Fix unnecessary_min_or_max for usize (rust-lang#16575)

Fixes: rust-lang#16555

changelog: [unnecessary_min_or_max]: Fix false negatives for
usize::{MIN,MAX} by handling usize bit width correctly during constant
evaluation, enabling the lint to trigger on cases like n.min(0) /
n.max(usize::MAX).

Author: dswij

clippy_lints/src/methods/unnecessary_min_or_max.rs

@@ -88,7 +88,17 @@ fn detect_extrema<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<
     match (cv.int_value(cx.tcx, ty)?, ty.kind()) {
         (FullInt::S(i), &ty::Int(ity)) if i == i128::MIN >> (128 - ity.bit_width()?) => Some(Extrema::Minimum),
         (FullInt::S(i), &ty::Int(ity)) if i == i128::MAX >> (128 - ity.bit_width()?) => Some(Extrema::Maximum),
-        (FullInt::U(i), &ty::Uint(uty)) if i == u128::MAX >> (128 - uty.bit_width()?) => Some(Extrema::Maximum),
+        (FullInt::U(i), &ty::Uint(uty))
+            if {
+                let bits = match uty {
+                    ty::UintTy::Usize => u32::try_from(cx.tcx.data_layout.pointer_size().bits()).ok()?,
+                    _ => u32::try_from(uty.bit_width()?).ok()?,
+                };
+                i == u128::MAX >> (128 - bits)
+            } =>
+        {
+            Some(Extrema::Maximum)
+        },
         (FullInt::U(0), &ty::Uint(_)) => Some(Extrema::Minimum),
         _ => None,
     }

tests/ui/unnecessary_min_or_max.fixed

@@ -77,6 +77,32 @@ fn main() {
     let _ = (X + 1).min(12);
     let _ = 12.min(X - 1);
     let _ = 12.max(X - 1);
+
+    let n: usize = 42;
+
+    let _ = 0;
+    //~^ unnecessary_min_or_max
+
+    let _ = 0usize;
+    //~^ unnecessary_min_or_max
+
+    let _ = (0usize);
+    //~^ unnecessary_min_or_max
+
+    let _ = usize::MIN;
+    //~^ unnecessary_min_or_max
+
+    let _ = usize::MAX;
+    //~^ unnecessary_min_or_max
+
+    let _ = (usize::MAX);
+    //~^ unnecessary_min_or_max
+
+    let _ = !0usize;
+    //~^ unnecessary_min_or_max
+
+    let _ = n;
+    //~^ unnecessary_min_or_max
 }
 fn random_u32() -> u32 {
     // random number generator

tests/ui/unnecessary_min_or_max.rs

@@ -77,6 +77,32 @@ fn main() {
     let _ = (X + 1).min(12);
     let _ = 12.min(X - 1);
     let _ = 12.max(X - 1);
+
+    let n: usize = 42;
+
+    let _ = n.min(0);
+    //~^ unnecessary_min_or_max
+
+    let _ = n.min(0usize);
+    //~^ unnecessary_min_or_max
+
+    let _ = (0usize).min(n);
+    //~^ unnecessary_min_or_max
+
+    let _ = n.min(usize::MIN);
+    //~^ unnecessary_min_or_max
+
+    let _ = n.max(usize::MAX);
+    //~^ unnecessary_min_or_max
+
+    let _ = (usize::MAX).max(n);
+    //~^ unnecessary_min_or_max
+
+    let _ = n.max(!0usize);
+    //~^ unnecessary_min_or_max
+
+    let _ = n.max(0);
+    //~^ unnecessary_min_or_max
 }
 fn random_u32() -> u32 {
     // random number generator

tests/ui/unnecessary_min_or_max.stderr

@@ -103,5 +103,53 @@ error: `x` is never smaller than `i32::MIN - 0` and has therefore no effect
 LL |     let _ = x.min(i32::MIN - 0);
    |             ^^^^^^^^^^^^^^^^^^^ help: try: `i32::MIN - 0`
 
-error: aborting due to 17 previous errors
+error: `n` is never smaller than `0` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:83:13
+   |
+LL |     let _ = n.min(0);
+   |             ^^^^^^^^ help: try: `0`
+
+error: `n` is never smaller than `0usize` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:86:13
+   |
+LL |     let _ = n.min(0usize);
+   |             ^^^^^^^^^^^^^ help: try: `0usize`
+
+error: `(0usize)` is never greater than `n` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:89:13
+   |
+LL |     let _ = (0usize).min(n);
+   |             ^^^^^^^^^^^^^^^ help: try: `(0usize)`
+
+error: `n` is never smaller than `usize::MIN` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:92:13
+   |
+LL |     let _ = n.min(usize::MIN);
+   |             ^^^^^^^^^^^^^^^^^ help: try: `usize::MIN`
+
+error: `n` is never greater than `usize::MAX` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:95:13
+   |
+LL |     let _ = n.max(usize::MAX);
+   |             ^^^^^^^^^^^^^^^^^ help: try: `usize::MAX`
+
+error: `(usize::MAX)` is never smaller than `n` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:98:13
+   |
+LL |     let _ = (usize::MAX).max(n);
+   |             ^^^^^^^^^^^^^^^^^^^ help: try: `(usize::MAX)`
+
+error: `n` is never greater than `!0usize` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:101:13
+   |
+LL |     let _ = n.max(!0usize);
+   |             ^^^^^^^^^^^^^^ help: try: `!0usize`
+
+error: `n` is never smaller than `0` and has therefore no effect
+  --> tests/ui/unnecessary_min_or_max.rs:104:13
+   |
+LL |     let _ = n.max(0);
+   |             ^^^^^^^^ help: try: `n`
+
+error: aborting due to 25 previous errors