Auto merge of #105446 - erikdesjardins:vt-size, r=nikic

Add 0..=isize::MAX range metadata to size loads from vtables

This is the (much belated) size counterpart to #91569.

Inspired by https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/Range.20metadata.20for.20.60size_of_val.60.20and.20other.20isize.3A.3AMAX.20limits. This could help optimize layout computations based on the size of a dyn trait. Though, admittedly, adding this to vtables wouldn't be as beneficial as adding it to slice len, which is used much more often.

Miri detects this UB already: https://github.com/rust-lang/rust/blob/b7cc99142ad0cfe47e2fe9f7a82eaf5b672c0573/compiler/rustc_const_eval/src/interpret/traits.rs#L119-L121
(In fact Miri goes further, [assuming a 48-bit address space on 64-bit platforms](https://github.com/rust-lang/rust/blob/9db224fc908059986c179fc6ec433944e9cfce50/compiler/rustc_abi/src/lib.rs#L312-L331), but I don't think we can assume that in an optimization.)

1 files changed, 12 insertions, 0 deletions

diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index 8c71332bfab..4582d3c6bad 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -774,6 +774,18 @@ impl Integer {
         }
     }
 
+    /// Returns the largest signed value that can be represented by this Integer.
+    #[inline]
+    pub fn signed_max(self) -> i128 {
+        match self {
+            I8 => i8::MAX as i128,
+            I16 => i16::MAX as i128,
+            I32 => i32::MAX as i128,
+            I64 => i64::MAX as i128,
+            I128 => i128::MAX,
+        }
+    }
+
     /// Finds the smallest Integer type which can represent the signed value.
     #[inline]
     pub fn fit_signed(x: i128) -> Integer {