Add a mir pre-codegen test for if T::ASSOC_CONST

Author: scottmcm

tests/mir-opt/pre-codegen/if_associated_const.check_bool.PreCodegen.after.mir

@@ -0,0 +1,23 @@
+// MIR for `check_bool` after PreCodegen
+
+fn check_bool() -> u32 {
+    let mut _0: u32;
+
+    bb0: {
+        switchInt(const <T as TraitWithBool>::FLAG) -> [0: bb1, otherwise: bb2];
+    }
+
+    bb1: {
+        _0 = const 456_u32;
+        goto -> bb3;
+    }
+
+    bb2: {
+        _0 = const 123_u32;
+        goto -> bb3;
+    }
+
+    bb3: {
+        return;
+    }
+}

tests/mir-opt/pre-codegen/if_associated_const.check_int.PreCodegen.after.mir

@@ -0,0 +1,33 @@
+// MIR for `check_int` after PreCodegen
+
+fn check_int() -> u32 {
+    let mut _0: u32;
+
+    bb0: {
+        switchInt(const <T as TraitWithInt>::VALUE) -> [1: bb1, 2: bb2, 3: bb3, otherwise: bb4];
+    }
+
+    bb1: {
+        _0 = const 123_u32;
+        goto -> bb5;
+    }
+
+    bb2: {
+        _0 = const 456_u32;
+        goto -> bb5;
+    }
+
+    bb3: {
+        _0 = const 789_u32;
+        goto -> bb5;
+    }
+
+    bb4: {
+        _0 = const 0_u32;
+        goto -> bb5;
+    }
+
+    bb5: {
+        return;
+    }
+}

tests/mir-opt/pre-codegen/if_associated_const.rs

@@ -0,0 +1,27 @@
+// skip-filecheck
+//@ compile-flags: -O -Zmir-opt-level=2 -Cdebuginfo=2
+
+#![crate_type = "lib"]
+
+pub trait TraitWithBool {
+    const FLAG: bool;
+}
+
+// EMIT_MIR if_associated_const.check_bool.PreCodegen.after.mir
+pub fn check_bool<T: TraitWithBool>() -> u32 {
+    if T::FLAG { 123 } else { 456 }
+}
+
+pub trait TraitWithInt {
+    const VALUE: i32;
+}
+
+// EMIT_MIR if_associated_const.check_int.PreCodegen.after.mir
+pub fn check_int<T: TraitWithInt>() -> u32 {
+    match T::VALUE {
+        1 => 123,
+        2 => 456,
+        3 => 789,
+        _ => 0,
+    }
+}