Add regression test for rust-lang#131901

Author: GuillaumeGomez

tests/rustdoc/footnote-ids.rs

@@ -0,0 +1,41 @@
+// This test ensures that footnotes ID are not duplicated across an item page.
+// This is a regression test for <https://github.com/rust-lang/rust/issues/131901>.
+
+#![crate_name = "foo"]
+
+//@ has 'foo/struct.Foo.html'
+
+pub struct Foo;
+
+impl Foo {
+    //@ has - '//a[@href="#fn1"]' '1'
+    //@ has - '//li[@id="fn1"]' 'Hiya'
+    //@ has - '//a[@href="#fn2"]' '2'
+    //@ has - '//li[@id="fn2"]' 'Tiya'
+    /// Link 1 [^1]
+    /// Link 1.1 [^2]
+    ///
+    /// [^1]: Hiya
+    /// [^2]: Tiya
+    pub fn l1(){}
+
+    //@ has - '//a[@href="#fn3"]' '1'
+    //@ has - '//li[@id="fn3"]' 'Yiya'
+    //@ has - '//a[@href="#fn4"]' '2'
+    //@ has - '//li[@id="fn4"]' 'Biya'
+    /// Link 2 [^1]
+    /// Link 3 [^2]
+    ///
+    /// [^1]: Yiya
+    /// [^2]: Biya
+    pub fn l2() {}
+}
+
+impl Foo {
+    //@ has - '//a[@href="#fn5"]' '1'
+    //@ has - '//li[@id="fn5"]' 'Ciya'
+    /// Link 3 [^1]
+    ///
+    /// [^1]: Ciya
+    pub fn l3(){}
+}