Rollup merge of #120504 - kornelski:try_with_capacity, r=Amanieu

Vec::try_with_capacity

Related to #91913

Implements try_with_capacity for `Vec`, `VecDeque`, and `String`. I can follow it up with more collections if desired.

`Vec::try_with_capacity()` is functionally equivalent to the current stable:

```rust
let mut v = Vec::new();
v.try_reserve_exact(n)?
```

However, `try_reserve` calls non-inlined `finish_grow`, which requires old and new `Layout`, and is designed to reallocate memory. There is benefit to using `try_with_capacity`, besides syntax convenience, because it generates much smaller code at the call site with a direct call to the allocator. There's codegen test included.

It's also a very desirable functionality for users of `no_global_oom_handling` (Rust-for-Linux), since it makes a very commonly used function available in that environment (`with_capacity` is used much more frequently than all `(try_)reserve(_exact)`).

1 files changed, 35 insertions, 0 deletions

diff --git a/tests/codegen/vec-with-capacity.rs b/tests/codegen/vec-with-capacity.rs
new file mode 100644
index 00000000000..47051f2eef8
--- /dev/null
+++ b/tests/codegen/vec-with-capacity.rs
@@ -0,0 +1,35 @@
+//@ compile-flags: -O
+//@ ignore-debug
+// (with debug assertions turned on, `assert_unchecked` generates a real assertion)
+
+#![crate_type = "lib"]
+#![feature(try_with_capacity)]
+
+// CHECK-LABEL: @with_capacity_does_not_grow1
+#[no_mangle]
+pub fn with_capacity_does_not_grow1() -> Vec<u32> {
+    let v = Vec::with_capacity(1234);
+    // CHECK: call {{.*}}__rust_alloc(
+    // CHECK-NOT: call {{.*}}__rust_realloc
+    // CHECK-NOT: call {{.*}}capacity_overflow
+    // CHECK-NOT: call {{.*}}finish_grow
+    // CHECK-NOT: call {{.*}}reserve
+    // CHECK-NOT: memcpy
+    // CHECK-NOT: memset
+    v
+}
+
+// CHECK-LABEL: @try_with_capacity_does_not_grow2
+#[no_mangle]
+pub fn try_with_capacity_does_not_grow2() -> Option<Vec<Vec<u8>>> {
+    let v = Vec::try_with_capacity(1234).ok()?;
+    // CHECK: call {{.*}}__rust_alloc(
+    // CHECK-NOT: call {{.*}}__rust_realloc
+    // CHECK-NOT: call {{.*}}capacity_overflow
+    // CHECK-NOT: call {{.*}}finish_grow
+    // CHECK-NOT: call {{.*}}handle_alloc_error
+    // CHECK-NOT: call {{.*}}reserve
+    // CHECK-NOT: memcpy
+    // CHECK-NOT: memset
+    Some(v)
+}