Shrink the const block shape by the width of the const keyword

src/expr.rs

@@ -180,6 +180,14 @@ pub(crate) fn format_expr(
             .unknown_error()
             .and_then(|control_flow| control_flow.rewrite_result(context, shape)),
         ast::ExprKind::ConstBlock(ref anon_const) => {
+            // 6 = "const ". The keyword sits on the first line of the rewrite, so the
+            // block has that many fewer columns to work with. Gated because widening
+            // the block by six columns is stable formatting on earlier style editions.
+            let shape = if context.config.style_edition() >= StyleEdition::Edition2027 {
+                shape.offset_left_opt(6).unwrap_or(shape)
+            } else {
+                shape
+            };
             let rewrite = match anon_const.value.kind {
                 ast::ExprKind::Block(ref block, opt_label) => {
                     // Inner attributes are associated with the `ast::ExprKind::ConstBlock` node,

tests/source/issue_7055_narrow_max_width.rs

@@ -0,0 +1,21 @@
+// rustfmt-style_edition: 2027
+// rustfmt-max_width: 30
+// rustfmt-error_on_line_overflow: false
+
+// Fewer than six columns of
+// budget left: still format
+// it, do not bail out and
+// emit it verbatim.
+fn deep() {
+    if a {
+        if b {
+            if c {
+                if d {
+                    if e {
+                        let q = const {   1 + 2   };
+                    }
+                }
+            }
+        }
+    }
+}

tests/source/issue_7055_style_edition_2021.rs

@@ -0,0 +1,74 @@
+// rustfmt-style_edition: 2021
+// rustfmt-max_width: 100
+// rustfmt-error_on_line_overflow: false
+
+struct S;
+
+impl S {
+    const fn new(_: &str) -> Self {
+        S
+    }
+
+    fn go(&self) {}
+}
+
+// The reported symptom. As the receiver of a method call the block was rewritten at
+// the full width, the `const ` prefix pushed the result past `max_width`, the chain
+// rejected it, and the whole expression was silently left unformatted.
+fn receiver_74() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+fn receiver_77() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+fn receiver_80() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+// Not a method receiver, so the over-wide rewrite was accepted and emitted. This is
+// the stable formatting the gate protects: on style editions below 2027 these keep
+// producing a line past `max_width`.
+fn statement_74() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+fn statement_77() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+fn statement_80() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+// The `const ` budget applies to a nested block too.
+fn nested() {
+    let _ = const {
+        const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  }
+    };
+}
+
+// Inner attributes belong to the `ConstBlock` node, not the `Block`, so they take the
+// `rewrite_block` path directly. Guard that the shape change leaves them alone.
+fn inner_attrs() {
+    let _ = const {
+        #![allow(unused)]
+        S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )
+    };
+}

tests/source/issue_7055_style_edition_2024.rs

@@ -0,0 +1,74 @@
+// rustfmt-style_edition: 2024
+// rustfmt-max_width: 100
+// rustfmt-error_on_line_overflow: false
+
+struct S;
+
+impl S {
+    const fn new(_: &str) -> Self {
+        S
+    }
+
+    fn go(&self) {}
+}
+
+// The reported symptom. As the receiver of a method call the block was rewritten at
+// the full width, the `const ` prefix pushed the result past `max_width`, the chain
+// rejected it, and the whole expression was silently left unformatted.
+fn receiver_74() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+fn receiver_77() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+fn receiver_80() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+// Not a method receiver, so the over-wide rewrite was accepted and emitted. This is
+// the stable formatting the gate protects: on style editions below 2027 these keep
+// producing a line past `max_width`.
+fn statement_74() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+fn statement_77() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+fn statement_80() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+// The `const ` budget applies to a nested block too.
+fn nested() {
+    let _ = const {
+        const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  }
+    };
+}
+
+// Inner attributes belong to the `ConstBlock` node, not the `Block`, so they take the
+// `rewrite_block` path directly. Guard that the shape change leaves them alone.
+fn inner_attrs() {
+    let _ = const {
+        #![allow(unused)]
+        S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )
+    };
+}

tests/source/issue_7055_style_edition_2027.rs

@@ -0,0 +1,74 @@
+// rustfmt-style_edition: 2027
+// rustfmt-max_width: 100
+// rustfmt-error_on_line_overflow: false
+
+struct S;
+
+impl S {
+    const fn new(_: &str) -> Self {
+        S
+    }
+
+    fn go(&self) {}
+}
+
+// The reported symptom. As the receiver of a method call the block was rewritten at
+// the full width, the `const ` prefix pushed the result past `max_width`, the chain
+// rejected it, and the whole expression was silently left unformatted.
+fn receiver_74() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+fn receiver_77() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+fn receiver_80() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+// Not a method receiver, so the over-wide rewrite was accepted and emitted. This is
+// the stable formatting the gate protects: on style editions below 2027 these keep
+// producing a line past `max_width`.
+fn statement_74() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+fn statement_77() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+fn statement_80() {
+    const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  } ;
+}
+
+// The `const ` budget applies to a nested block too.
+fn nested() {
+    let _ = const {
+        const {  S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )  }
+    };
+}
+
+// Inner attributes belong to the `ConstBlock` node, not the `Block`, so they take the
+// `rewrite_block` path directly. Guard that the shape change leaves them alone.
+fn inner_attrs() {
+    let _ = const {
+        #![allow(unused)]
+        S::new( "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" )
+    };
+}

tests/target/issue_7055_narrow_max_width.rs

@@ -0,0 +1,23 @@
+// rustfmt-style_edition: 2027
+// rustfmt-max_width: 30
+// rustfmt-error_on_line_overflow: false
+
+// Fewer than six columns of
+// budget left: still format
+// it, do not bail out and
+// emit it verbatim.
+fn deep() {
+    if a {
+        if b {
+            if c {
+                if d {
+                    if e {
+                        let q = const {
+                            1 + 2
+                        };
+                    }
+                }
+            }
+        }
+    }
+}

tests/target/issue_7055_style_edition_2021.rs

@@ -0,0 +1,70 @@
+// rustfmt-style_edition: 2021
+// rustfmt-max_width: 100
+// rustfmt-error_on_line_overflow: false
+
+struct S;
+
+impl S {
+    const fn new(_: &str) -> Self {
+        S
+    }
+
+    fn go(&self) {}
+}
+
+// The reported symptom. As the receiver of a method call the block was rewritten at
+// the full width, the `const ` prefix pushed the result past `max_width`, the chain
+// rejected it, and the whole expression was silently left unformatted.
+fn receiver_74() {
+    const { S::new("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") }
+        .go();
+}
+
+fn receiver_77() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+fn receiver_80() {
+    const {
+        S::new(
+            "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+        )
+    }
+    .go();
+}
+
+// Not a method receiver, so the over-wide rewrite was accepted and emitted. This is
+// the stable formatting the gate protects: on style editions below 2027 these keep
+// producing a line past `max_width`.
+fn statement_74() {
+    const { S::new("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") };
+}
+
+fn statement_77() {
+    const { S::new("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") };
+}
+
+fn statement_80() {
+    const { S::new("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") };
+}
+
+// The `const ` budget applies to a nested block too.
+fn nested() {
+    let _ = const {
+        const { S::new("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") }
+    };
+}
+
+// Inner attributes belong to the `ConstBlock` node, not the `Block`, so they take the
+// `rewrite_block` path directly. Guard that the shape change leaves them alone.
+fn inner_attrs() {
+    let _ = const {
+        #![allow(unused)]
+        S::new("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+    };
+}