stderr
+++ b/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 `{`
-   |         |
-   |         this `if` expression has a condition, but no block
+   |                     ^^^^^^ expected `{`
 ...
 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
    |
diff --git a/src/test/ui/missing/missing-block-hint.stderr b/src/test/ui/missing/missing-block-hint.stderr
index 148e214e58e..16954223a45 100644
--- a/src/test/ui/missing/missing-block-hint.stderr
+++ b/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 `{`
-   |         |
-   |         this `if` expression has a condition, but no block
+   |                  ^^ expected `{`
+   |
+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; }
diff --git a/src/test/ui/parser/bad-if-statements.rs b/src/test/ui/parser/bad-if-statements.rs
new file mode 100644
index 00000000000..2c501e3a5b3
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/parser/bad-if-statements.stderr b/src/test/ui/parser/bad-if-statements.stderr
new file mode 100644
index 00000000000..ee839db6455
--- /dev/null
+++ b/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
+
diff --git a/src/test/ui/parser/if-block-unreachable-expr.rs b/src/test/ui/parser/if-block-unreachable-expr.rs
new file mode 100644
index 00000000000..4063a337084
--- /dev/null
+++ b/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; }; } {}
+}
diff --git a/src/test/ui/parser/issue-61858.stderr b/src/test/ui/parser/issue-61858.stderr
index 8b95d9c6ae4..03f51c6e3a8 100644
--- a/src/test/ui/parser/issue-61858.stderr
+++ b/src/test/ui/parser/issue-61858.stderr
@@ -2,9 +2,13 @@ error: expected `{`, found `)`
   --> $DIR/issue-61858.rs:2:15
    |
 LL |     (if foobar)
-   |      --       ^ expected `{`
-   |      |
-   |      this `if` expression has a condition, but no block
+   |               ^ expected `{`
+   |
+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
 
diff --git a/src/test/ui/parser/issue-68091-unicode-ident-after-if.rs b/src/test/ui/parser/issue-68091-unicode-ident-after-if.rs
index 00f90cc73b3..57d36feb37b 100644
--- a/src/test/ui/parser/issue-68091-unicode-ident-after-if.rs
+++ b/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
-    };             //~| ERROR mismatched types
+        $($c)ö* {}
+        //~^ ERROR missing condition for `if` expression
+    };
 }
 
 fn main() {
diff --git a/src/test/ui/parser/issue-68091-unicode-ident-after-if.stderr b/src/test/ui/parser/issue-68091-unicode-ident-after-if.stderr
index cdd4c670500..6674b924e9c 100644
--- a/src/test/ui/parser/issue-68091-unicode-ident-after-if.stderr
+++ b/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
-  --> $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
+error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/parser/issue-91421.rs b/src/test/ui/parser/issue-91421.rs
index 9959df56638..8bba27f3724 100644
--- a/src/test/ui/parser/issue-91421.rs
+++ b/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
-    //~| HELP: maybe you forgot the right operand of the condition?
+    //~^ ERROR: this `if` expression is missing a block after the condition
+    //~| HELP: this binary operation is possibly unfinished
         3
-        //~^ ERROR: mismatched types [E0308]
     } else { 4 };
 }
diff --git a/src/test/ui/parser/issue-91421.stderr b/src/test/ui/parser/issue-91421.stderr
index 04284d5e3b2..2d9652051dd 100644
--- a/src/test/ui/parser/issue-91421.stderr
+++ b/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?
-  --> $DIR/issue-91421.rs:4:25
+help: this binary operation is possibly unfinished
+  --> $DIR/issue-91421.rs:4:20
    |
 LL |     let value = if true && {
-   |                         ^^
+   |                    ^^^^^^^
 
-error[E0308]: mismatched types
-  --> $DIR/issue-91421.rs:7:9
-   |
-LL |         3
-   |         ^ expected `bool`, found integer
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/parser/issues/issue-13483.rs b/src/test/ui/parser/issues/issue-13483.rs
index a2fd9264b15..4e32fcab3f0 100644
--- a/src/test/ui/parser/issues/issue-13483.rs
+++ b/src/test/ui/parser/issues/issue-13483.rs
@@ -1,15 +1,15 @@
 fn main() {
     if true {
-    } else if { //~ ERROR missing condition
-    //~^ ERROR mismatched types
+    } else if {
+    //~^ ERROR missing condition for `if` expression
     } else {
     }
 }
 
 fn foo() {
     if true {
-    } else if { //~ ERROR missing condition
-    //~^ ERROR mismatched types
+    } else if {
+    //~^ ERROR missing condition for `if` expression
     }
     bar();
 }
diff --git a/src/test/ui/parser/issues/issue-13483.stderr b/src/test/ui/parser/issues/issue-13483.stderr
index 5fd05b18ce0..f5534090f18 100644
--- a/src/test/ui/parser/issues/issue-13483.stderr
+++ b/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
-  --> $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
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/parser/issues/issue-51602.stderr b/src/test/ui/parser/issues/issue-51602.stderr
index d800890bca3..4a5653fdb51 100644
--- a/src/test/ui/parser/issues/issue-51602.stderr
+++ b/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 `{`
-   |     |
-   |     this `if` expression has a condition, but no block
+   |          ^^ expected `{`
+   |
+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
 
diff --git a/src/test/ui/parser/issues/issue-62554.stderr b/src/test/ui/parser/issues/issue-62554.stderr
index 3589016e1dc..9e62572e388 100644
--- a/src/test/ui/parser/issues/issue-62554.stderr
+++ b/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 `{`
-   |                 |
-   |                 this `if` expression has a condition, but no block
+   |                       ^^^^^^^^^^^ expected `{`
    |
+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 { }
diff --git a/src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.rs b/src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.rs
index e24649ea044..e66caa19ec9 100644
--- a/src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.rs
+++ b/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;
     };
 
diff --git a/src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr b/src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr
index aebfc1a72b7..eea8ed0c963 100644
--- a/src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr
+++ b/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
-  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:23:7
+error: this `if` expression is missing a block after the condition
+  --> $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
-  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:27:7
+error: this `if` expression is missing a block after the condition
+  --> $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
-  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:31:7
+error: this `if` expression is missing a block after the condition
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:31:5
+   |
+LL |     if let Some(n) = opt && let another = n else {
+   |     ^^
+   |
+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 {
-   |       ^ expected if condition here
+   |                                            ^
 
 error: expected `{`, found keyword `else`
   --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:37:33