mir: unused_generic_params query

This commit implements the `unused_generic_params` query, an initial
version of polymorphization which detects when an item does not use
generic parameters and is being needlessly monomorphized as a result.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/polymorphization/const_parameters/closures.rs

@@ -0,0 +1,61 @@
+// build-fail
+// compile-flags: -Zpolymorphize-errors
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+
+// This test checks that the polymorphization analysis correctly detects unused const
+// parameters in closures.
+
+// Function doesn't have any generic parameters to be unused.
+pub fn no_parameters() {
+    let _ = || {};
+}
+
+// Function has an unused generic parameter in parent and closure.
+pub fn unused<const T: usize>() -> usize {
+//~^ ERROR item has unused generic parameters
+    let add_one = |x: usize| x + 1;
+//~^ ERROR item has unused generic parameters
+    add_one(3)
+}
+
+// Function has an unused generic parameter in closure, but not in parent.
+pub fn used_parent<const T: usize>() -> usize {
+    let x: usize = T;
+    let add_one = |x: usize| x + 1;
+//~^ ERROR item has unused generic parameters
+    x + add_one(3)
+}
+
+// Function uses generic parameter in value of a binding in closure.
+pub fn used_binding<const T: usize>() -> usize {
+    let x = || {
+        let y: usize = T;
+        y
+    };
+
+    x()
+}
+
+// Closure uses a value as an upvar, which used the generic parameter.
+pub fn unused_upvar<const T: usize>() -> usize {
+    let x: usize = T;
+    let y = || x;
+//~^ ERROR item has unused generic parameters
+    y()
+}
+
+// Closure uses generic parameter in substitutions to another function.
+pub fn used_substs<const T: usize>() -> usize {
+    let x = || unused::<T>();
+    x()
+}
+
+fn main() {
+    no_parameters();
+    let _ = unused::<1>();
+    let _ = used_parent::<1>();
+    let _ = used_binding::<1>();
+    let _ = unused_upvar::<1>();
+    let _ = used_substs::<1>();
+}

src/test/ui/polymorphization/const_parameters/closures.stderr

@@ -0,0 +1,44 @@
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/closures.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
+
+error: item has unused generic parameters
+  --> $DIR/closures.rs:17:19
+   |
+LL | pub fn unused<const T: usize>() -> usize {
+   |                     - generic parameter `T` is unused
+LL |
+LL |     let add_one = |x: usize| x + 1;
+   |                   ^^^^^^^^^^^^^^^^
+
+error: item has unused generic parameters
+  --> $DIR/closures.rs:15:8
+   |
+LL | pub fn unused<const T: usize>() -> usize {
+   |        ^^^^^^       - generic parameter `T` is unused
+
+error: item has unused generic parameters
+  --> $DIR/closures.rs:25:19
+   |
+LL | pub fn used_parent<const T: usize>() -> usize {
+   |                          - generic parameter `T` is unused
+LL |     let x: usize = T;
+LL |     let add_one = |x: usize| x + 1;
+   |                   ^^^^^^^^^^^^^^^^
+
+error: item has unused generic parameters
+  --> $DIR/closures.rs:43:13
+   |
+LL | pub fn unused_upvar<const T: usize>() -> usize {
+   |                           - generic parameter `T` is unused
+LL |     let x: usize = T;
+LL |     let y = || x;
+   |             ^^^^
+
+error: aborting due to 4 previous errors; 1 warning emitted
+

src/test/ui/polymorphization/const_parameters/functions.rs

@@ -0,0 +1,33 @@
+// build-fail
+// compile-flags: -Zpolymorphize-errors
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+
+// This test checks that the polymorphization analysis correctly detects unused const
+// parameters in functions.
+
+// Function doesn't have any generic parameters to be unused.
+pub fn no_parameters() {}
+
+// Function has an unused generic parameter.
+pub fn unused<const T: usize>() {
+//~^ ERROR item has unused generic parameters
+}
+
+// Function uses generic parameter in value of a binding.
+pub fn used_binding<const T: usize>() -> usize {
+    let x: usize = T;
+    x
+}
+
+// Function uses generic parameter in substitutions to another function.
+pub fn used_substs<const T: usize>() {
+    unused::<T>()
+}
+
+fn main() {
+    no_parameters();
+    unused::<1>();
+    used_binding::<1>();
+    used_substs::<1>();
+}

src/test/ui/polymorphization/const_parameters/functions.stderr

@@ -0,0 +1,17 @@
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/functions.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
+
+error: item has unused generic parameters
+  --> $DIR/functions.rs:13:8
+   |
+LL | pub fn unused<const T: usize>() {
+   |        ^^^^^^       - generic parameter `T` is unused
+
+error: aborting due to previous error; 1 warning emitted
+