[rustc_parse] Forbid lets in certain places

compiler/rustc_parse/src/parser/expr.rs

@@ -7,6 +7,7 @@ use super::{
 };
 use crate::maybe_recover_from_interpolated_ty_qpath;
 
+use core::mem;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
 use rustc_ast::tokenstream::Spacing;
@@ -26,7 +27,6 @@ use rustc_session::lint::BuiltinLintDiagnostics;
 use rustc_span::source_map::{self, Span, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Pos};
-use std::mem;
 
 /// Possibly accepts an `token::Interpolated` expression (a pre-parsed expression
 /// dropped into the token stream, which happens while parsing the result of
@@ -2343,7 +2343,9 @@ impl<'a> Parser<'a> {
 
     /// Parses the condition of a `if` or `while` expression.
     fn parse_cond_expr(&mut self) -> PResult<'a, P<Expr>> {
-        let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+        let cond = self.with_let_management(true, |local_self| {
+            local_self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)
+        })?;
 
         if let ExprKind::Let(..) = cond.kind {
             // Remove the last feature gating of a `let` expression since it's stable.
@@ -2356,6 +2358,13 @@ impl<'a> Parser<'a> {
     /// Parses a `let $pat = $expr` pseudo-expression.
     /// The `let` token has already been eaten.
     fn parse_let_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        if !self.let_expr_allowed {
+            self.struct_span_err(
+                self.prev_token.span,
+                "expected expression, found `let` statement",
+            )
+            .emit();
+        }
         let lo = self.prev_token.span;
         let pat = self.parse_pat_allow_top_alt(
             None,
@@ -2672,6 +2681,8 @@ impl<'a> Parser<'a> {
     }
 
     pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
+        // Used to check the `let_chains` and `if_let_guard` features mostly by scaning
+        // `&&` tokens.
         fn check_let_expr(expr: &Expr) -> (bool, bool) {
             match expr.kind {
                 ExprKind::Binary(_, ref lhs, ref rhs) => {
@@ -2694,7 +2705,7 @@ impl<'a> Parser<'a> {
             )?;
             let guard = if this.eat_keyword(kw::If) {
                 let if_span = this.prev_token.span;
-                let cond = this.parse_expr()?;
+                let cond = this.with_let_management(true, |local_this| local_this.parse_expr())?;
                 let (has_let_expr, does_not_have_bin_op) = check_let_expr(&cond);
                 if has_let_expr {
                     if does_not_have_bin_op {
@@ -3256,4 +3267,17 @@ impl<'a> Parser<'a> {
             Ok((res, trailing))
         })
     }
+
+    // Calls `f` with the internal `let_expr_allowed` set to `let_expr_allowed` and then
+    // sets the internal `let_expr_allowed` back to its original value.
+    fn with_let_management<T>(
+        &mut self,
+        let_expr_allowed: bool,
+        f: impl FnOnce(&mut Self) -> T,
+    ) -> T {
+        let last_let_expr_allowed = mem::replace(&mut self.let_expr_allowed, let_expr_allowed);
+        let rslt = f(self);
+        self.let_expr_allowed = last_let_expr_allowed;
+        rslt
+    }
 }

compiler/rustc_parse/src/parser/mod.rs

@@ -147,12 +147,15 @@ pub struct Parser<'a> {
     /// This allows us to recover when the user forget to add braces around
     /// multiple statements in the closure body.
     pub current_closure: Option<ClosureSpans>,
+    /// Used to track where `let`s are allowed. For example, `if true && let 1 = 1` is valid
+    /// but `[1, 2, 3][let _ = ()]` is not.
+    let_expr_allowed: bool,
 }
 
 // This type is used a lot, e.g. it's cloned when matching many declarative macro rules. Make sure
 // it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(Parser<'_>, 328);
+rustc_data_structures::static_assert_size!(Parser<'_>, 336);
 
 /// Stores span information about a closure.
 #[derive(Clone)]
@@ -455,6 +458,7 @@ impl<'a> Parser<'a> {
                 inner_attr_ranges: Default::default(),
             },
             current_closure: None,
+            let_expr_allowed: false,
         };
 
         // Make parser point to the first token.

src/test/ui/mir/issue-92893.rs

@@ -1,6 +1,7 @@
 struct Bug<A = [(); (let a = (), 1).1]> {
     //~^ `let` expressions are not supported here
-    //~^^ `let` expressions in this position are unstable [E0658]
+    //~| `let` expressions in this position are unstable [E0658]
+    //~| expected expression, found `let` statement
     a: A
 }
 

src/test/ui/mir/issue-92893.stderr

@@ -1,3 +1,9 @@
+error: expected expression, found `let` statement
+  --> $DIR/issue-92893.rs:1:22
+   |
+LL | struct Bug<A = [(); (let a = (), 1).1]> {
+   |                      ^^^
+
 error: `let` expressions are not supported here
   --> $DIR/issue-92893.rs:1:22
    |
@@ -15,6 +21,6 @@ LL | struct Bug<A = [(); (let a = (), 1).1]> {
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2294-if-let-guard/feature-gate.rs

@@ -58,8 +58,10 @@ fn _macros() {
     }
     use_expr!((let 0 = 1 && 0 == 0));
     //~^ ERROR `let` expressions in this position are unstable
+    //~| ERROR expected expression, found `let` statement
     use_expr!((let 0 = 1));
     //~^ ERROR `let` expressions in this position are unstable
+    //~| ERROR expected expression, found `let` statement
     match () {
         #[cfg(FALSE)]
         () if let 0 = 1 => {}

src/test/ui/rfc-2294-if-let-guard/feature-gate.stderr

@@ -1,5 +1,17 @@
+error: expected expression, found `let` statement
+  --> $DIR/feature-gate.rs:59:16
+   |
+LL |     use_expr!((let 0 = 1 && 0 == 0));
+   |                ^^^
+
+error: expected expression, found `let` statement
+  --> $DIR/feature-gate.rs:62:16
+   |
+LL |     use_expr!((let 0 = 1));
+   |                ^^^
+
 error: no rules expected the token `let`
-  --> $DIR/feature-gate.rs:69:15
+  --> $DIR/feature-gate.rs:71:15
    |
 LL |     macro_rules! use_expr {
    |     --------------------- when calling this macro
@@ -58,7 +70,7 @@ LL |         () if let Range { start: _, end: _ } = (true..true) && false => {}
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
 error[E0658]: `if let` guards are experimental
-  --> $DIR/feature-gate.rs:65:12
+  --> $DIR/feature-gate.rs:67:12
    |
 LL |         () if let 0 = 1 => {}
    |            ^^^^^^^^^^^^
@@ -203,14 +215,14 @@ LL |     use_expr!((let 0 = 1 && 0 == 0));
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are unstable
-  --> $DIR/feature-gate.rs:61:16
+  --> $DIR/feature-gate.rs:62:16
    |
 LL |     use_expr!((let 0 = 1));
    |                ^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
 
-error: aborting due to 23 previous errors
+error: aborting due to 25 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs

@@ -81,9 +81,11 @@ fn _macros() {
     use_expr!((let 0 = 1 && 0 == 0));
     //~^ ERROR `let` expressions are not supported here
     //~| ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
     use_expr!((let 0 = 1));
     //~^ ERROR `let` expressions are not supported here
     //~| ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
 }
 
 fn nested_within_if_expr() {
@@ -147,7 +149,8 @@ fn nested_within_if_expr() {
     //~| ERROR mismatched types
     //~| ERROR mismatched types
 
-    if let true = let true = true {} //~ ERROR `let` expressions are not supported here
+    if let true = let true = true {}
+    //~^ ERROR `let` expressions are not supported here
 }
 
 fn nested_within_while_expr() {
@@ -211,7 +214,8 @@ fn nested_within_while_expr() {
     //~| ERROR mismatched types
     //~| ERROR mismatched types
 
-    while let true = let true = true {} //~ ERROR `let` expressions are not supported here
+    while let true = let true = true {}
+    //~^ ERROR `let` expressions are not supported here
 }
 
 fn not_error_because_clarified_intent() {
@@ -225,45 +229,85 @@ fn not_error_because_clarified_intent() {
 }
 
 fn outside_if_and_while_expr() {
-    &let 0 = 0; //~ ERROR `let` expressions are not supported here
+    &let 0 = 0;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
 
-    !let 0 = 0; //~ ERROR `let` expressions are not supported here
-    *let 0 = 0; //~ ERROR `let` expressions are not supported here
-    //~^ ERROR type `bool` cannot be dereferenced
-    -let 0 = 0; //~ ERROR `let` expressions are not supported here
-    //~^ ERROR cannot apply unary operator `-` to type `bool`
+    !let 0 = 0;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
+    *let 0 = 0;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR type `bool` cannot be dereferenced
+    //~| ERROR expected expression, found `let` statement
+    -let 0 = 0;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR cannot apply unary operator `-` to type `bool`
+    //~| ERROR expected expression, found `let` statement
 
     fn _check_try_binds_tighter() -> Result<(), ()> {
         let 0 = 0?;
         //~^ ERROR the `?` operator can only be applied to values that implement `Try`
         Ok(())
     }
-    (let 0 = 0)?; //~ ERROR `let` expressions are not supported here
-    //~^ ERROR the `?` operator can only be used in a function that returns `Result`
+    (let 0 = 0)?;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR the `?` operator can only be used in a function that returns `Result`
     //~| ERROR the `?` operator can only be applied to values that implement `Try`
+    //~| ERROR expected expression, found `let` statement
 
-    true || let 0 = 0; //~ ERROR `let` expressions are not supported here
-    (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
-    true && (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
+    true || let 0 = 0;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
+    (true || let 0 = 0);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
+    true && (true || let 0 = 0);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
 
     let mut x = true;
-    x = let 0 = 0; //~ ERROR `let` expressions are not supported here
+    x = let 0 = 0;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
 
-    true..(let 0 = 0); //~ ERROR `let` expressions are not supported here
-    ..(let 0 = 0); //~ ERROR `let` expressions are not supported here
-    (let 0 = 0)..; //~ ERROR `let` expressions are not supported here
+    true..(let 0 = 0);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
+    ..(let 0 = 0);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
+    (let 0 = 0)..;
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
 
     (let Range { start: _, end: _ } = true..true || false);
     //~^ ERROR `let` expressions are not supported here
     //~| ERROR mismatched types
+    //~| ERROR expected expression, found `let` statement
 
     (let true = let true = true);
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR expected expression, found `let` statement
+    //~| ERROR expected expression, found `let` statement
+
+    {
+        #[cfg(FALSE)]
+        let x = true && let y = 1;
+        //~^ ERROR expected expression, found `let` statement
+    }
+
+    #[cfg(FALSE)]
+    {
+        [1, 2, 3][let _ = ()]
+        //~^ ERROR expected expression, found `let` statement
+    }
 
     // Check function tail position.
     &let 0 = 0
     //~^ ERROR `let` expressions are not supported here
     //~| ERROR mismatched types
+    //~| ERROR expected expression, found `let` statement
 }
 
 // Let's make sure that `let` inside const generic arguments are considered.
@@ -335,4 +379,14 @@ fn with_parenthesis() {
     let fun = || true;
     if let true = (true && fun()) && (true) {
     }
+
+    #[cfg(FALSE)]
+    let x = (true && let y = 1);
+    //~^ ERROR expected expression, found `let` statement
+
+    #[cfg(FALSE)]
+    {
+        ([1, 2, 3][let _ = ()])
+        //~^ ERROR expected expression, found `let` statement
+    }
 }