feat: add more tests for the const_is_empty lint

Suggested by @xFredNet and @matthiaskrgr.

tests/ui/const_is_empty.rs

@@ -1,4 +1,6 @@
+#![feature(inline_const)]
 #![warn(clippy::const_is_empty)]
+#![allow(clippy::needless_late_init, unused_must_use)]
 
 fn test_literal() {
     if "".is_empty() {
@@ -99,3 +101,69 @@ fn main() {
     let _ = b"".is_empty();
     //~^ ERROR: this expression always evaluates to true
 }
+
+fn str_from_arg(var: &str) {
+    var.is_empty();
+    // Do not lint, we know nothiny about var
+}
+
+fn update_str() {
+    let mut value = "duck";
+    value = "penguin";
+
+    let _ = value.is_empty();
+    // Do not lint since value is mutable
+}
+
+fn macros() {
+    // Content from Macro
+    let file = include_str!("const_is_empty.rs");
+    let _ = file.is_empty();
+    // No lint because initializer comes from a macro result
+
+    let var = env!("PATH");
+    let _ = var.is_empty();
+    // No lint because initializer comes from a macro result
+}
+
+fn conditional_value() {
+    let value;
+
+    if true {
+        value = "hey";
+    } else {
+        value = "hej";
+    }
+
+    let _ = value.is_empty();
+    // Do not lint, current constant folding is too simple to detect this
+}
+
+fn cfg_conditioned() {
+    #[cfg(test)]
+    let val = "";
+    #[cfg(not(test))]
+    let val = "foo";
+
+    let _ = val.is_empty();
+    // Do not lint, value depend on a #[cfg(…)] directive
+}
+
+fn not_cfg_conditioned() {
+    let val = "";
+    #[cfg(not(target_os = "inexistent"))]
+    let _ = val.is_empty();
+    //~^ ERROR: this expression always evaluates to true
+}
+
+const fn const_rand() -> &'static str {
+    "17"
+}
+
+fn const_expressions() {
+    let _ = const { if true { "1" } else { "2" } }.is_empty();
+    // Do not lint, we do not recurse into boolean expressions
+
+    let _ = const_rand().is_empty();
+    // Do not lint, we do not recurse into functions
+}

tests/ui/const_is_empty.stderr

@@ -1,5 +1,5 @@
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:4:8
+  --> tests/ui/const_is_empty.rs:6:8
    |
 LL |     if "".is_empty() {
    |        ^^^^^^^^^^^^^
@@ -8,148 +8,154 @@ LL |     if "".is_empty() {
    = help: to override `-D warnings` add `#[allow(clippy::const_is_empty)]`
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:7:8
+  --> tests/ui/const_is_empty.rs:9:8
    |
 LL |     if "foobar".is_empty() {
    |        ^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:13:8
+  --> tests/ui/const_is_empty.rs:15:8
    |
 LL |     if b"".is_empty() {
    |        ^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:16:8
+  --> tests/ui/const_is_empty.rs:18:8
    |
 LL |     if b"foobar".is_empty() {
    |        ^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:33:8
+  --> tests/ui/const_is_empty.rs:35:8
    |
 LL |     if empty2.is_empty() {
    |        ^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:36:8
+  --> tests/ui/const_is_empty.rs:38:8
    |
 LL |     if non_empty2.is_empty() {
    |        ^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:58:13
+  --> tests/ui/const_is_empty.rs:60:13
    |
 LL |     let _ = EMPTY_STR.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:60:13
+  --> tests/ui/const_is_empty.rs:62:13
    |
 LL |     let _ = NON_EMPTY_STR.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:62:13
+  --> tests/ui/const_is_empty.rs:64:13
    |
 LL |     let _ = EMPTY_BSTR.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:64:13
+  --> tests/ui/const_is_empty.rs:66:13
    |
 LL |     let _ = NON_EMPTY_BSTR.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:66:13
+  --> tests/ui/const_is_empty.rs:68:13
    |
 LL |     let _ = EMPTY_ARRAY.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:68:13
+  --> tests/ui/const_is_empty.rs:70:13
    |
 LL |     let _ = EMPTY_ARRAY_REPEAT.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:70:13
+  --> tests/ui/const_is_empty.rs:72:13
    |
 LL |     let _ = EMPTY_U8_SLICE.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:72:13
+  --> tests/ui/const_is_empty.rs:74:13
    |
 LL |     let _ = NON_EMPTY_U8_SLICE.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:74:13
+  --> tests/ui/const_is_empty.rs:76:13
    |
 LL |     let _ = NON_EMPTY_ARRAY.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:76:13
+  --> tests/ui/const_is_empty.rs:78:13
    |
 LL |     let _ = NON_EMPTY_ARRAY_REPEAT.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:78:13
+  --> tests/ui/const_is_empty.rs:80:13
    |
 LL |     let _ = EMPTY_REF_ARRAY.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:80:13
+  --> tests/ui/const_is_empty.rs:82:13
    |
 LL |     let _ = NON_EMPTY_REF_ARRAY.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:82:13
+  --> tests/ui/const_is_empty.rs:84:13
    |
 LL |     let _ = EMPTY_SLICE.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:84:13
+  --> tests/ui/const_is_empty.rs:86:13
    |
 LL |     let _ = NON_EMPTY_SLICE.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:86:13
+  --> tests/ui/const_is_empty.rs:88:13
    |
 LL |     let _ = NON_EMPTY_SLICE_REPEAT.is_empty();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:92:13
+  --> tests/ui/const_is_empty.rs:94:13
    |
 LL |     let _ = value.is_empty();
    |             ^^^^^^^^^^^^^^^^
 
 error: this expression always evaluates to false
-  --> tests/ui/const_is_empty.rs:95:13
+  --> tests/ui/const_is_empty.rs:97:13
    |
 LL |     let _ = x.is_empty();
    |             ^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:97:13
+  --> tests/ui/const_is_empty.rs:99:13
    |
 LL |     let _ = "".is_empty();
    |             ^^^^^^^^^^^^^
 
 error: this expression always evaluates to true
-  --> tests/ui/const_is_empty.rs:99:13
+  --> tests/ui/const_is_empty.rs:101:13
    |
 LL |     let _ = b"".is_empty();
    |             ^^^^^^^^^^^^^^
 
-error: aborting due to 25 previous errors
+error: this expression always evaluates to true
+  --> tests/ui/const_is_empty.rs:155:13
+   |
+LL |     let _ = val.is_empty();
+   |             ^^^^^^^^^^^^^^
+
+error: aborting due to 26 previous errors