Tiny-C compiler, in Rust

As an exercize in Rust, rewrote Marc Feeley's Tiny-C compile (Copyright (C) 2001 by Marc Feeley, All Rights Reserved). The Rust translation has been kept as close in spirit as possible to the original, while still being reasonable example of Rust (please do let me know what could be improved).

Reproducing the language description here:

This is a compiler for the Tiny-C language. Tiny-C is a considerably stripped down version of C and it is meant as a pedagogical tool for learning about compilers. The integer global variables "a" to "z" are predefined and initialized to zero, and it is not possible to declare new variables. The compiler reads the program from standard input and prints out the value of the variables that are not zero. The grammar of Tiny-C in EBNF is:

 <program> ::= <statement>
 <statement> ::= "if" <paren_expr> <statement> |
                 "if" <paren_expr> <statement> "else" <statement> |
                 "while" <paren_expr> <statement> |
                 "do" <statement> "while" <paren_expr> ";" |
                 "{" { <statement> } "}" |
                 <expr> ";" |
                 ";"
 <paren_expr> ::= "(" <expr> ")"
 <expr> ::= <test> | <id> "=" <expr>
 <test> ::= <sum> | <sum> "<" <sum>
 <sum> ::= <term> | <sum> "+" <term> | <sum> "-" <term>
 <term> ::= <id> | <int> | <paren_expr>
 <id> ::= "a" | "b" | "c" | "d" | ... | "z"
 <int> ::= <an_unsigned_decimal_integer>

Here are a few invocations of the compiler:

$ echo "a=b=c=2<3;" | cargo run
a = 1
b = 1
c = 1
$ echo "{ i=1; while (i<100) i=i+i; }" | cargo run
i = 128
$ echo "{ i=125; j=100; while (i-j) if (i<j) j=j-i; else i=i-j; }" | cargo run
i = 25
j = 25
$ echo "{ i=1; do i=i+10; while (i<50); }" | cargo run
i = 51
$ echo "{ i=1; while ((i=i+10)<50) ; }" | cargo run
i = 51
$ echo "{ i=7; if (i<5) x=1; if (i<10) y=2; }" | cargo run
i = 7
y = 2

The compiler does a minimal amount of error checking to help highlight the structure of the compiler.

tommythorn/tinyc-in-rust

Tiny-C compiler, in Rust