Don't warn about snake case for field puns that don't introduce a new name.

Author: jumbatm

src/librustc_lint/nonstandard_style.rs

@@ -350,7 +350,20 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonSnakeCase {
     }
 
     fn check_pat(&mut self, cx: &LateContext<'_, '_>, p: &hir::Pat<'_>) {
-        if let &PatKind::Binding(_, _, ident, _) = &p.kind {
+        if let &PatKind::Binding(_, hid, ident, _) = &p.kind {
+            if let hir::Node::Pat(parent_pat) = cx.tcx.hir().get(cx.tcx.hir().get_parent_node(hid))
+            {
+                if let PatKind::Struct(_, field_pats, _) = &parent_pat.kind {
+                    for field in field_pats.iter() {
+                        if field.ident != ident {
+                            // Only check if a new name has been introduced, to avoid warning
+                            // on both the struct definition and this pattern.
+                            self.check_snake_case(cx, "variable", &ident);
+                        }
+                    }
+                    return;
+                }
+            }
             self.check_snake_case(cx, "variable", &ident);
         }
     }

src/test/ui/lint/issue-66362-no-snake-case-warning-for-field-puns.rs

@@ -0,0 +1,29 @@
+#![deny(non_snake_case)]
+#![allow(unused_variables)]
+#![allow(dead_code)]
+
+enum Foo {
+    Bad {
+        lowerCamelCaseName: bool,
+        //~^ ERROR structure field `lowerCamelCaseName` should have a snake case name
+    },
+    Good {
+        snake_case_name: bool,
+    },
+}
+
+fn main() {
+    let b = Foo::Bad { lowerCamelCaseName: true };
+
+    match b {
+        Foo::Bad { lowerCamelCaseName } => {}
+        Foo::Good { snake_case_name: lowerCamelCaseBinding } => { }
+        //~^ ERROR variable `lowerCamelCaseBinding` should have a snake case name
+    }
+
+    if let Foo::Good { snake_case_name: anotherLowerCamelCaseBinding } = b { }
+    //~^ ERROR variable `anotherLowerCamelCaseBinding` should have a snake case name
+
+    if let Foo::Bad { lowerCamelCaseName: yetAnotherLowerCamelCaseBinding } = b { }
+    //~^ ERROR variable `yetAnotherLowerCamelCaseBinding` should have a snake case name
+}

src/test/ui/lint/issue-66362-no-snake-case-warning-for-field-puns.stderr

@@ -0,0 +1,32 @@
+error: structure field `lowerCamelCaseName` should have a snake case name
+  --> $DIR/issue-66362-no-snake-case-warning-for-field-puns.rs:7:9
+   |
+LL |         lowerCamelCaseName: bool,
+   |         ^^^^^^^^^^^^^^^^^^ help: convert the identifier to snake case: `lower_camel_case_name`
+   |
+note: lint level defined here
+  --> $DIR/issue-66362-no-snake-case-warning-for-field-puns.rs:1:9
+   |
+LL | #![deny(non_snake_case)]
+   |         ^^^^^^^^^^^^^^
+
+error: variable `lowerCamelCaseBinding` should have a snake case name
+  --> $DIR/issue-66362-no-snake-case-warning-for-field-puns.rs:20:38
+   |
+LL |         Foo::Good { snake_case_name: lowerCamelCaseBinding } => { }
+   |                                      ^^^^^^^^^^^^^^^^^^^^^ help: convert the identifier to snake case: `lower_camel_case_binding`
+
+error: variable `anotherLowerCamelCaseBinding` should have a snake case name
+  --> $DIR/issue-66362-no-snake-case-warning-for-field-puns.rs:24:41
+   |
+LL |     if let Foo::Good { snake_case_name: anotherLowerCamelCaseBinding } = b { }
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert the identifier to snake case: `another_lower_camel_case_binding`
+
+error: variable `yetAnotherLowerCamelCaseBinding` should have a snake case name
+  --> $DIR/issue-66362-no-snake-case-warning-for-field-puns.rs:27:43
+   |
+LL |     if let Foo::Bad { lowerCamelCaseName: yetAnotherLowerCamelCaseBinding } = b { }
+   |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert the identifier to snake case: `yet_another_lower_camel_case_binding`
+
+error: aborting due to 4 previous errors
+